Radiant Intensity Calculation Using Irradiance Measurement

Some conditions have to be matched to get a precise result: 

• The distance between the light source and the measuring plane has to be at least 10 times the diameter (or largest illuminating dimension) of the light source to get a measuring error below 1 %.
• The Luxmeter has to be positioned exactly in line with the centre of the light source. If a specbos 1201 or 1211 in Irradiance mode is used, the alignment can be done using the target laser in Radiance mode and attaching the diffusor after alignment.
• Stray light has to be avoided using appropriate apertures between source and measuring probe (see the figure above).
• The measurement of the distance has to be the more precise the smaller the distance d. It is 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.

It can be used for the radiometric (E,I), photometric (E_ν,I_ν) or spectral data (E(λ),I(λ)).