What is photometry?
Measuring light the way the eye responds to it, in flux, intensity and illuminance.
Photometry is the measurement of light as the human eye responds to it. A lamp or LED gives out a quantity of light, but what matters to a pilot is how much of it is seen, so photometry weights light by the eye’s daytime (photopic) sensitivity, which peaks near 555 nm, rather than counting raw energy.
Three quantities do most of the work. Luminous flux, in lumen, is the total light a source emits. Luminous intensity, in candela, is the flux sent into one direction per unit solid angle. Illuminance, in lux, is the light reaching a surface, and for a small source it falls with the square of the distance. AGL fixtures are rated mainly by intensity in candela, measured in many directions, because a pilot sees them from a distance and along set angles.
Frequently Asked Questions
Q1What is photometry?
The measurement of light as the human eye responds to it. It weights light by the eye's sensitivity, so the results describe how bright a light looks and not just how much energy it gives out.
Q2What is the difference between luminous flux and luminous intensity?
Flux, in lumen, is the total light a source emits in all directions. Intensity, in candela, is the part of that flux sent into one direction per unit solid angle. A narrow beam has high intensity for the same flux.
Q3What is illuminance?
The light arriving at a surface, in lux, which is lumen per square metre. For a small source it equals the intensity divided by the square of the distance, for a surface facing the source.
Q4Why are AGL fixtures rated in candela?
A pilot sees an AGL light from a distance and along particular directions. Intensity in candela describes how bright the light is in each direction, which is what matters at that distance.
Q5Why does distance matter?
Illuminance falls with the square of the distance, so doubling the distance cuts it to a quarter. This is why light is measured at a stated distance in photometric testing.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.