Focused Grid

For radiography and fluoroscopy of thicker objects, where higher kV and shorter exposure times are necessary, focused grids, which are more effective at higher kV, can be used at short focus-film distances. This applies even to general surveys.
When focused grids are used, the central beam of the X-rays is centered at the focal distance. This in order to get the highest possible grid efficiency. The degree of deviation from the indicated focal distance that is admissible is dependent on the grid ratio. The higher the grid ratio, the narrower the deviation limits.