Int_to_Irr.txt

https://www.jeti.com/cms/index.php/general/radiant-intensity-calculation

Application of JETI Spectroradiometers 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 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 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.

[object]<-----------------d----------------->[diff][Luxmeter] 


The Inverse square law reads as follows:

I : The Radiant intensity

E :  Irradiance

d : The distance d between the measuring plane and the source has to be measured before

I = E x d^2



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