What is illuminance?
How much light arrives on a surface, per unit area, and why it falls away with distance from the source.
Illuminance is the luminous flux arriving on a surface, per unit area, measured in lux, where one lux is one lumen per square metre. It belongs to the surface that receives the light, not to the source that sends it.
For a small source and a surface facing it, E = I ÷ d². This is the inverse-square rule: the same light spreads over a larger area as it travels, so doubling the distance cuts illuminance to a quarter. In AGL, signal fixtures are specified by intensity in candela, while area lighting such as aprons is specified in lux, and a photometer measures illuminance from which intensity is worked out.
Frequently Asked Questions
Q1What is illuminance?
The luminous flux arriving on a surface, per unit area, measured in lux. It describes the surface that receives the light, not the source that sends it.
Q2What is the unit of illuminance?
The lux (lx). One lux is one lumen spread over one square metre of surface.
Q3How is illuminance different from luminous intensity?
Intensity belongs to the source and a direction, is measured in candela and does not change with distance. Illuminance belongs to the surface and changes with distance, following E = I ÷ d² for a small source.
Q4Why does illuminance fall with distance?
The light from a small source spreads over a larger area the farther it travels, and that area grows with the square of the distance. Doubling the distance spreads the same light over four times the area, so illuminance falls to a quarter.
Q5Where is illuminance used in AGL?
Area lighting such as aprons is specified in lux, measured on a surface. Signal fixtures such as runway edge lights are specified in candela. During photometric measurement, a photometer reads illuminance at a known distance and the intensity is worked out from it.
Q6Does the angle of the surface matter?
Yes. A surface tilted away from the light receives less, by the cosine of the angle between the light and the surface normal. The inverse-square rule is written for a surface facing the source.
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