# Calculating Irradiance from BPW21R Photodiode Current

Discussion in 'The Projects Forum' started by pochd, Feb 19, 2011.

1. ### pochd Thread Starter New Member

May 6, 2010
13
0
I am using a BPW21R photodiode to measure irradiance instead of a BPW21 photodiode in a pyranometer circuit that could be found in the pdf link below. How should I calculate irradiance(W/m^2) from the current produced in a transimpedance amp circuit if there is no Spectral sensitivity(Ampere/Watt) value given?

Pyranometer Circuit Design

http://www.mdpi.com/1424-8220/9/6/4615/pdf

Photodiode Datasheet

http://www.vishay.com/docs/81519/81519.pdf

Apr 20, 2004
15,815
283
3. ### pochd Thread Starter New Member

May 6, 2010
13
0
I believe i need more than just an estimate. Is it ok to use the sensitivity value (nA/lx) given in the optical characteristics to get an A/W value? (0.81) Below is the calculation i used:

Sensitivity x lumens/Watt * Area^-1

9x10-9 [A*m^2/lumen] x 683 [lumens/Watt] x (7.5x10-6)^-1 [m^2]