A TIME-SCALE SENSITOMETRIC METHOD FOR EVALUATING SCREEN-FILM SYSTEMS

An x-ray sensitometer is used to measure the characteristic curve of r adiographic films exposed with fluorescent intensifying screens. The s eries of relative exposures, necessary to cover the full density range of the film, can be obtained by either time-scale or intensity-scale sensitometric methods. We have developed a convenient method of exposi ng film-screen systems for time-scale sensitometry. In this method, du ring exposure the x-ray kilovoltage, tube current and x-ray intensity remain constant and a geometric series of exposures of the film is mod ulated by varying the exposure time. This time variation can be obtain ed when a lead disc with different sector openings is rotated in front of the film system by a stepping motor. The conditions normally used are 70 kVp x-rays, 3.5 mm Al total filtration at the tube, and 2.4 m f ocal spot-film distance. This exposure latitude gives a complete chara cteristic curve of film-screen systems.