Application of JETI Spectroradiometers specbos 1201 and specbos 1211
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 used to convert the Irradiance into Radiant intensity. The distance d between the measuring plane and the source has to be measured before. 
The following figure shows the set up. 
Some conditions have to be matched to get a precise result:

The Inverse square law reads as follows: 
It can be used for the radiometric (E,I), photometric (E_{ν},I_{ν}) or spectral data (E(λ),I(λ)). 