A room 10 m by 8 m requires 300 lux. With a utilisation factor of 0.5 and a maintenance factor of 0.8, the total flux needed is
Answer: C
The area is 80 square metres, so 24 000 lumens would suffice at the working plane, and dividing by the product of the two factors, 0.4, gives 60 000 from the lamps. The two factors together double the requirement, which is why they must never be omitted.
Please log in or register to report this question.
For that same room, with luminaires of 4000 lm each, the number required is
Answer: D
Sixty thousand divided by four thousand is fifteen, which would be arranged as three rows of five for uniformity. The arrangement matters as much as the count, since the standard sets a uniformity as well as an average.
Please log in or register to report this question.
A measured illuminance of 750 lux where the standard requires 300 indicates
Answer: A
Correct answer: over-lighting, and an opportunity to remove or dim luminaires
Two and a half times the required illuminance is energy spent on light nobody needs, and it can be recovered by removing lamps, dimming, or reducing the maintained level at the next relamping. Note that a design allows some margin for depreciation, but not a margin of this size.
Please log in or register to report this question.
An occupancy sensor in a store lit 10 hours a day but occupied 2 hours saves about
Answer: B
Correct answer: 80 per cent of that room's lighting energy
\[ \frac{10-2}{10} = 80\ \mathrm{per\ cent}. \] Eight of the ten hours are lit with nobody present, and the sensor removes them. This is why stores, washrooms and meeting rooms are where sensors pay, and why fitting one in a continuously occupied space saves nothing.
Please log in or register to report this question.
Removing one tube in three from an over-lit office of 300 twin fittings, each tube taking 46 circuit watts and running 2500 hours, saves about
Answer: C
Two hundred tubes removed at 46 watts is 9.2 kilowatts, and over 2500 hours that is 23 000 units. The remaining fittings should be checked to confirm the illuminance still meets the standard and the uniformity is acceptable.
Please log in or register to report this question.
A luminaire whose diffuser has become dirty enough to absorb 30 per cent of the output is delivering, per watt consumed,
Answer: D
Correct answer: 70 per cent of its original light
Thirty per cent of the output is absorbed by the dirt, so seventy per cent emerges - while the power drawn is unchanged. The effective efficacy of the installation has therefore fallen by nearly a third, which is what a cleaning programme recovers for the cost of the labour.
Please log in or register to report this question.
Match the quantity with its unit.
1. Luminous flux 2. Illuminance 3. Luminous efficacy
P. Lumens per watt Q. Lumen R. Lux
Answer: A
Correct answer: 1-Q, 2-R, 3-P
Three quantities and three units. Luminous flux is measured in lumens, the total light emitted. Illuminance is measured in lux, the flux arriving per square metre. And efficacy is lumens per watt, the flux obtained per unit of electrical power - the figure of merit for a lamp.
Please log in or register to report this question.
Match the measure with what it reduces.
1. Occupancy sensor 2. Electronic ballast 3. Light-coloured decoration
P. The flux needed for the same illuminance Q. The hours of operation R. The circuit watts per lamp
Answer: B
Correct answer: 1-Q, 2-R, 3-P
Three measures acting on three different terms of the same product. A sensor reduces the hours of operation. An electronic ballast reduces the circuit watts per lamp. And light decoration raises the utilisation factor, so less flux is needed for the same illuminance. Energy is hours times watts, and the third measure reduces how many lamps are required at all.
Please log in or register to report this question.
Put the steps of a lighting audit in order.
1. Measure the illuminance at the working plane 2. Compare against the standard for the task 3. Record the lamp types, wattages and hours 4. Identify over-lighting, relamping and control opportunities
Answer: C
Correct answer: 3, 1, 2, 4
Recording what is installed and how long it runs comes first, since the energy cannot be computed without it. The illuminance is then measured, compared against the standard for the task, and the opportunities identified from the gap between what is provided and what is needed.
Please log in or register to report this question.
Consider the following statements about lighting.
1. The lux is a lumen per square metre 2. The marked lamp wattage excludes the ballast loss 3. Colour temperature and colour rendering are the same thing
Answer: D
Correct answer: 1 and 2 only are correct
Statements 1 and 2 are correct: a lux is one lumen per square metre, and the wattage marked on a lamp excludes the ballast loss, which is why circuit watts must be used. Statement 3 confuses two independent properties - colour temperature is how warm the light looks, and rendering is how truly it shows colours.
Please log in or register to report this question.
