Parallel Grid

Parallel grids are normally made on the "prismatic section" principle, i.e. the strip heights taper off towards the edges. This reduces the cut-off of primary radiation at the edges - an essential, when large cassettes and short focus-film distances are used.
Parallel grids are also made, where the strip heights even at the edges are retained. Such grids are classified as grids with ratio r, whereas the grids made on the "prismatic section" principle have the ratio ro.
Parallel grids give satisfactory results, when the required image size is small enough, and the exposure admits a low grid ratio. This can be advantageous particularly with bed side exposures, where the absence of a grid/focal distance allow a greater latitude in positioning the X-ray tube.