The commonly used sizes of filter grids are 5 “X18” – 18 “X18”, and the grid ratios are 8:1, 10:1 and 12:1. This kind of filter grate is a square filter grate. The filter grating used for image intensifier screen is different from ordinary filter grating. Image intensifier screen is a device for perspective. The output screen of image intensifier is circular, so the shape of filter grating is circular.
The filter grating is placed between the human body and the film to filter out the effect of scattered rays on the film. At present, the filter grating is used for camera racks, cameras, and image intensifier screen. It is square on camera racks and cameras, and circular at the input end of image intensifier screen. The focal length varies with the position of the filter grating.