Visibility of the overlaid virtual image is sensitively affected by surrounding illumination in OST (optical see-through) displays. Ambient light may especially deteriorate the visibility of low gray levels, whose luminance is comparable to ambient light. Therefore, we first derive a luminance model based on actual measurements under various ambient lights, and use this model to extract low gray-level region (LGR), which suffers severely from contrast loss. It was experimentally found that gamma tone mapping is the most appropriate for LGR contrast enhancement of OST displays. The gamma value is optimally determined by cost minimization. Visibility enhancement is verified by experiments on a practical setup with a variety of ambient light levels and images.