How is light intensity measured?
A photometer reads the light arriving at a known distance, and a simple calculation turns that reading into candela.
AGL fixtures are rated in candela, the light sent in one direction. A photometer cannot read candela directly. It reads illuminance in lux at a known distance, and intensity is found from I = E × d².
The result is valid only when the photometer is square to the beam and far enough away for the fixture to act as a point source. One reading gives one direction, so the fixture is measured at many angles to build the full beam pattern.
Frequently Asked Questions
Q1What is light-intensity measurement?
Finding how strongly a fixture shines in a given direction, in candela. A photometer reads the light arriving at a known distance and the result is converted to intensity.
Q2What is a candela?
The unit of luminous intensity: the light a source sends in one direction, per unit of solid angle. AGL fixtures are rated in candela at set angles, not in lumens.
Q3How is candela calculated from a lux reading?
Intensity equals illuminance multiplied by the square of the distance. A reading of 50 lux at 10 m gives 50 x 10 x 10 = 5,000 cd. The calculation holds only when the photometer is far enough away for the fixture to act as a point source.
Q4Why is the photometer kept square to the beam?
The photometer measures light falling on its face. If it is tilted, the reading is reduced and the calculated intensity is too low. It is aligned so that its face is square to the line from the fixture.
Q5Is one measurement enough?
No. A single reading gives intensity in one direction only. The fixture is measured at many angles to build the full beam pattern, and the on-axis value is only one point of it.
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