For design criteria of displayed images and for the judgment of image quality, it is very important to dispose of a trustful formula for the contrast sensitivity of the human eye. The contrast sensitivity function or CSF depends on a number of conditions. Most important are the luminance and the viewing angle of the object, but surround illumination can also play a role. In the paper a practical formula is given for a standard observer. This formula is derived from a more general physical formula for the contrast sensitivity. In this paper also the effects of orientation angle and surround luminance will be treated. The orientation angle will be incorporated in the formula and a correction factor will be given for the dependence on surround luminance.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.