The method of proximity photolithography is a pratical and available way in the manufactrue of grating and radial encode. The distance from mask to surface of photoresist coated on ruled blank is very important

The size of it effects on the ruling ctuality directly. such as causing edge scattering As it is very difficult to produce the exact parallel light in usual optical systems

the acteal image we get is't an ideal self-image. Therefore.a better position of the mask should be found in order to ensure the ruling quality. The relationshiop between image contrast and lithographic gap is deduced in the viwe of image contrast ui this paper

A reliable basis is prosided for selecting the best gap in actual photolithography.