Radiant intensity calculation using irradiance measurement
The Radiant intensity I of a (point like) source can be obtained by measuring the Irradiance E in a certain distance. Afterwards the inverse square law has to be applied to convert the measured Irradiance into Radiant intensity. The distance d between the measuring plane and the source must be known. The following figure shows the set up.
Some conditions must be matched to get a precise result:
- The distance between the light source and the measuring plane must be at least 10 times the diameter (or largest lucid dimension) of the light source to get a measuring error below 1 %.
- The Luxmeter must be positioned exactly in line with the centre of the light source. If a specbos 1211-2 in Irradiance mode is used, the alignment can be done using the target laser in Radiance mode and attaching the diffusor after the device alignment.
- Stray light must be avoided using appropriate apertures between source and measuring probe (see the figure above).
- The measurement of the distance must be the more precise the smaller the distance d is. It will be measured from the top of the light source to the reference plane of the Luxmeter. If this plane is not known, the front plane of the diffusor can be used.
The inverse square law is as follows:
It can be used for the radiometric (E,I), photometric (Eν,Iν) or spectral data (E(λ),I(λ)).
A specbos 1211-2 or a spectraval 1501 can be configured to measure Radiant/ Luminous Intensity directly:
The Irradiance calibration file must get a new header title: Radiant intensity