It's common practice in radiography, especially medical radiography where radiation doses are relevant, to use a fluorescent screen to expose photographic film, rather than rely on the effect of x-rays on the film itself. I understand the concept of fluorescence and the overall photon gain/quantum efficiency gain of using a heavy metal fluorescent material (e.g. Calcium Tungstate or Gadolinium oxysulphide) to absorb the x-rays photons and emit numerous visible light photons. This subsequently increases the overall sensitivity (speed) of the system. However, a fluorescent screen is also stated to increase the film contrast (gamma) significantly. As gamma is a log-log measurement this would suggest that there is a non-linear process in the screen. As surely, if the conversion was linear (e.g. 100 light photons for 1 x-ray photon) then the system gamma would be unaffected. Does anyone know what this non-linearity is, and what factors affect it? I've already been through about half-a-dozen radiography texts, all of which mention the effect on gamma, but none of which explain it.