I use a TaoTronics 2 watt LED grow bulb in a cheap hang on socket from the hardware store. It is mounted 6 inches above the water of my sump. Just below the water surface (6.5 inches from the light) the PAR read 1150, at a depth of 6 inches from the water surface (12 inches from the source) the PAR is 550 and at 12 inches from the water surface (18 inches from source) the PAR reads 499 using the Apogee SQ-420 sensor