Which unit measures the total visible light output of a bulb?
AnswerLumen
Lumens measure light output; watts measure the electrical power used.
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Lighting facts & stats
8 fact-checked facts about lighting, each with the reason behind it. 4 more are in today's round and join this page after it closes.
The basics
In an incandescent bulb, what glows to produce light?
AnswerA heated filament
Current heats a thin tungsten filament until it glows white-hot.
Going deeper
What does a lux measure?
AnswerIlluminance: one lumen per square metre
Lux measures how much light falls on a surface.
In three-point lighting for photos and film, what are the three lights?
AnswerKey, fill and back light
The key light is the main source, the fill softens shadows and the back light separates the subject.
Fluorescent tubes contain a small amount of which element as vapor?
AnswerMercury
Mercury vapor gives off UV light that makes the tube's phosphor coating glow.
What color do low-pressure sodium street lamps glow?
AnswerYellow-orange
Sodium emits light near 589 nm, giving those lamps their strong yellow-orange color.
Expert level
Which SI base unit measures luminous intensity?
AnswerCandela
The candela is one of the seven SI base units; lumens and lux are derived from it.
What color temperature is typical of 'warm white' household bulbs?
AnswerAbout 2700 K
Warm white bulbs are around 2700 K; daylight-style bulbs are closer to 5000–6500 K.
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