In principle the photocathode can be defined by three parameters; the thickness and the real and imaginary parts of the refractive index. In practice, the reflective index is itself a function of wavelength, and is modelled in the usual way. Measurements on several Hamamatsu tubes have been used to estimate mean values for these parameters.
Given these parameters, Fresnel's laws, as applied to a thin absorptive film can then be applied to deduce the reflectivity, transmissivity and absorptivity of the photocathode; the absorptivity has been shown to scale with the quantum efficiency, and from this the probability of a `hit' being scored can be calculated [14].