I think I am finally satisfied with my design, after a lot more work. I finally thought to look up the spectral response for the Apogee Quantum PAR meter, to see what they use and how good it is. I was surprised to find that they also have a far from perfect match with the 400-700 nM PAR spectral range, and, as a result their meter is only good to about +/- 4% for typical aquarium lights, and less accurate for incandescent lights. So, I tried to match their spectral response as best I could with the photodiode and Roscolux filter gels that I have. That proved to be relatively easy, once I worked out the details of what the filter response should look like. I settled on using the Roscolux # 4815 filter gel, giving this spectral response vs the Qualtum meter spectral response:
That left the problem of how to adjust the calibration of the meter without changing the spectral response. So, I noticed that in the Roscolux sample book are 19 different diffuser filters, white diffusers which would not alter the color of the light passing through them. I set up a PAR meter and checked the reduction in the reading that each of those diffusers gave me when I put one on top of the PAR meter sensor. The transmissivities ranged from 45% to 90%, so by using different combinations of those diffuser filters I could adjust the sensitivity of the meter over a very wide range.
With that problem solved, I made some modifications to the last design of the sensor, all aimed at making it easier to make the parts and to assemble them with minimal variation from assembly to assembly. My final design, which I prototyped, was:
With this design I can make 5+ of each part in about 30 minutes, with the aid of a few very simple fixtures. Assembling the parts was very easy, even with the tiny parts.
Today I received a batch of 5 more lux meters, so I was able to assemble a complete PAR meter for testing and to determine what diffuser filters I needed. It took just 2 diffuser filters, Roscolux # 102 and #114, to get a near perfect match with a Quantum meter. This PAR meter is now well within my goal of +/- 10% accuracy. I have two more just about assembled now.
If you have reasonable mechanical skills you should be able to easily duplicate this and make them to sell, if you wish to do so. The Roscolux filter sample books are free at a dealer, or you can buy one at Amazon. The acrylic tubes are available on EBay. You should have little difficulty finding usable flexible 2 conductor cable at a local electronics supply store, although Radioshack doesn't seem to have any.
I have a list of 7 people who have asked that I make them one, so I will finish off that list, and maybe make a few more, then I hope someone else picks up this relatively easy, high demand "product" for others to buy.
Here is the assembly sequence I am following:
Assemble Photodiode to item 5.
Apply acrylic cement to holes in item 5 to attach diode.
Thread cable through hole in item 6, and solder cable wires to leads.
Assemble above assembly to item 6, with acrylic cement.
Paint outside cylindrical surface of above assembly with black nail polish.
Glue item 4 to item 1 with acrylic cement.
Paint outside cylindrical surface of that assembly with black nail polish.
Load filters into item 4-1 assembly and push item 3 in to retain.
Use masking tape to assemble above assemblies for testing.
Adjust diffuser filters to get correct PAR reading.
Apply 2 drops of acrylic cement to hold item 3 in place.
Glue the two sub-assemblies together with acrylic cement.
Glue above assembly to item 7, and paint bottom of item 7 under item 6 with black
nail polish. Glue on a “socket” for holding wand made of piece of acrylic tube to
This modified meter works on the maximum scale of the lux meter, the 50,000lux scale. I just use a couple of drops of acrylic cement to lock the range selector slide switch in the right position.