Page "Local hidden variable theory" Paragraph 30

We might perhaps suppose the polarisers to be perfect, with output intensity of polariser A proportional to cos < sup > 2 </ sup >( a − λ ), but reject the quantum-mechanical assumption that the function relating this intensity to the probability of detection is a straight line through the origin.

Real detectors, after all, have " dark counts " that are there even when the input intensity is zero, and become saturated when the intensity is very high.

It is not possible for them to produce outputs in exact proportion to input intensity for all intensities.