About 2 years ago I began looking for a combination of Roscolux filters that would change the spectral response of an Excelitas photodiode to equal that of the Apogee Quantum PAR meter. I found many combinations that were usable, but until today I never found a combination that almost exactly duplicated the Apogee response. Here is that Apogee spectral response that is the goal:
The colored area is the ideal PAR meter response, the definition of PAR being all of the radiation between 400 and 700 nm wave length.
An Excelitas VTB8441BH photodiode has built in filters to give this spectral response:
Today I found a 3 filter combination that works very well, using the spectrum plots shown on the Rosco website, http://www.rosco.com/filters/roscolux.cfm
Here is how I calculated the response:
These are the three filters, along with their transmission at each of 20 wavelengths. I used a spreadsheet to figure out the sensitivity of the diode at each wave length by multiplying the transmission percentages at each wave length together and multiplying that by the relative sensitivity of the diode at those wave lengths. Then to be able to plot that on a graph I converted each of those sensitivities so the the highest one is 1.0 and the others are fractions of 1.0.
When this is plotted on a graph, along with the Apogee Quantum PAR meter spectral sensitivity it looks like this:
You can see that a PAR meter using that photodiode and those filters would cover the whole PAR range better than the Apogee meter, but at the cost of including a little of the radiation outside the PAR range. I think that is a good trade off.
If the 3D printed parts here work out right, and if it is possible to adjust the assembly to calibrate it to read PAR accurately with just geometry changes, this should make a very good PAR meter.