9. Photometry Basics
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Photometry is the branch of optics concerned with measuring the intensity of visible light as perceived by the human eye. A few basic quantities and their SI units are frequently asked as one-line factual questions in competitive examinations.
Quantity | Definition (brief) | SI Unit |
|---|---|---|
Luminous flux | The total quantity of visible light energy emitted by a source per unit time, as perceived by the human eye | Lumen (lm) |
Luminous intensity | The luminous flux radiated by a source per unit solid angle in a given direction — essentially, how 'bright' a source appears from a particular direction | Candela (cd) |
Illuminance (illumination) | The amount of luminous flux falling per unit area of a surface — how well-lit a surface is | Lux (lx); 1 lux = 1 lumen per square metre |
Luminous efficacy | The ratio of luminous flux emitted by a source to the electrical power it consumes — a measure of how efficiently a lamp converts electricity into visible light | Lumens per watt (lm/W) |
A simple way to remember the relationship among these quantities is to think of a light bulb: the bulb's total light output (how much visible light energy it emits every second, in all directions) is its luminous flux, measured in lumens — this is the figure increasingly printed on LED bulb packaging today instead of wattage, since wattage measures electrical power consumed, not light actually produced. How intensely that light shines in one particular direction (say, straight down from a ceiling fitting) is its luminous intensity, measured in candela. And how well-lit your desk actually becomes, depending on both the bulb's intensity and its distance from the desk, is the illuminance falling on the desk's surface, measured in lux — office and study spaces are typically recommended to have an illuminance of a few hundred lux for comfortable reading.
The candela is one of the seven base SI units. It is worth remembering that luminous intensity (candela) describes a source, while illuminance (lux) describes how much of that light actually falls on a surface — a bright source far away can illuminate a surface just as weakly as a dim source held close, because illuminance falls off with the square of the distance from the source (the inverse square law).