A null happens when you look at red through a red lense and blue through blue blockers and green ect. When the three different colors are emmited in the right combination of intensities a color will be zip. The info is collected by a a/d and data placed in a file and processed.
using ohms law the current is inversely proportional to voltage. The accuracy involves temperature coefficients.
If you start with tight voltage and compensated op-amp
the result is I=V/R +/- delta T
Some of the finest start very low and stable and amplify.
Example: you have 10uA that is...