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The loss in contrast discrimination ability of the human visual system was estimated under a variety of ambient illumination levels first. Then it was modelled as a non-linear weighting function defined in spatial frequency domain to determine which of parts of the image, whatever their spatial frequency, will appear under a given surround luminance level. The weighting function was adopted as a filter for developing an image enhancement algorithm adaptive to surround luminance. The algorithm aims to improve the image contrast under various surround levels especially for small-sized mobile phone displays through gain control of a 2D contrast sensitivity function.