Anyone know what unit of measurement these measure in? It is graduated 0-2000 but it just says light on one side and dark on the other. Its a 2 prong model that you stick in the soil, I have found it useless for reading soil ph so I use it as a moisture gauge, and to check amount of light getting to the lower part of the plants. I just don't know what its measuring lumens, lux, foot candle, or whatever