So if I use the 4 filters in your final versions, how would I tell that the lux meter is reading par correctly (or close to it)?
You would have to borrow a Quantum PAR meter and verify the accuracy. However, each of the Rosco filters has a percent transmission number, which is shown on the Rosco website. If you multiply those 4 numbers together, .88 x .64 x .40 x .61 = .1374 which is 1 divided by 7.28. If there were no other factors involved, it should be 1 divided by 7.6, or .1316. However, since the lux meter photodiode isn't equally sensitive to all wave lengths, it worked out that the .1374 was a near perfect total transmissivity.
The only things this modification does are:
Filter the light striking the photodiode to reduce it by a factor of .1374.
Adjust the spectral response curve to make is a bit more consistent with what a PAR meter response curve should look like.
Using the same optical path, make a waterproof sensor, so it can be used in the aquarium.
The resulting "PAR meter" will be as good as the Mastech lux meter is, but it will just read differently. And, as far as I can tell so far, the Mastech lux meter is all that it needs to be to be a good meter for our uses.