Photometry
Advanced
Luminous intensity, flux and illuminance.
Measures light as perceived by the human eye: luminous flux (lumen), illuminance (lux = lm/m²) and luminous intensity (candela). Weighted by the eye's sensitivity (peak at 555 nm).
A 800 lm bulb over 1 m² gives 800 lux.
Radiometry measures energy not perception.
In more detail
Photometry measures light as perceived by the eye, using units such as lumens and candelas. It weights wavelengths by the eye's sensitivity, unlike radiometry which measures physical power.
Real-world uses
- Lighting designers specify brightness in lumens.
- Photographers use light meters based on photometric principles.
- Lighting designers measure luminous intensity and illuminance to make sure rooms and streets are bright enough.
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Frequently asked questions
What is a lumen?
The unit of luminous flux, the perceived light output of a source.
Why is green brighter than red for the same power?
The eye is most sensitive to green, so equal physical power looks brighter in green.
Is luminous intensity the same as power?
No. Luminous intensity weights light by the sensitivity of the eye, so it is measured in candela rather than watts.
Related topics in Optics
Fibre optics numerical aperture · Colour and the eye · Additive and subtractive colour mixing · Geometrical vs wave optics