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Illumination: board exam questions with solutions

On the REE exam under Electrical Engineering Professional Subjects and the RME exam under Technical Subjects. Try each question before you open its solution, then practise with fresh numbers below: 2 computed question types on this topic.

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Questions

1. The lux, the SI unit of illuminance, is equal to which of the following?

  1. One lumen per square metre
  2. One candela per square metre
  3. One lumen per watt
  4. One candela per steradian
Show the answer and solution

Answer: A. One lumen per square metre

Illuminance E = Φ/A, so 1 lx = 1 lm/m². Candela per square metre is luminance, and lumen per watt is efficacy.

2. A lamp with a luminous intensity of 1000 cd hangs 2 m directly above a work surface. What is the illuminance on the surface directly below it?

  1. 500 lx
  2. 250 lx
  3. 125 lx
  4. 1000 lx
Show the answer and solution

Answer: B. 250 lx

Inverse-square law: E = I/d² = 1000/2² = 250 lx.

3. The same 1000 cd lamp is 2 m above the floor. What is the horizontal illuminance at a floor point 1.5 m to the side of the point directly below the lamp?

  1. 160 lx
  2. 96 lx
  3. 128 lx
  4. 250 lx
Show the answer and solution

Answer: C. 128 lx

Distance d = √(2² + 1.5²) = 2.5 m and cos θ = 2/2.5 = 0.8. E = I cos θ/d² = 1000 × 0.8/6.25 = 128 lx.

4. An 18 W LED tube produces 1800 lm. What is its luminous efficacy?

  1. 32 400 lm/W
  2. 0.01 lm/W
  3. 10 lm/W
  4. 100 lm/W
Show the answer and solution

Answer: D. 100 lm/W

Efficacy = luminous flux/electrical power = 1800/18 = 100 lm/W.

5. A 10 m × 8 m office needs an average maintained illuminance of 500 lx. Each luminaire emits 3600 lm, the coefficient of utilization is 0.6, and the maintenance factor is 0.8. Using the lumen method, what is the minimum number of luminaires?

  1. 24
  2. 23
  3. 12
  4. 19
Show the answer and solution

Answer: A. 24

N = E × A/(Φ × CU × MF) = 500 × 80/(3600 × 0.6 × 0.8) = 23.15. Round up to 24 luminaires.

6. If the distance from a point source to a surface facing it is doubled, the illuminance on the surface becomes:

  1. One-half of its original value
  2. One-fourth of its original value
  3. Twice its original value
  4. Unchanged
Show the answer and solution

Answer: B. One-fourth of its original value

By the inverse-square law E ∝ 1/d², so doubling d gives E/4.

7. A small lamp radiates uniformly in all directions with a luminous intensity of 100 cd. What is its total luminous flux?

  1. 100 lm
  2. 628 lm
  3. 1257 lm
  4. 314 lm
Show the answer and solution

Answer: C. 1257 lm

A sphere subtends 4π sr, so Φ = 4π × I = 4π × 100 = 1257 lm.

8. A 20 m² room is lit by 4 lamps of 2500 lm each. The coefficient of utilization is 0.5 and the maintenance factor is 0.8. What is the average maintained illuminance on the work plane?

  1. 500 lx
  2. 250 lx
  3. 50 lx
  4. 200 lx
Show the answer and solution

Answer: D. 200 lx

E = N Φ CU MF/A = 4 × 2500 × 0.5 × 0.8/20 = 200 lx.

Worked examples with computed numbers

These numbers are fixed on this page; in practice they change every time.

9. A lamp of 100 cd shines straight down onto a surface 1 m below. What is the illuminance?

  1. 100 lx
  2. 7.96 lx
  3. 150 lx
  4. 50 lx
Show the answer and solution

Answer: A. 100 lx

Inverse square law (normal incidence): E = I/d² = 100/1² = 100 lx.

10. A 60 m² room needs 500 lx. Each luminaire gives 2000 lm, with UF 0.5 and MF 0.8. How many luminaires are needed?

  1. 30
  2. 41
  3. 38
  4. 15
Show the answer and solution

Answer: C. 38

N = E·A/(Φ·UF·MF) = 500 × 60/(2000 × 0.5 × 0.8) = 37.5 → 38 luminaires.

Other topics in the same subjects

DC circuits · AC circuits · Three-phase systems · Transients (RC, RL, RLC) · Semiconductors and diodes · Power electronics · Instrumentation and measurement · Control systems · Transformers · DC machines · Induction motors · Synchronous machines · Power systems and transmission · Protection: fuses, breakers, relays · Power plant engineering · PEC: branch circuits, ampacity and protection · Tools and test equipment · Motors, starters and control circuits · RA 7920 and ethics (electrical)

An independent, free reviewer: not affiliated with the PRC or the Boards. Subject weights and passing rules are from the PRC examination programs and the laws (RA 9292, RA 7920); check the current PRC program before you file. Code questions follow the Philippine Electrical Code 2017 Part 1; check them against your own copy of the Code.