I have copied this from the net and I will one day buy the sensor.
If you have a good quality multi-meter you can purchase the SQ-120 quantum sensor from Apogee instruments
http://www.apogeeinstruments.com/man..._300manual.pdf
The sensor which costs $139.00 measures Milli-volts which you just multiply the reading on your Multi-meter by 5 to give you a very accurate par reading without having to spend hundreds more on the actual Quantum meter.