Photodiode voltage as a function of relative polarizer angle for light at 650 nm passing through two polarizers. \(V_{pd}\) corresponds to the voltage measured across a resistor placed in series with a photodiode and is linearly proportional to light intensity under these conditions. A change in polarization angle of the light (due to the Faraday effect, for example) can be detected as a change in photodiode voltage but the size of the change in voltage for a given change in polarization angle depends on the relative orientation of the two polarizers.