100 questions — 82 objective (1 mark), 10 short (5 marks), 8 long (10 marks). Every answer is checked against the 2014 BEE guidebook and carries its book section reference plus an explanation. ▶ Practice this chapter interactively (timer, read-aloud, progress saving).
Objective questions (1 mark) — 82
📖 §8.2 Basic parameters – Illuminance (lux)
1. One lux is defined as:
One lumen per watt
One lumen per square metre
One candela per square metre
One watt per square metre
Answer: B) One lumen per square metre
Confirmed vs Book-3 §8.2 — Illuminance (E) is the luminous flux incident on a surface per unit area; lux (lx) is its metric unit and one lux = one lumen per square metre. (a) lumen per watt is luminous efficacy — flux EMITTED per watt of lamp power, not flux received on a surface. (c) candela/m2 is luminance (brightness of a surface) and (d) W/m2 is a radiant, not photometric, quantity.
Source: AI practice
📖 §8.2 Luminous efficacy
2. Luminous efficacy of a lamp is expressed in:
Lumens (lm)
Lux (lm/m2)
Lumens per Watt (lm/W)
Candela (cd)
Answer: C) Lumens per Watt (lm/W)
Confirmed vs Book-3 §8.2 — Luminous efficacy = luminous flux emitted by the lamp / power consumed by the lamp, so the unit is lumens per lamp Watt (lm/W); §8.1 gives the theoretical maximum as 683 lm/W at 555 nm. (b) lux (lm/m2) is illuminance ON a surface, (a) lumen alone is luminous flux and (d) candela is luminous intensity. Lamp CIRCUIT efficacy is the same ratio but per circuit watt, i.e. including control gear losses.
Source: AI practice
📖 §8.2 Inverse square law
3. According to the inverse square law, if the distance between a point source and a surface is doubled, the illuminance on the surface becomes:
Half (1/2)
Double (2x)
One quarter (1/4)
Four times (4x)
Answer: C) One quarter (1/4)
Confirmed vs Book-3 §8.2 — The inverse square law E = I/d2 makes illuminance inversely proportional to the SQUARE of the distance, so doubling d gives E2 = E1/2^2 = one quarter of E1. (a) 'half' is the trap of treating the fall-off as linear in d. (d) four times is what happens when the distance is HALVED — the book's own example: 10 lm/m2 at 1 m becomes 40 lm/m2 at 0.5 m.
Source: AI practice
📖 §8.2 Inverse square law
4. A surface receives an illuminance of 16 lm/m2 at a distance of 2 m from a source. Using the inverse square law, the illuminance at 1 m would be:
8 lm/m2
32 lm/m2
64 lm/m2
4 lm/m2
Answer: C) 64 lm/m2
Confirmed vs Book-3 §8.2 — Apply E1 x d1^2 = E2 x d2^2: 16 x 2^2 = E2 x 1^2, so E2 = 16 x 4 = 64 lm/m2. Moving closer to the source INCREASES illuminance by the square of the distance ratio. (a) 8 and (d) 4 come from dividing instead of multiplying; (b) 32 comes from scaling by d instead of d2.
5. Per Table 8.1, which of the following lamps has the HIGHEST average luminous efficacy?
Metal Halide
Low Pressure Sodium Vapour (LPSV / SOX)
Compact Fluorescent Lamp (CFL)
Incandescent (GLS)
Answer: B) Low Pressure Sodium Vapour (LPSV / SOX)
Confirmed vs Book-3 §8.3, Table 8.1 — Average efficacies: LPSV (SOX) 150 lm/W (range 101-175) > metal halide 100 (75-125) > CFL 60 (40-70) > incandescent 14 (8-18). LPSV is the most efficacious source in the book's table. (a) metal halide is the trap — it is second, not first. The trade-off: LPSV has the worst colour rendering (CRI 10, monochromatic), so the book limits it to roadways, tunnels and canals.
Source: AI practice
📖 §8.3 Table 8.1 (Colour Rendering Index column)
6. Which lamps have the best (highest) Colour Rendering Index, CRI = 100?
Incandescent and Halogen
LPSV and HPSV
Metal Halide and LED
CFL and Fluorescent tube
Answer: A) Incandescent and Halogen
Confirmed vs Book-3 §8.3, Table 8.1 — Only incandescent (GLS) and halogen are listed as 'Excellent (100)'. (c) metal halide is 'Good (70)' and LED 'Very good (80)'; (d) FTL is 67-77 and CFL 85; (b) HPSV is 'Fair (22)' and LPSV 'Poor (10)' — the worst of all. Note the inverse trade-off: the two CRI-100 lamps are also the least efficacious (14 and 20 lm/W average).
Source: AI practice
📖 §8.3 Fluorescent tube lamp (T-numbers)
7. Fluorescent tube designations such as T12, T8 and T5 are categorized on the basis of the tube's:
Length
Wattage
Diameter
Colour temperature
Answer: C) Diameter
Confirmed vs Book-3 §8.3 — The T-number is the tube DIAMETER in eighths of an inch: T12 = 38 mm (12/8 in), T8 = 25 mm (1 in), T5 = 16 mm (5/8 in), T2 = 6 mm. Length (a) and wattage (b) vary independently of the T-number. The book adds that T5 and T8 lamps offer about 5% higher efficacy than the 40 W T12.
Source: AI practice
📖 §8.2 Luminaire / Control gear
8. A device that distributes, filters or transforms the light emitted from one or more lamps (but excludes the lamps themselves) is called a:
Ballast
Luminaire
Photocell
Reflector lamp
Answer: B) Luminaire
Confirmed vs Book-3 §8.2 — Book definition: a luminaire is a device that distributes, filters or transforms the light emitted from one or more lamps, and includes all parts for fixing and protecting the lamps, EXCEPT the lamps themselves. (a) a ballast is control gear — a current-limiting device that counters the negative resistance of a discharge lamp; (d) a reflector lamp is itself a lamp (an incandescent with an internal mirror), not a luminaire.
Source: AI practice
📖 §8.6(f) Electronic ballasts
9. A high-frequency electronic ballast typically operates the lamp at a frequency of about:
50 Hz
500-1000 Hz
20-30 kHz
1-2 MHz
Answer: C) 20-30 kHz
Confirmed vs Book-3 §8.6(f) — Electronic ballasts convert the supply frequency to about 20,000 to 30,000 Hz, i.e. 20-30 kHz. (a) 50 Hz is the supply frequency at which a conventional electromagnetic choke operates. Book-stated benefits: loss of only about 1 Watt against 10-15 Watts in a standard choke, the starter is eliminated, the tube lights up instantly without flickering, and efficacy improves at the higher frequency.
Source: AI practice
📖 §8.7 Daylight linked control
10. A daylight-linked lighting control that switches or dims the electric lighting according to available natural light uses a:
Infra-red occupancy sensor
Photoelectric cell (photocell)
Spring-wound dial timer
Ultrasonic motion detector
Answer: B) Photoelectric cell (photocell)
Confirmed vs Book-3 §8.7 — Daylight-linked control uses photoelectric cells, either simply to switch lighting on and off or for dimming, so that daylight plus electric light always reaches the design level; if daylight alone meets the requirement the electric lighting is switched off. (a) infra-red and (d) ultrasonic sensors belong to OCCUPANCY-linked control (they detect movement or noise), and (c) the spring-wound dial timer is the cheapest form of time-based control.
Source: AI practice
📖 §8.5 Lumen method – number of fittings
11. A room of area 100 m2 requires 300 lux on the work plane. Each fitting delivers 4000 lumens, with UF = 0.5 and LLF = 0.75. The number of fittings required (rounded up) is:
15
20
25
30
Answer: B) 20
Confirmed vs Book-3 §8.5 — N = (E x A)/(F x UF x LLF) = (300 x 100)/(4000 x 0.5 x 0.75) = 30,000/1500 = 20 fittings. (a) 15 comes from omitting the light loss factor (30,000/2000); (c) 25 from using UF = 0.6. LLF covers lamp lumen depreciation, luminaire dirt and room surface maintenance; the computed N is always rounded UP.
Source: AI practice
📖 §8.3 Compact fluorescent lamp (CFL)
12. A Compact Fluorescent Lamp (CFL) is primarily intended as a direct replacement for which lamp?
Fluorescent tube light (FTL)
Incandescent / GLS lamp
High pressure sodium vapour lamp
Metal halide lamp
Answer: B) Incandescent / GLS lamp
Confirmed vs Book-3 §8.3 — CFLs are compact/miniature versions of fluorescent lamps that are 'designed to replace an incandescent lamp' and fit into most existing incandescent fixtures. (a) FTL is the classic trap — a CFL IS a miniature FTL, so it does not replace one. Table 8.1 gain: CFL 60 lm/W average and 8000-10,000 h life against GLS 14 lm/W and 1000 h.
Source: AI practice
📖 §8.2 Luminaire definition
13. What is the primary function of a luminaire in a lighting system?
To generate light
To store electrical energy
To distribute light emitted from lamps
To control the voltage supply
Answer: C) To distribute light emitted from lamps
Confirmed vs Book-3 §8.2 — A luminaire is 'a device that distributes, filters or transforms the light emitted from one or more lamps' and includes everything for fixing/protecting the lamps except the lamps themselves. The lamp (not the luminaire) generates the light, so option (a) is wrong; storage/voltage control are not lighting functions.
Source: Sep 2024
📖 §8.3 Table 8.1 Luminous performance of lamps
14. Which type of lamp has the highest luminous efficacy among the following?
Low pressure sodium vapour lamp
Halogen lamp
LED lamp
Compact fluorescent lamp (CFL)
Answer: A) Low pressure sodium vapour lamp
Confirmed vs Book-3 §8.3 — Table 8.1 gives LPSV (SOX) 101–175 lm/W (avg 150), the highest of every lamp listed; the text calls LPSV 'the most efficacious light sources'. LED is 50–130 (avg 90), CFL 40–70 (avg 60), halogen 18–24 (avg 20). LPSV's penalty is CRI 10 (monochromatic light), not efficacy.
Source: Sep 2024
📖 §8.6(d) Selection of high efficiency lamps & luminaires
15. How does the use of high-efficiency luminaries contribute to energy conservation?
By decreasing the power consumption
By increasing the luminous efficacy
By improving light distribution characteristics
All of the above
Answer: D) All of the above
Confirmed vs Book-3 §8.6(d) — Efficient lamps/luminaires save energy by (i) drawing less power for the same lumens, (ii) higher luminous efficacy (lm/W, Table 8.1) and (iii) better light distribution so more of the flux reaches the working plane (higher utilisation factor, §8.5). All three mechanisms apply, hence 'all of the above'.
Source: Sep 2024
📖 §8.2 Installed power density (W/m²) — LPD arithmetic (ECBC context)
16. A hotel building has 14 floors, each of 1000m2 area, If the Lighting Power Density is 10.8 per m2 the interior lighting power allowance for the hotel building is_________
110800 W
129600 W
151200 W
186600 W
Answer: C) 151200 W
Confirmed vs Book-3 §8.2 (power density) — Lighting power allowance = total floor area × LPD = (14 floors × 1000 m²) × 10.8 W/m² = 14,000 × 10.8 = 151,200 W. Option (b) 129,600 W corresponds to 12 floors and (a)/(d) do not match any multiple of 10.8.
Source: Sep 2024
📖 §8.2 Luminous efficacy (lm/W)
17. Luminous efficacy is:
Ratio of lumens to watts
Measured in candela
Same as luminance
Measures reflection of light
Answer: A) Ratio of lumens to watts
Confirmed vs Book-3 §8.2 — Luminous efficacy is 'the ratio of luminous flux emitted by a lamp to the power consumed by the lamp', unit lumens per Watt (lm/W). Candela is the unit of luminous intensity, luminance is brightness of a surface, and reflectance concerns reflected light — none is efficacy.
Source: Sep 2025
📖 §8.2 Colour rendering index (CRI) + Table 8.1
18. Which lamp is most suitable for color-critical applications?
Halogen lamps
LED lamps
CFLs
Metal halide lamps
Answer: A) Halogen lamps
Confirmed vs Book-3 §8.3 Table 8.1 — Halogen (like incandescent) has CRI 'Excellent (100)', versus LED 80, CFL 85 and metal halide 70. Colour-critical work needs the highest CRI, so halogen is correct; LED tempts because it is efficient, but efficacy is not colour rendering.
Source: Sep 2025
📖 §8.2 Luminaire definition
19. Luminaires are used to:
Store electrical energy
Distribute and control light from lamps
Produce light directly
Increase lamp wattage
Answer: B) Distribute and control light from lamps
Confirmed vs Book-3 §8.2 — The luminaire 'distributes, filters or transforms the light emitted from one or more lamps' (i.e. controls it). It does not produce light (the lamp does), store energy, or change lamp wattage.
Source: Sep 2025
📖 §8.6(g) Lighting controllers / §8.7 Energy efficient lighting controls
20. Examples of lighting controls include:
Dimmer switches
Timers
Photo-sensors
All of the above
Answer: D) All of the above
Confirmed vs Book-3 §8.6(g) — 'Lighting controllers ... includes dimmers, motion & occupancy sensors, photosensors and timers.' Dimmer switches, timers and photo-sensors are all named, so 'all of the above'.
Source: Sep 2025
📖 §8.2 Illuminance & Lux
21. The unit of one lux is
1000 lumen per square meter
10 lumen per square meter
1 lumen per square meter
1 lumen per square feet
Answer: C) 1 lumen per square meter
Confirmed vs Book-3 §8.2 — 'Lux (lx) is the metric unit of measure for illuminance of a surface. One lux is equal to one lumen per square meter.' Lumen per square foot is the foot-candle (imperial), and the 10/1000 multiples have no basis.
Source: Book EOC
📖 §8.2 Luminous efficacy (lm/W)
22. Luminous efficacy of a lamp is given by
Lux/Watt
lumens/Watt
Watt/Lux
Watt/lumens
Answer: B) lumens/Watt
Confirmed vs Book-3 §8.2 — Luminous efficacy = luminous flux emitted (lumens) ÷ power consumed (Watt), i.e. lumens/Watt. Lux/Watt is not efficacy (lux is illuminance on a surface, not lamp output); the inverse ratios are meaningless.
Source: Book EOC
📖 §8.6(f) Electronic ballasts
23. A fluorescent tube light fitted with an electronic choke will
operate at 50 Hz
not need a starter
operate at 0.5 power factor
none of the above
Answer: B) not need a starter
Confirmed vs Book-3 §8.6(f) — 'With electronic ballast, the starter is eliminated and the tube light lights up instantly without flickering.' It converts supply to about 20–30 kHz (so not 50 Hz) and has ~1 W loss; it does not run at 0.5 PF.
Source: Book EOC
📖 §8.2 Luminaire definition
24. A device that distributes and filters the light emitted from one or more lamps is
control gear
lamp
luminaire
starter
Answer: C) luminaire
Confirmed vs Book-3 §8.2 — Verbatim definition: a luminaire 'distributes, filters or transforms the light emitted from one or more lamps'. Control gear (ballast, ignitor) limits current/starts the discharge; the lamp produces light; a starter only initiates an FTL.
25. The T2, T5, T8 and T12 fluorescent lamps are categorized based on
diameter of the tube
length of the tube
both diameter and length
none of the above
Answer: A) diameter of the tube
Confirmed vs Book-3 §8.3 — 'These four lamps vary in diameter': T12 = 38 mm (1.5" = 12/8"), T8 = 25 mm (1"), T5 = 16 mm (5/8"), T2 = 6 mm (1/4"). The T-number is the diameter in eighths of an inch; length is not part of the designation.
Source: Book EOC
📖 §8.6(e) Reduction of lighting feeder voltage + Table 8.3
26. Which of the following options reduces the lighting consumption in a wide spread plant?
replacing 150 W HPSV lamps with 250 W HPMV lamps
maintaining 260 V for the lighting circuit with 220 V rated lamps
installing separate lighting transformer and maintaining optimum voltage
none of the above
Answer: C) installing separate lighting transformer and maintaining optimum voltage
Confirmed vs Book-3 §8.6(e) — Reducing/optimising lighting feeder voltage with reactors/transformers gives 5–15 % saving, and higher voltage shortens lamp life. Option (b) over-voltage (260 V on 220 V lamps) raises power input (Table 8.3: +8.1 % for 10 % over-voltage); option (a) swaps 150 W HPSV for 250 W HPMV, i.e. more watts at lower efficacy (Table 8.1: HPMV 50 vs HPSV 90 lm/W).
Source: Book EOC
📖 §8.3(5) HPSV vs metal halide + Table 8.1
27. Which of the following has the highest efficacy?
Metal halide
halogen lamps
HPMV
HPSV
Answer: D) HPSV
Confirmed vs Book-3 §8.3 — On HPSV the book states 'Its higher efficacy makes it a better choice than metal halide', and Table 8.1 shows HPSV 67–121 lm/W vs HPMV 44–57 and halogen 18–24, so HPSV is the key. Caution: Table 8.1's averages (MH 100 vs HPSV 90) look reversed — follow the text statement for this book question; overall highest efficacy of all lamps is LPSV (150).
Source: Book EOC
📖 §8.3(4) Compact fluorescent lamp
28. A CFL is an energy efficient replacement for
FTL
GLS
HPMV
HPSV
Answer: B) GLS
Confirmed vs Book-3 §8.3 — CFLs 'are designed to replace an incandescent lamp and can fit into most existing light fixtures formerly used for incandescent' — the incandescent lamp is the GLS (General Lighting Service) lamp. CFL is itself a miniature FTL, so (a) is wrong; HPMV/HPSV are HID outdoor/industrial lamps.
Source: Book EOC
📖 §8.7 Occupancy sensors
29. Which method uses infrared, acoustic, ultrasonic or microwave sensors for lighting control?
time-based control
daylight-linked control
occupancy-linked control
localized switching
Answer: C) occupancy-linked control
Confirmed vs Book-3 §8.7 — 'Occupancy-linked control can be achieved using infra-red, acoustic, ultrasonic or microwave sensors, which detect either movement or noise.' Time-based control uses timed-turnoff switches, daylight-linked uses photoelectric cells, localised switching uses local switches.
Source: Book EOC
📖 §8.2 Illuminance (E)
30. Which of the following is the best definition of illuminance?
Time rate of flow of light energy
Luminous flux incident on an object per unit area
Flux density emitted from an object without regard for direction
Flux density emitted from an object in a given direction
Answer: B) Luminous flux incident on an object per unit area
Confirmed vs Book-3 §8.2 — Illuminance 'is the quotient of the luminous flux incident on an element of the surface ... by the area of that element', i.e. flux incident per unit area (lux). Option (a) describes luminous flux; (c)/(d) describe light emitted from (not incident on) a surface — luminous exitance/luminance.
Source: Book EOC
📖 §8.2 Illuminance & Lux
31. Lux is defined as __________.
ratio of luminous flux emitted by a lamp to the power consumed by the lamp
lux per square meter
lumen per square feet
none of the above
Answer: D) none of the above
Confirmed vs Book-3 §8.2 — One lux = one lumen per square metre. Option (a) is the definition of luminous efficacy, (b) 'lux per square metre' is circular/wrong, (c) lumen per square foot is the foot-candle; hence none of the above.
Source: Mar 2023
📖 §8.3 Table 8.1 Luminous performance of lamps
32. Which among the following is the most energy efficient lamp for the same wattage rating?
HPMV
GLS
CFL
Metal halide
Answer: D) Metal halide
Corrected (was c) — Book-3 §8.3 Table 8.1: 'most energy efficient for the same wattage' means highest lumens per Watt. Metal halide = 75–125 lm/W (avg 100) beats CFL 40–70 (avg 60), HPMV 44–57 (avg 50) and GLS 8–18 (avg 14). CFL tempts because it is the efficient replacement for GLS, but it is not the most efficacious lamp in this list.
Source: 15th Exam
📖 §8.2 Colour rendering index (CRI) + Table 8.1
33. _____ is a measure of effect of light on the perceived colour of objects
lux
lumens
CRI
lamp circuit efficacy
Answer: C) CRI
Confirmed vs Book-3 §8.2 — 'Colour rendering index (CRI): is a measure of the effect of light on the perceived color of objects.' Lux is illuminance, lumens is luminous flux, and lamp circuit efficacy is lumens per circuit Watt including control-gear losses.
34. The occupancy sensors in a lighting installation are best suited for
conference halls
large production shops/hangars
entrances of offices/buildings
street lighting
Answer: A) conference halls
Corrected (was c) — Book-3 §8.7: occupancy sensors detect movement or noise 'in room spaces', switch lights ON when occupied and OFF after a set delay, with a built-in time delay because 'occupants often remain still or quiet for short periods' — i.e. intermittently used rooms such as conference halls. Large production shops/hangars call for localized switching (§8.7: 'applications which contain large spaces'); street lighting uses timer/daylight/lux-based controls (§8.7); building entrances need continuous lighting for security and see only transient movement, so a sensor there gives little saving.
35. In a T-5 Fluorescent Lamp, "5" is indicative of:
Tube diameter
5 watt loss
5% Energy Saving with respect to T8
5th generation lamp
Answer: A) Tube diameter
Confirmed vs Book-3 §8.3 — The T-number is the tube diameter in eighths of an inch: T5 = 5/8" = 16 mm, T8 = 1" = 25 mm, T12 = 1.5" = 38 mm. It is not a wattage, a loss figure or a generation; the book's separate fact is that T5/T8 give ~5 % higher efficacy than 40 W T12.
Source: 14th Exam
📖 §8.6(f) Electronic ballasts
36. Which of the following is not true for a fluorescent lamp with electronic ballast
presence of stroboscopic effect
energy savings
increased light output
no starter required
Answer: A) presence of stroboscopic effect
Confirmed vs Book-3 §8.6(f) — An electronic ballast runs the tube at 20–30 kHz, so the tube 'lights up instantly without flickering' — no stroboscopic effect. The other three ARE true: ~1 W loss vs 10–15 W (energy saving of 15–20 W/tube), efficacy improves at high frequency (more light), and the starter is eliminated.
Source: 14th Exam
📖 §8.6(e) Reduction of lighting feeder voltage + Table 8.3; Book EOC Q6
37. To optimize the voltage level fed to the lighting feeder, the best option is to install
servo stabilizer for lighting feeder
'exclusive' transformer for lighting
microprocessor based controllers
high frequency (HF) electronic ballasts
Answer: B) 'exclusive' transformer for lighting
Confirmed vs Book-3 §8.6(e) — The book's measure for optimising lighting voltage is a dedicated reactor/transformer on the lighting feeder ('manufacturers supply reactors/transformers as standard products', 5–15 % saving), and the book's own question keys 'installing separate lighting transformer and maintaining optimum voltage'. An exclusive transformer isolates lighting from power feeders and lets the tap be set at the optimum level; a servo stabiliser only corrects fluctuation, while HF ballasts and microprocessor controllers do not set feeder voltage.
Source: Set-A
📖 §8.2 Control gear (ballast); §8.6(f) Electronic ballasts
38. Which one of the following is an incorrect statement?
fluorescent lamp is an electric discharge lamp
electronic ballasts make use of semi-conductor devices
electronic ballasts have very low internal loss
fluorescent lamps can produce light by direct connection to the power source
Answer: D) fluorescent lamps can produce light by direct connection to the power source
Confirmed vs Book-3 §8.2 — 'Ballast is needed to start and operate fluorescent lamps, because of the characteristics of a gaseous arc' (negative resistance), so an FTL cannot be connected directly to the supply — (d) is the incorrect statement. (a) is verbatim ('basically an electric discharge lamp'), and electronic ballasts use semiconductor circuits with only ~1 W loss (§8.6f).
Source: Set-A
📖 §8.8 Standards & Labeling for FTL — Table 8.4
39. Energy Star Label Rating scheme for Fluorescent lamp is based on
Lumens per Watt at 100, 2000 and 3500 hours of use
End of Lamp Life in terms of burning hours
Lumen depreciation at 2000 hours
Color Rendering Index
Answer: A) Lumens per Watt at 100, 2000 and 3500 hours of use
Confirmed vs Book-3 §8.8 Table 8.4 — The FTL star rating is set by lumens per Watt measured at 100, 2000 and 3500 hours of use (e.g. 5-star ≥ 92, ≥ 83 and ≥ 78 lm/W respectively). Lamp life, lumen depreciation alone or CRI are not the rating criteria.
Source: Jul 2022
📖 §8.2 Colour rendering index (CRI) + Table 8.1
40. Which of the following is wrong with respect to Color Rendering Index (CRI)?
The CRI is expressed in a relative scale ranging from 0 - 100
CRI indicates how perceived colors match actual colors
CRI of Sodium Vapour lamp is much higher than that of a normal Incandescent Lamp
The higher the color rendering index, the less color shift or distortion occurs
Answer: C) CRI of Sodium Vapour lamp is much higher than that of a normal Incandescent Lamp
Confirmed vs Book-3 §8.2/Table 8.1 — CRI is a 0–100 scale showing how perceived colours match the reference source; higher CRI = less colour shift (statements a, b, d are true). Sodium vapour lamps have CRI 22 (HPSV) or 10 (LPSV) whereas incandescent is 100, so (c) is the wrong statement.
Source: 17th Sep-2016
📖 §8.3(8) LED lamp advantages; §8.9 Case study; §8.7 Street lighting
41. Which of the following is wrong statement with reference to LED lamps?
LED lamps are as energy efficient as CFL bulbs or better
LED lamps are more durable than CFLs
LED lamps has no hazardous material like mercury
LED lamps are not suitable for Street Lighting purpose
Answer: D) LED lamps are not suitable for Street Lighting purpose
Confirmed vs Book-3 §8.3/§8.9 — Table 8.1 lists LED applications as 'office, industry, outdoor, retail...' and the §8.9 case study replaces street/security FTLs with 18 W LEDs (payback 3.4 years), so LEDs ARE suitable for street lighting — (d) is the wrong statement. LEDs are mercury-free, vibration-resistant (durable) and 50–130 lm/W vs CFL 40–70, so (a)–(c) are true.
Source: 17th Sep-2016
📖 §8.3(1) Incandescent lamp (resistive filament); §8.2 Control gear
42. Power factor is highest in case of
sodium vapour lamps
LED lamps
tube lights
incandescent lamps
Answer: D) incandescent lamps
Confirmed vs Book-3 §8.3 — An incandescent lamp is a plain heated filament, a purely resistive load, so its power factor is ~1. Sodium vapour lamps and tube lights need inductive ballasts/ignitors (§8.2 control gear) and LED lamps need electronic drivers, all of which lower the power factor.
43. Which of the following devices do not produce any harmonics ?
UPS
incandescent bulb
arc furnace
electronic ballast
Answer: B) incandescent bulb
Confirmed vs Book-3 §8.3 — The incandescent filament is a linear resistive load, so current is sinusoidal and no harmonics are generated. UPS, arc furnaces and electronic ballasts (which convert supply to 20–30 kHz, §8.6f) are non-linear switching/arcing loads that inject harmonics.
Source: Mar 2021
📖 §8.2 Inverse square law (E1·d1² = E2·d2²)
44. As per the Inverse Square Law of illumination what will be the illuminance at half the distance?
50%
4 times
double
No change
Answer: B) 4 times
Confirmed vs Book-3 §8.2 — Illuminance is inversely proportional to the square of distance (E = I/d²). At half the distance E ∝ 1/(0.5)² = 4 times; the book's own example: 10 lm/m² at 1 m becomes 40 lm/m² at 0.5 m. 'Double' is the common trap (that would be a linear law).
45. The nomenclature T2, T5, T8 and T12 for fluorescent lamps are categorized based on:
diameter of the tube
length of the tube
both diameter and length of the tube
power consumption
Answer: A) diameter of the tube
Confirmed vs Book-3 §8.3 — T12 = 38 mm, T8 = 25 mm, T5 = 16 mm, T2 = 6 mm: 'these four lamps vary in diameter' (the T-number is eighths of an inch). Length and wattage are not encoded in the designation.
Source: 16th Exam
📖 §8.2 Inverse square law (E1·d1² = E2·d2²)
46. The illuminance of a lamp at one meter distance is 10 Lm/m2. What will be the corresponding value at 0.7 meter distance?
14.28
20.41
10
none of these
Answer: B) 20.41
Confirmed vs Book-3 §8.2 — E1·d1² = E2·d2² → E2 = 10 × (1/0.7)² = 10/0.49 = 20.41 lm/m². Option (a) 14.28 is the linear (1/0.7) trap; illuminance rises with the square of the distance ratio.
Source: 16th Exam (alt set)
📖 §8.2 Illuminance & Lux
47. Illuminance of a surface is expressed in
Radians
Lux
Lumens
LPD
Answer: B) Lux
Confirmed vs Book-3 §8.2 — 'Lux (lx) is the metric unit of measure for illuminance of a surface' (1 lm/m²). Lumens measure luminous flux emitted by the source, LPD is lighting power density in W/m², and radians are angles.
Source: 19th Exam
📖 §8.2 Installed power density (W/m²) — LPD arithmetic (ECBC context)
48. A hotel building has four floors each of 1000m2 area. If the interior lighting power allowance for the hotel building is 43,000 W. The Lighting Power Density (LPD) is
10.75
0.09
43
data insufficient
Answer: A) 10.75
Confirmed vs Book-3 §8.2 (power density) — LPD = lighting power ÷ floor area = 43,000 W ÷ (4 × 1000 m²) = 10.75 W/m². Option (b) 0.09 is the inverted ratio; the data are sufficient.
Source: Mar 2021
📖 §8.2 Colour rendering index (CRI) + Table 8.1
49. Which of the following type of lamps is most suitable for color critical applications?
halogen lamps
LED lamps
CFLs
Low pressure sodium vapour lamp
Answer: A) halogen lamps
Confirmed vs Book-3 §8.3 Table 8.1 — Halogen CRI = Excellent (100), LED 80, CFL 85, LPSV Poor (10 — monochromatic, colours appear grey). Colour-critical work needs the highest CRI, so halogen.
50. The T2, T5, T8 and T12 fluorescent tube light are categorized based on
diameter of the tube
length of the tube
both diameter and length of the tube
power consumption
Answer: A) diameter of the tube
Confirmed vs Book-3 §8.3 — T12 = 38 mm (1.5"), T8 = 25 mm (1"), T5 = 16 mm (5/8"), T2 = 6 mm (1/4"); the classification is by tube diameter only.
Source: 18th Exam
📖 §8.3(9) Induction lamp
51. The lamp based on high frequency electromagnetic field from outside, exciting the mercury gas sealed in the bulb, to produce UV radiation and light is
Induction lamp
Fluorescent lamp
Mercury vapour lamp
Metal halide lamp
Answer: A) Induction lamp
Confirmed vs Book-3 §8.3 — Induction lamp: 'high frequency electromagnetic fields are induced from outside the sealed chamber', electrons collide with mercury atoms to produce UV, which the phosphor converts to visible light — no electrodes inside the bulb. A fluorescent lamp uses internal filaments/electrodes; HPMV and metal halide are HID arc lamps.
52. A parameter that indicates adequacy of lighting for a particular application is
installed load efficacy
installed power density
lux
lumens
Answer: C) lux
Confirmed vs Book-3 §8.2/§8.4 — Adequacy of lighting for a task is judged by the illuminance on the working plane in lux, compared with the recommended lux ranges (IS 3646, Table 8.2). Installed load efficacy (lux/W/m²) and installed power density (W/m²/100 lux) measure energy efficiency, and lumens is lamp output, not what reaches the task.
53. Which of the following is not an example of lighting controls?
dimmers
timers
photosensors
daylight harvesting
Answer: D) daylight harvesting
Confirmed vs Book-3 §8.6(g) — The book lists lighting controllers as 'dimmers, motion & occupancy sensors, photosensors and timers'. Daylight harvesting is the strategy of using natural daylight (§8.6a / §8.7 daylight-linked control); it is implemented BY photosensors and dimmers, so it is not itself a control device.
Source: 18th Exam
📖 §8.3(1) Incandescent lamp — P = V²/R
54. Which of the following incandescent bulbs will have the least resistance?
220 V, 60 W
220 V, 100 W
115 V, 60 W
115 V, 100 W
Answer: D) 115 V, 100 W
Confirmed vs Book-3 §8.3 — For a resistive filament R = V²/P. 220 V/60 W → 807 Ω; 220 V/100 W → 484 Ω; 115 V/60 W → 220 Ω; 115 V/100 W → 132 Ω, the least. Lower voltage and higher wattage both reduce resistance.
55. The T5, T8 and T12 fluorescent tube lights are categorized based on
Diameter of the tube
Length of the tube
Both diameter and length of the tube
Power consumption
Answer: A) Diameter of the tube
Confirmed vs Book-3 §8.3 — T5 = 16 mm, T8 = 25 mm, T12 = 38 mm: the T-number is the tube diameter in eighths of an inch; length and power are not part of the designation.
Source: 19th Exam
📖 §8.2 Luminous efficacy (lm/W)
56. Ratio of luminous flux (lumen) emitted by a lamp to the power consumed (watt) by the lamp is called
Luminous intensity
Luminous efficacy
Reflectance
Luminance
Answer: B) Luminous efficacy
Confirmed vs Book-3 §8.2 — Verbatim: luminous efficacy 'is the ratio of luminous flux emitted by a lamp to the power consumed by the lamp' (lm/W). Luminous intensity is flux per solid angle (candela), luminance is surface brightness, reflectance is the reflected fraction.
Source: 19th Exam
📖 §8.7 Occupancy sensors
57. __________ can be achieved using infrared, acoustic, ultrasonic or microwave sensors for energy efficient lighting control.
Time-based control
Daylight-linked control
Occupancy-linked control
Localized switching
Answer: C) Occupancy-linked control
Confirmed vs Book-3 §8.7 — 'Occupancy-linked control can be achieved using infra-red, acoustic, ultrasonic or microwave sensors, which detect either movement or noise.' Time-based control uses timed-turnoff switches; daylight-linked control uses photoelectric cells; localized switching uses local switches in large spaces.
Source: 19th Exam
📖 §8.2 Inverse square law (E1·d1² = E2·d2²)
58. The illuminance is 10 lm/m² from a lamp at 1 meter distance. The illuminance at half the distance will be
40 lm/m²
10 lm/m²
5 lm/m²
None of the above
Answer: A) 40 lm/m²
Confirmed vs Book-3 §8.2 — This is the book's own worked example: E = (d1/d2)² × E1 = (1.0/0.5)² × 10 = 40 lm/m². Halving the distance quadruples illuminance (inverse square law); 5 lm/m² is the trap of halving instead.
Source: 19th Exam
📖 §8.8 Standards & Labeling for FTL — Table 8.4
59. The star rating scheme of Fluorescent Tube light as per BEE Standards & Labelling Scheme is based on
Lumen Output
Lux per Watt
Lux per Watt per m2
Lumen per Watt at different operating hours
Answer: D) Lumen per Watt at different operating hours
Confirmed vs Book-3 §8.8 Table 8.4 — The FTL star rating is defined by lumens per Watt at 100, 2000 and 3500 hours of use (5-star ≥ 92 / ≥ 83 / ≥ 78 lm/W). Lumen output alone or lux-based ratios are not the criterion.
Source: 19th Exam
📖 §8.2 Colour rendering index (CRI) + Table 8.1
60. Which of the following is not true with respect to Color Rendering Index (CRI)?
The CRI is expressed in a relative scale ranging from 0-100.
CRI indicates, how perceived colors match with actual colors.
LED lamps are having comparatively higher CRI than Incandescent Lamps.
The higher the color rendering index, the less color shift or distortion occurs
Answer: C) LED lamps are having comparatively higher CRI than Incandescent Lamps.
Confirmed vs Book-3 §8.2/Table 8.1 — CRI is a 0–100 relative scale, indicates how perceived colours match actual colours, and a higher CRI means less colour shift (a, b, d true). Incandescent CRI = 100 vs LED 80, so 'LEDs have higher CRI than incandescent' is the untrue statement.
Source: Sep 2019
📖 §8.3(1) Incandescent lamp (resistive filament); §8.2 Control gear
61. Power factor is highest in the case of _________________.
Sodium vapour lamps
Induction lamps
LED Lamps
Incandescent lamps
Answer: D) Incandescent lamps
Confirmed vs Book-3 §8.3 — The incandescent filament is a purely resistive load, giving power factor ≈ 1. Sodium vapour and induction lamps need ballasts/ignitors and LEDs need electronic drivers, all of which draw reactive/distorted current and lower the power factor.
Confirmed vs Book-3 §8.5 — 'LLF = Light Loss Factor. This factor takes account of the depreciation over time of lamp output and dirt accumulation on the fitting and walls'; LLF = lamp-lumen MF × luminaire MF × room-surface MF (typical 0.8 AC office, 0.7 clean industrial, 0.6 dirty industrial).
Source: Sep 2019
📖 §8.6(e) Reduction of lighting feeder voltage + Table 8.3
63. If voltage is increased from 230 V to 250 V for a fluorescent tube light, it will result in
reduced power consumption
increased power consumption
decreased light levels
no change in power consumption and light levels
Answer: B) increased power consumption
Confirmed vs Book-3 §8.6(e) Table 8.3 — For fluorescent lamps a 10 % higher voltage increases power input by 8.1 % (and light output by only 8 %); higher voltage also reduces lamp life. So 230 → 250 V raises power consumption; it does not reduce light level.
Source: 9th Dec-2009
📖 §8.6(f) Electronic ballasts
64. The electronic ballast fitted in a tube light fitting does not have one of the following characteristics
lower operational losses than conventional ballasts
tuned circuit to deliver power at 28-32 KHz
requiring a starter
low temperature rise
Answer: C) requiring a starter
Confirmed vs Book-3 §8.6(f) — Electronic ballasts have ~1 W loss (vs 10–15 W), run the lamp at high frequency (book: 20–30 kHz; exam quotes 28–32 kHz) and hence low temperature rise, and 'the starter is eliminated'. Requiring a starter is the characteristic they do NOT have.
Source: 9th Dec-2009
📖 §8.6(f) Electronic ballasts
65. The basic functions of an electronic ballast fitted to a fluorescent tube light exclude one of the following
to stabilize the gas discharge
to supply power to the lamp at supply frequency
to ignite the tube light
to supply power to the lamp at very high frequency
Answer: B) to supply power to the lamp at supply frequency
Confirmed vs Book-3 §8.6(f) — Basic functions of an electronic ballast: 'to ignite the lamp, to stabilize the gas discharge, and to supply the power to the lamp' — and it does so after converting the supply frequency to about 20,000–30,000 Hz. Supplying the lamp at supply (50 Hz) frequency is what a conventional electromagnetic choke does, so (b) is excluded.
Source: 10th Jul-2010
📖 §8.6(e) Reduction of lighting feeder voltage + Table 8.3; Book EOC Q6
66. Which of the following options reduces the electricity consumption in lighting system in a wide spread plant?
installing separate lighting transformer and maintaining optimum voltage
maintaining 260 V for the lighting circuit with 220 V rated lamps
replacing 150 W HPSV lamps with 250 W HPMV lamps
none of the above
Answer: A) installing separate lighting transformer and maintaining optimum voltage
Confirmed vs Book-3 §8.6(e) — A separate lighting transformer maintaining optimum voltage saves 5–15 % and extends lamp life. Running 220 V lamps at 260 V increases power input (Table 8.3) and shortens life; swapping 150 W HPSV for 250 W HPMV increases watts and lowers efficacy (Table 8.1: HPSV 90 vs HPMV 50 lm/W).
Source: 10th Jul-2010
📖 §8.6(e) Reduction of lighting feeder voltage + Table 8.3
67. The advantage of installing a dedicated servo transformer for lighting feeders is;
'Voltage' fluctuations in lighting circuit can be minimized by isolating from the power feeders.
reduction of voltage related problems, which in turn increases the efficiency of the lighting system.
with proper control device 'over voltage' that might occur during lean load or off-peak can be avoided, in turn less energy consumption and improved lamp life can be achieved
all the above
Answer: D) all the above
Confirmed vs Book-3 §8.6(e) — The book notes that 'higher night-time voltage reduces lamp life' and that reactors/transformers on the lighting feeder save 5–15 %. A dedicated servo/lighting transformer isolates lighting from power-feeder fluctuations, avoids lean-load over-voltage, improves efficiency and lamp life — all three statements hold, so 'all the above'.
Source: 11th Feb-2011
📖 §8.2 Inverse square law (E1·d1² = E2·d2²)
68. The illuminance is 10 lm/m2 from a lamp at 1 meter distance. The illuminance at half the distance will be
40 lm/m2
10 lm/m2
5 lm/m2
none of the above
Answer: A) 40 lm/m2
Confirmed vs Book-3 §8.2 — Book's worked example: E = (1.0/0.5)² × 10 = 40 lm/m². Halving the distance multiplies illuminance by 4 (inverse square law), not by 2.
Source: 11th Feb-2011
📖 §8.3(1) Incandescent lamp — Figure 8.3 energy flow
69. The lamp which gives 10% visible radiation is
CFL
flourescent tube light
HPSV
incandescent lamp
Answer: D) incandescent lamp
Confirmed vs Book-3 §8.3 Figure 8.3 — Incandescent lamp energy flow: ≈10 % visible radiation, ~20 % conduction/convection loss, ~70 % infrared. Fluorescent/CFL are 3–5 times as efficient and HPSV is 67–121 lm/W, so only the incandescent lamp gives just 10 % visible output.
Source: 11th Feb-2011
📖 §8.6(f) Electronic ballasts
70. The electronic ballast in lighting application does not have one of the following characteristics
lower operational losses than conventional ballasts
tuned circuit to deliver power at 28-32 kHz
requiring a starter
low temperature rise
Answer: C) requiring a starter
Confirmed vs Book-3 §8.6(f) — Electronic ballasts: losses ~1 W vs 10–15 W (lower losses, low temperature rise), operate the lamp at high frequency (book 20–30 kHz), and 'the starter is eliminated' — so 'requiring a starter' is the characteristic it does not have.
Source: 11th Feb-2011
📖 §8.3 Table 8.1 Luminous performance of lamps (LED row)
71. The lumens output varies from _______ Lumens/Watt in case of White LED lamps.
30-50
75-125
101-175
67-121
Answer: A) 30-50
Confirmed vs Book-3 §8.3 Table 8.1 — By elimination: 75–125 lm/W is metal halide, 101–175 is LPSV and 67–121 is HPSV, so 30–50 (the older-edition figure for white LEDs used by this 2011 paper) is the intended key. Note the 2014 Table 8.1 now lists LED at 50–130 lm/W (avg 90; up to 200 in the laboratory) — quote 50–130 if asked directly about LED efficacy.
Source: 11th Feb-2011
📖 §8.2 Colour rendering index (CRI) + Table 8.1
72. Which one of the following lamps has maximum CRI ?
Incandescent lamp
LED lamp
CFL lamp
HPSV lamp
Answer: A) Incandescent lamp
Confirmed vs Book-3 §8.3 Table 8.1 — Incandescent CRI = Excellent (100): it renders the standard colour chips identically to the reference source. LED = 80, CFL = 85, HPSV = Fair (22).
Source: Mar 2021
📖 §8.2 Inverse square law (E1·d1² = E2·d2²)
73. The illuminance is 10 lm/m2 from a lamp at 1 meter distance. What will be the illuminance (in lm/m2) at 2 meter distance from lamp ?
2.75
2.5
40
20
Answer: B) 2.5
Confirmed vs Book-3 §8.2 — E1·d1² = E2·d2² → E2 = 10 × (1/2)² = 2.5 lm/m². Doubling the distance cuts illuminance to one quarter; 40 would be the answer for halving the distance and 20 assumes a (wrong) linear law.
Source: Mar 2021
📖 §8.2 Installed power density (W/m²) — LPD arithmetic (ECBC context)
74. An Energy Conservation Building Code has four floors each of 1000m2 area. If the interior lighting power allowance for the hotel building is 43,000 W. The Lighting Power Density (LPD) is ___________
10.75
0.09
43
data insufficient
Answer: A) 10.75
Confirmed vs Book-3 §8.2 (power density) — LPD = 43,000 W ÷ (4 × 1000 m²) = 10.75 W/m². 0.09 is the inverted ratio (m²/W) and 43 ignores the number of floors.
Source: Mar 2021 (Set B)
📖 §8.2 Colour rendering index (CRI) + Table 8.1
75. Which one of the following has the maximum CRI?
Incandescent lamp
LED lamp
CFL lamp
HPSV lamp
Answer: A) Incandescent lamp
Confirmed vs Book-3 §8.3 Table 8.1 — Incandescent = CRI 100 (excellent); LED 80, CFL 85, HPSV 22. Maximum CRI is therefore the incandescent lamp.
Source: Mar 2021 (Set B)
📖 §8.2 Inverse square law (E1·d1² = E2·d2²)
76. The illuminance is 20 lm/m² from a lamp at 1 meter distance. The illuminance at half the distance would be
77. In T-5 Fluorescent Lamp, '5' is indicative of:
5 watt power rating
5% energy saving with respect to T8
5/8 generation lamp
Tube diameter
Answer: D) Tube diameter
Confirmed vs Book-3 §8.3 — T5 means a tube of 5/8 inch (16 mm) diameter; T8 = 1 inch (25 mm), T12 = 1.5 inch (38 mm). The number is neither wattage nor a saving percentage (the book's 5 % figure is the T5/T8 efficacy gain over T12, unrelated to the name).
Source: Jul 2022
📖 §8.6(e) Voltage & losses (general electrical: P = VI, I²R loss)
78. For the same quantity of power handled by a distribution line, lower the voltage
lower the current drawn and lower the distribution loss
lower the voltage drop and lower the distribution loss
higher the current drawn and higher the distribution loss
higher the voltage drop and lower the distribution loss
Answer: C) higher the current drawn and higher the distribution loss
Confirmed (general principle; not specific to Book-3 Ch.8) — For the same power P = V·I, a lower voltage means a proportionally higher current, and distribution loss I²R rises with the square of the current, so both current and losses are higher.
Source: Jul 2022
📖 §8.2 Colour rendering index (CRI) + Table 8.1
79. __________ is a measure of effect of light on the perceived colour appearance of objects.
lux
lumens
CRI
lamp circuit efficacy
Answer: C) CRI
Confirmed vs Book-3 §8.2 — 'Colour rendering index (CRI): is a measure of the effect of light on the perceived color of objects' (100 = identical to the reference source). Lux is illuminance, lumens is flux, lamp circuit efficacy is lumens per circuit Watt.
Source: Mar 2023
📖 §8.7 Occupancy sensors
80. Which of the following is not used as a sensor for lighting occupancy linked control?
Infrared
acoustic
ultrasonic
pressure
Answer: D) pressure
Confirmed vs Book-3 §8.7 — Occupancy-linked control uses 'infra-red, acoustic, ultrasonic or microwave sensors' that detect movement or noise. Pressure sensing is not among the book's sensor types.
Source: Mar 2023
📖 §8.2 Installed power density (W/m²) — LPD arithmetic (ECBC context)
81. A hotel building has four floors each of 1000 m² area. If the Lighting Power Density (LPD) is 10.8 W/m², the interior lighting power allowance for the hotel building is __________.
1000 W
21600 W
43200 W
none of the above
Answer: C) 43200 W
Confirmed vs Book-3 §8.2 (power density) — Allowance = area × LPD = (4 × 1000 m²) × 10.8 W/m² = 43,200 W. 21,600 W would be two floors; 1000 W ignores the LPD.
Metal halide lamp can be considered as a variant of high pressure mercury vapour lamp (HPMV)
Efficacy of fluorescent tube light (FTL) remains constant throughout its operational life
HPSV lamps differ from mercury and metal-halide lamps in that they do not contain starting electrodes
LPSV lamps are the most efficacious light sources, but they produce the poorest quality light of all the lamp types
Answer: B) Efficacy of fluorescent tube light (FTL) remains constant throughout its operational life
Confirmed vs Book-3 §8.3 — Every lamp has 'the percent of output that a lamp loses over its life' and §8.6(h) says light output falls with ageing lamps, so FTL efficacy does NOT stay constant — (b) is the incorrect statement (Table 8.4 even rates FTLs at 100/2000/3500 h for this reason). (a), (c) and (d) are verbatim book statements: metal halide is a variant of HPMV, HPSV lamps contain no starting electrodes, and LPSV is the most efficacious but poorest-quality light.
Source: 11th Feb-2011
Short questions (5 marks) — 10
📖 §8.2 Basic parameters and terms in lighting
1. Define illuminance and luminous efficacy, giving the unit of each.
Model answer: Illuminance (E) is the luminous flux incident on a surface per unit area; its unit is the lux (1 lux = 1 lumen/m2). Luminous efficacy is the luminous flux emitted by a lamp divided by the electrical power consumed by the lamp; its unit is lumens per Watt (lm/W). The theoretical maximum efficacy is 683 lm/W at a wavelength of 555 nm.
Confirmed vs Book-3 §8.2 — Method: illuminance = flux INCIDENT per unit area (lux = lm/m2); efficacy = flux EMITTED per watt consumed (lm/W). Quote the units for full marks and, if asked, add the 683 lm/W maximum at 555 nm and the related term 'lamp circuit efficacy' (lumens per CIRCUIT watt, i.e. including control gear).
Source: AI practice
📖 §8.5 Lighting design for interiors – number of fittings
2. A room of 8 m x 6 m requires 250 lux. Each fitting provides 5000 lumens with a utilisation factor of 0.5 and a light loss factor of 0.8. Calculate the number of fittings required.
Model answer: Area A = 8 x 6 = 48 m2. N = (E x A) / (F x UF x LLF) = (250 x 48) / (5000 x 0.5 x 0.8) = 12000 / 2000 = 6 fittings. Since the result is exactly 6, six fittings are required (otherwise round up to the next whole number).
Confirmed vs Book-3 §8.5 — Method: A = L x W, then N = (E x A)/(F x UF x LLF), and round UP to a whole number of fittings. Here (250 x 48)/(5000 x 0.5 x 0.8) = 12,000/2000 = 6 exactly. Compare the book's worked example: N = (200 x 100)/(2 x 3050 x 0.66 x 0.8) = 6.2, taken as 6 twin-tube fixtures = 12 lamps of 36 W.
3. A commercial training hall with dimensions 18 m × 12 m is being planned. Calculate the number of 18 W LED lamps, each providing 1800 lumens, required to achieve an illuminance level of 300 Lux. The lamps will be installed at a height of 3 meters from the working plane. The utilisation factor (UF) of the system is 0.70, and the light loss factor (LLF) is 0.80.
Model answer: Area of room (A): 18 × 12 = 216 m²
Total lumens required (Φ_total): Φ_total = E × A = 300 × 216 = 64800 lumens
Effective lumens per lamp (Φ_lamp_effective): Φ_lamp_effective = Lumen Output × UF × LLF = 1800 × 0.70 × 0.80 = 1008 lumens
Number of lamps required (N): N = Φ_total / Φ_lamp_effective = 64800 / 1008 = 64.3 ≈ 65 lamps
Book-3 §8.5 lumen method: N = (E × A)/(F × UF × LLF) = (300 × 216)/(1800 × 0.70 × 0.80) = 64,800/1008 = 64.3 → 65 lamps. Height (3 m) is only needed for the room index/UF, which is already given as 0.70.
Source: Sep 2025
📖 §8.6(f) Electronic ballasts; §8.2 Control gear (ballast)
4. What is the function of an electronic choke in a lighting system?
Model answer: A choke (ballast) is the control gear of a discharge lamp: a current-limiting device that counters the negative-resistance characteristic of the discharge, aids the initial voltage build-up needed for starting and acts as a stabiliser in the circuit (§8.2). An electronic choke performs these basic functions — ignite the lamp, stabilise the gas discharge and supply power to the lamp — but does so by converting the supply frequency to about 20,000–30,000 Hz. Benefits (Book-3 §8.6f): ballast loss only ~1 W against 10–15 W in an electromagnetic choke; tube efficacy improves at high frequency, giving a saving of about 15–20 W per tube light; the starter is eliminated and the tube lights instantly without flicker; lower temperature rise and longer lamp life.
Book-3 §8.6(f): electronic ballast = ignite + stabilise discharge + supply power at 20–30 kHz; ~1 W loss vs 10–15 W; saves 15–20 W per tube; no starter, instant flicker-free start.
Source: Book EOC
📖 §8.6 General energy saving opportunities (a)–(h)
5. List five energy saving measures in a lighting system.
Model answer: Any five of the Book-3 §8.6 measures: (1) Use natural daylighting — north-light glazing, FRP skylights/atrium domes, windows, solar tubes (8–10 h/day). (2) De-lamping to remove excess lighting (with lower mounting height and efficient luminaires). (3) Task lighting — high illuminance only at the task, lower general level, low-wattage lamps. (4) Select high-efficiency lamps and luminaires per Table 8.1 (e.g. GLS → CFL/LED, T12 → T5/T8, HPMV → HPSV/metal halide/LED), subject to colour-rendering needs. (5) Reduce lighting feeder voltage with reactors/transformers — 5–15 % saving reported. (6) Replace electromagnetic chokes with electronic ballasts — ~1 W loss instead of 10–15 W, saving 15–20 W per tube. (7) Lighting controllers — dimmers, occupancy sensors, photosensors, timers (§8.7). (8) Lighting maintenance — clean lamps/fixtures, repaint interiors, relamp (dirt can cut illumination by 50 % at full power).
Model answer: Lux (lx): the metric unit of illuminance of a surface; one lux equals one lumen of luminous flux incident per square metre (1 lx = 1 lm/m²). Illuminance falls with the square of the distance (inverse square law). CRI (Colour Rendering Index): a measure of the effect of light on the perceived colour of objects. A set of standard colour chips is viewed under the lamp and under a reference source of the same correlated colour temperature; if the lamp renders the chips identically to the reference its CRI is 100, otherwise less than 100 — a low CRI means some colours appear unnatural. Luminous efficacy: the ratio of luminous flux emitted by a lamp to the power consumed by the lamp, in lumens per lamp Watt (lm/W); it reflects the efficiency of converting electricity to light (maximum possible 683 lm/W at 555 nm).
Book-3 §8.2 definitions: Lux = lm/m²; CRI = effect of light on perceived colour (100 = identical to reference); efficacy = lumens emitted per Watt consumed.
Source: Book EOC
📖 §8.2 Inverse square law (E1·d1² = E2·d2²)
7. The illuminance is 17 lm/m2 from a lamp at 2 meter distance. What will be the illuminance at 1.5 m distance?
Model answer: Inverse square law (Book-3 §8.2): E1·d1² = E2·d2². E2 = E1 × (d1/d2)² = 17 × (2/1.5)² = 17 × (4/2.25) = 17 × 1.778 = 30.2 lm/m². The illuminance at 1.5 m is about 30.2 lux (higher, because the point is nearer the source).
E2 = 17 × (2/1.5)² = 30.2 lux by the inverse square law.
Model answer: Lux (lx): the illuminance produced by a luminous flux of one lumen uniformly distributed over a surface area of one square meter (SI unit of illumination = one lumen per square meter). Luminous efficacy: the ratio of luminous flux emitted by a lamp to the power consumed by the lamp (lumens/Watt) — the energy efficiency of conversion from electricity to light.
Book-3 §8.2 definitions: one lux = one lumen per square metre of illuminance; luminous efficacy = luminous flux emitted ÷ power consumed by the lamp (lm/W), the efficiency of electricity-to-light conversion.
Model answer: Lux (lx): the illuminance produced by a luminous flux of one lumen uniformly distributed over a surface area of one square metre; the SI unit of illumination, equal to one lumen per square metre. Luminous efficacy: the ratio of luminous flux emitted by a lamp to the power consumed by the lamp (lumen/Watt); it is the energy efficiency of conversion from electricity to light.
Book-3 §8.2 definitions (2.5 marks each): one lux = one lumen per square metre; luminous efficacy = lumens emitted per Watt consumed by the lamp.
10. The size of an air-conditioned office is 12 m × 7 m. Desired illuminance level is 200 Lux. An architect has suggested to install 24 no's of 20 W LED lights at a height of 3 m from ground level. The working plane is 0.75 m above the floor. The other details of 20W LED lamps are: Output of LED Lamps 2000 lumens; Utilization factor 0.65; Light Loss Factor (LLF) 0.75. Calculate floor index & number of LED lights required to get the desired illuminance. As an energy manager do you agree with the architect decision = why?
Model answer: Mounting Height, Hm = 3 - 0.75 = 2.25 m. Room Index (RI) = (L×W)/[Hm×(L+W)] = (12×7)/[2.25×(12+7)] = 84/42.75 = 1.97. Number of LED lights = (E×A)/(F×UF×LLF) = (200×12×7)/(2000×0.65×0.75) = 16800/975 = 17.23 ≈ 18 lights. So total number of 20W LED lights required is 18. No, I don't agree with the architect because the number of LED lights required is only 18 against suggested 24 nos, which is an energy inefficient design.
Book-3 §8.5: Hm = 3 − 0.75 = 2.25 m; RI = (12×7)/[2.25×(12+7)] = 1.97; N = (200×84)/(2000×0.65×0.75) = 17.2 → 18 lamps, so 24 lamps would over-light the office (energy inefficient).
Source: Jul 2022
Long questions (10 marks) — 8
📖 §8.6 General energy saving opportunities in lighting
1. List any five energy-saving opportunities in a lighting system as given in the BEE guidebook.
Model answer: Five energy-saving opportunities from Section 8.6 are: (1) Use natural day lighting - north-light glazing (saw-tooth trusses), skylights of FRP material, atrium with FRP dome, windows and roof-mounted solar tubes giving 8-10 hrs/day. (2) De-lamping to remove excess lighting, combined with reduced mounting height and efficient luminaires so illuminance is hardly affected. (3) Selection of high-efficiency lamps and luminaires using Table 8.1 (limitation: the lamp may not be changed where colour rendering is important). (4) Install electronic ballasts - losses of only about 1 W against 10-15 W in an electromagnetic choke, no starter, instant flicker-free start and better efficacy at 20-30 kHz. (5) Lighting controllers - dimmers, motion and occupancy sensors, photosensors and timers. (Other valid answers: task lighting with low wattage lamps; reduction of lighting feeder voltage, reported savings of 5% to 15%; and lighting maintenance - cleaning of lamps/fixtures, repainting interiors and relamping, since dirt and ageing can cut illumination by 50% or more while full power is still drawn.)
Confirmed vs Book-3 §8.6 — Recall the eight-item list in order: day lighting, de-lamping, task lighting, high-efficiency lamps/luminaires, feeder voltage reduction (5-15%), electronic ballasts, lighting controllers, maintenance. Any five, each with one supporting figure or example, earns full marks.
2. L4 State whether True or False:
1. The lumen (lm) is the photometric equivalent of the Watt, weighted to match the eye response of the "standard observer," with blue light receiving the greatest weight.
2. The CRI of a lamp is 100 if it renders the color of the chips identical to the reference light source, indicating perfect color rendering.
3. A commercial building with a high window-to-wall ratio (WWR) and low SHGC glazing will experience higher cooling loads, as more solar heat will be transmitted through the windows.
4. Rotary screw compressors are preferable for fluctuating air demand.
5. Operating compressors at lower delivery pressures always results in higher energy efficiency.
6. Heat of compression dryers have higher operating costs compared to heatless purge dryers.
7. Using variable speed drives in compressors can eliminate unloaded running conditions and save energy.
8. Motor efficiency generally increases as the motor's rated capacity increases.
9. The power factor of an induction motor improves as the load on the motor decreases.
10. A decrease in supply voltage by 10% will decrease the torque of the motor by approximately 19%.
Lighting items: (1) False — Book-3 §8.1: yellowish-green (555 nm) light, not blue, receives the greatest weight (683 lm/W). (2) True — §8.2: if the lamp renders the standard colour chips identical to the reference source its CRI is 100. Item 3 (glazing) and items 4–10 (compressors, motors) belong to other chapters; keys as per the printed model solution.
3. In a municipal area each 250 W HPSV lamp is proposed to be replaced by 2 numbers of 28 W CFL lamps. What is the annual saving if the lamp is operated for 3000 hours in a year? Do you recommend or reject such a measure? Give reason.
Model answer: Present load per point = 250 W (HPSV). Proposed = 2 × 28 W = 56 W. Saving per point = 250 − 56 = 194 W = 0.194 kW. Annual energy saving per point = 0.194 kW × 3000 h = 582 kWh/year (≈ Rs 2,910/yr at Rs 5/kWh). Recommendation: REJECT. Using Book-3 Table 8.1 averages, the 250 W HPSV gives about 250 × 90 = 22,500 lumens, whereas 2 × 28 W CFL give only about 56 × 60 = 3,360 lumens — roughly 15 % of the present light output — so the street/area illuminance would collapse far below the required level. The measure only cuts wattage, not energy per lumen; a genuine saving must maintain the design illuminance, e.g. by replacing HPSV with an equal-lumen higher-efficacy source (LED) or by using street-lighting controls (§8.7: timers, daylight control, alternate switching and voltage controllers after midnight).
Saving = (250 − 56) W × 3000 h = 582 kWh/yr per point, but reject: 2 × 28 W CFL ≈ 3,360 lm vs 250 W HPSV ≈ 22,500 lm (Table 8.1), so illuminance would be inadequate.
4. Explain the principle and features of LED lamps, Incandescent lamps, Fluorescent lamps and metal halide lamps.
Model answer: Incandescent (GLS) lamp — Principle: a tungsten filament is heated to incandescence by the electric current through it; the filament sits in an evacuated bulb filled with inert gas (argon, krypton, nitrogen) to increase brilliance and stop burn-out. Features (Book-3 Fig. 8.3, Table 8.1): only ~10 % of input becomes visible light (~70 % IR, ~20 % conduction/convection loss); efficacy 8–18 lm/W (avg 14); CRI excellent (100); life ~1000 h; cheapest, instant start; used in homes, restaurants, emergency lighting. Fluorescent lamp (FTL/CFL) — Principle: a glass tube with a trace of mercury vapour and a phosphor coating; when switched on the starter contacts open, the filaments heat the gas and an arc discharge is set up; the ionised gas (plasma) excites the fluorescent coating which emits visible light. A ballast is needed to start and operate it, and flux depends on ambient temperature. Features: 3–5 times as efficient as incandescent and lasts 10–20 times longer; efficacy 46–60 lm/W (avg 50), CRI 67–77 depending on coating, life ~5000 h; tubes classed by diameter (T12 38 mm, T8 25 mm, T5 16 mm; T5/T8 ~5 % more efficacious than T12); CFLs are compact versions (40–70 lm/W, CRI 85, 8000–10,000 h) that replace GLS lamps. Metal halide lamp — Principle: a variant of the high-pressure mercury vapour lamp; the discharge tube contains mercury, argon and metal halides (rare-earth iodides or sodium/scandium iodide). As the arc heats up the halide dissociates, releasing the metal into the arc and augmenting the mercury discharge; the halides protect the quartz wall and the spectrum can be tailored by the choice of halides. Needs a starter/ignitor (some use a third electrode). Features: efficacy 75–125 lm/W (avg 100), CRI good (70) — much better than HPMV; life ~8000 h; used for industrial bays, spot/flood lighting and retail. LED lamp — Principle: electroluminescence in a semiconductor p-n junction; N-type material (extra electrons) and P-type (extra holes) — when excited by current electrons cross into holes and light is emitted. White light is obtained by phosphor conversion, RGB colour mixing or a hybrid of both. Features: efficacy 50–130 lm/W typical (up to 200 lm/W in the laboratory), CRI very good (80), life 30,000–60,000 h (up to ~100,000 h with controlled junction temperature); low power, directional output, instant switching with no warm-up, high vibration/impact resistance, no UV/IR, dimmable, mercury-free — ideal with solar/battery for remote applications and regarded as the future of green lighting.
Book-3 §8.3: incandescent = heated filament (14 lm/W, CRI 100, ~10 % visible); fluorescent = mercury discharge exciting phosphor (50 lm/W, 3–5× GLS efficiency, ballast needed); metal halide = HPMV variant with metal halides (100 lm/W, CRI 70); LED = p-n junction electroluminescence (50–130 lm/W, 30,000–60,000 h, no UV/IR, mercury-free).
Source: Book EOC
📖 Book EOC L-2; §8.7 Street lighting & controls; §8.3 Table 8.1; §8.9 case-study method
5. A municipal corporation has 30000 numbers of 250 W HPMV lamps operating 3600 hours/yr. Calculate the annual energy consumption and annual electricity cost at Rs 4.75/kWh. Estimate the demand cost if the present demand cost is Rs 150/kVA (assume 0.7 PF). Discuss energy saving opportunities in the municipal lighting system with investment and simple payback for at least 3 options.
Model answer: Connected load = 30,000 × 250 W = 7,500,000 W = 7,500 kW (7.5 MW; note the book says municipal lighting loads are typically 2–7 MW). Annual energy = 7,500 kW × 3,600 h = 27,000,000 kWh = 270 lakh kWh/yr. Annual energy cost = 27,000,000 × Rs 4.75 = Rs 12.825 crore/yr. Demand: kVA = kW/PF = 7,500/0.7 = 10,714 kVA; demand cost = 10,714 × Rs 150 = Rs 16.07 lakh per billing month, i.e. about Rs 1.93 crore/yr if the Rs 150/kVA charge is monthly. Energy-saving options (state assumptions): (1) Replace 250 W HPMV (Table 8.1: 44–57 lm/W, avg 50) with 150 W HPSV (67–121 lm/W, avg 90) of similar lumen output — saving 100 W/lamp = 30,000 × 0.1 kW × 3,600 h = 10.8 million kWh/yr ≈ Rs 5.13 crore/yr; at ~Rs 3,000 per fitting investment ≈ Rs 9 crore → simple payback ≈ 1.8 years. (2) Replace HPMV with ~100 W LED luminaires (50–130 lm/W, 30,000–60,000 h life, low maintenance) — saving ~150 W/lamp = 16.2 million kWh/yr ≈ Rs 7.7 crore/yr; at ~Rs 8,000 per fitting investment ≈ Rs 24 crore → payback ≈ 3.1 years (plus large maintenance saving from long life). (3) Street-lighting controls per §8.7 — timer/daylight (lux-based) switching, selective/alternate switching after midnight, voltage controllers after midnight (5–15 % saving, §8.6e) and PLC-controlled lighting panels — say 10 % of energy = 2.7 million kWh/yr ≈ Rs 1.28 crore/yr for ~Rs 1 crore investment → payback < 1 year. Also improve power factor (0.7 → 0.95) to cut kVA demand by ~26 % (10,714 → 7,895 kVA), saving ≈ Rs 4.2 lakh/month in demand charges. Simple payback in each case = investment ÷ annual saving.
📖 §8.6 General energy saving opportunities; §8.7 Controls & street lighting
6. List five measures to reduce energy consumption in lighting systems for buildings, industry and street lighting.
Model answer: 1. Reduce excessive illumination to standard levels (switching, delamping). 2. Control lighting aggressively with clock/delay timers, photocells and occupancy sensors. 3. Install efficient lamp alternatives to incandescent/mercury vapour (low/high pressure sodium, metal halide, fluorescent). 4. Select high power factor, long-life ballasts and lamps. 5. Upgrade obsolete fluorescents to CFLs and electronic ballasts. (Also: lower fixtures, use daylighting/skylights, lighter wall colours, task lighting, LED exit signs, re-evaluate exterior lighting.)
Book-3 §8.6(a)–(h): daylighting, de-lamping, task lighting, high-efficiency lamps/luminaires (Table 8.1), feeder-voltage reduction (5–15 %), electronic ballasts (15–20 W/tube), lighting controllers, maintenance; §8.7 street lighting: timers, daylight/lux control, alternate switching and voltage controllers after midnight, PLC panels.
Source: 14th Exam
📖 §8.3(1) Incandescent lamp; §8.2 Control gear (discharge lamps); §8.6 Energy saving opportunities
7. a) Briefly explain the difference between a 'filament lamp' and a 'gas discharge lamp'. b) State any 3 best practices in a lighting system for energy savings.
Model answer: a) Filament lamps (e.g. incandescent) produce light by a filament heated to incandescence by the flow of electric current through it. A gas discharge lamp produces light not by heating a filament but by the excitation of gas contained in a tubular or elliptical outer bulb. b) Any three of: install energy efficient fluorescent lamps in place of conventional ones; CFLs in place of incandescent lamps; metal halide lamps in place of mercury/sodium vapour lamps; HPSV lamps where colour rendering is not critical; LED indicator lamps in place of filament lamps; optimum daylighting; grouping of lighting for control flexibility; microprocessor based controllers; exclusive lighting transformer; servo stabilizer on lighting feeder; high-frequency electronic ballasts in place of conventional ballasts.
Book-3 §8.3: filament lamp = wire heated to incandescence by current; discharge lamp = current through low-pressure mercury vapour/gas between electrodes (needs ballast/ignitor, §8.2). Best practices from §8.6/§8.7: efficient lamps per Table 8.1, electronic ballasts, feeder-voltage optimisation, daylighting, controls.
Source: 9th Dec-2009
📖 §8.7 Energy efficient lighting controls — Daylight linked control (part b: Ch.7 cooling towers)
8. a) List any three energy efficient lighting controls. Describe briefly about daylight linked control (5 marks). b) Explain briefly about various water losses in cooling towers and how they can be minimized (5 marks).
Model answer: a) Refer Guidebook-3 Page 243-244. Three energy efficient lighting controls: occupancy/motion sensors, timers/time-scheduled controls, and daylight linked (photoelectric) controls. Daylight linked control uses a photo sensor to measure available natural daylight and automatically dims or switches off artificial lighting near windows/skylights so total illuminance is maintained at the design level, saving energy when daylight is sufficient. b) Refer Guidebook-3 Page 205. Cooling tower water losses: Evaporation loss (water evaporated to provide cooling - unavoidable, inherent to process); Drift/windage loss (fine droplets carried out by air - minimized by efficient drift eliminators); Blowdown loss (water bled off to control dissolved solids - minimized by increasing cycles of concentration with proper water treatment); plus leakage/overflow losses minimized by good maintenance.
Book-3 §8.7: occupancy sensors, timed-turnoff switches and daylight-linked (photoelectric) control; photocells switch or dim so that daylight + electric light always reaches the design level, switching electric light off when daylight alone suffices. Part (b) is from the cooling-tower chapter.