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
Pyranometer Circuit Design
http://www.mdpi.com/1424-8220/9/6/4615/pdf
Photodiode Datasheet
http://www.vishay.com/docs/81519/81519.pdf