The lumen is the unit of
Answer: A
Correct answer: luminous flux
The lumen measures the total light a source emits, weighted by the eye’s sensitivity across the spectrum. Illuminance is measured in lux, which is lumens per square metre; intensity in candelas; and luminance in candelas per square metre. Keeping the four apart is the beginning of lighting calculation.
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The lux is defined as
Answer: B
It is the quantity a light meter reads and the quantity every lighting standard is written in, because it describes what falls on the task rather than what the lamp produces. The candela per square metre is luminance, which describes brightness as seen.
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Luminous efficacy is expressed in
Answer: C
It states how much light a lamp gives for the electricity it consumes and is therefore the single figure by which lamp families are ranked. Comparing two installations on wattage alone, without it, says nothing about the light delivered.
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The quantity that a lighting standard for an office actually specifies is
Answer: D
What matters is the light on the desk, and the standard sets it in lux together with a uniformity and a glare limit. Installed wattage per square metre is a separate efficiency benchmark rather than a lighting requirement.
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Of the following lamp types, the highest luminous efficacy is offered by
Answer: A
Modern diode luminaires reach well over a hundred lumens a watt and the best exceed two hundred, against twelve to seventeen for a filament lamp. That factor of ten is why filament lighting has been withdrawn almost everywhere.
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A tungsten filament lamp has a low efficacy because most of its input appears as
Answer: B
Correct answer: heat and infrared radiation
A filament emits by being hot, and a black body at 2,800 degrees radiates overwhelmingly in the infrared - only two or three per cent of the input emerges as visible light. The efficacy is therefore about 15 lumens per watt against 120 or more for a diode source, and the difference is radiated heat.
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A high pressure sodium lamp is used for road lighting despite its poor colour rendering because it offers
Answer: C
A hundred to a hundred and thirty lumens a watt and twenty thousand hours suited an application where recognising colour hardly mattered. Diode luminaires now beat it on efficacy as well as on colour and control, which is why it is being displaced.
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The colour rendering index of a lamp describes how faithfully it
Answer: D
Correct answer: reveals the colours of the objects it lights
The index compares the appearance of test colours under the lamp with their appearance under a reference source at the same colour temperature. A score of 100 means no shift at all. It is a property of the spectrum: a lamp emitting in three narrow bands can look white and still render a red badly.
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Colour temperature and colour rendering index are independent because colour temperature describes
Answer: A
Correct answer: the apparent warmth of the light rather than its faithfulness
Colour temperature says what colour the light is - warm and yellowish at 2,700 K, cool and bluish at 6,500 - while rendering says how truly it shows the colours of objects. A lamp may be any temperature and render well or badly, which is why both must be specified.
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Three of the following are reasons to reject a lamp of high efficacy for a particular room. The exception is
Answer: B
Correct answer: its lumens per watt are higher than the incumbent's
Higher efficacy is the reason to choose a lamp, not to reject one. The other three are genuine grounds for rejection: a long restrike leaves the space dark after a momentary interruption, poor rendering fails the task, and incompatibility with the existing controls costs more than the lamp saves.
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