In a photorefractive medium the total fluence of light (the product of the intensity of light and the time of exposure) determines the extent of
towards the final, steady-state response of the material to an optical pattern. The end result, the ultimate response of the material is determined by the relative contrast of the optical pattern and not the intensity. This behavior is rather different to that of most nonlinear optical materials, which usually require a high intensity of light to function well. A photorefractive material can respond to weak patterned illumination if sufficient exposure time is possible to obtain an adequate response.