There are a lot of cheap Lux meters available now that look like they could become a hobby grade PAR meter with some modifications. Just for the fun of it I bought one. It is a Mastech LX1010BS, from Cybertech, in California, for only $14.99, $21.53 after shipping and tax. I ordered it last Saturday and it arrived yesterday, Tuesday. You can't beat that!
Here it is:
It takes a 9 volt battery, which I don't have right now, so I haven't tested it at all. I did partly disassemble the sensor though - 2 screws.
I have a LED light and a CFL-in-a-dome reflector light to calibrate it against a PAR meter. I'm guessing there will be a different conversion factor for the different types of light, but that's fine for what I want.
My requirement is to be able to use this under water, so that is the challenge. Ebay has a nice 4 inch diameter acrylic box that I could just seal the sensor in, but that has problems: temperature and atmospheric pressure changes would flex the box and probably break the seal, It would float so would be hard to push down to where I want it, it is too big for convenience. Another option is to discard the sensor housing and use a small 2 inch diameter screw top acrylic box as the housing. For the moment that is my plan.
What am I overlooking? And, is there a better way?
To help those who don't want to read through 10 pages of frustrating failures, the final successful project starts at post #158