I have conducted an experiment to measure the changes in voltage/current due to the resistance changes of a photocell present in an LDR. I wanted to know how I could use those values in order to calculate the illuminance present on the surface of the photocell. I did not use a voltage differential circuit. I looked at some websites and all the related experiments were simply using differential circuits. Is there an LDR resistance to luminance conversion equation or something similar. Thanks.