Malus' law exactly. We might perhaps suppose the polarisers to be perfect, with output intensity of polariser A proportional to cos2(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. By varying our assumptions, it seems possible that the realist prediction could approach the quantum-mechanical one within the limits of experimental error, though clearly a compromise must be reached. We have to match both the behaviour of the individual light beam on passage through a polariser and the observed coincidence curves. The former would be expected to follow Malus' law fairly closely, though experimental evidence here is not so easy to obtain. We are interested in the behaviour of very weak light and the law may be slightly different from that of stronger light. See also EPR paradox Bohr–Einstein debates References Further reading Figure 5 highlights experimental data points inexplicable by local hidden variable theory. Quantum measurement Hidden variable theory