HERETOFORE, the sensitometric determinations of X-ray intensifying screen factors have not given reliable results owing to the fact that the intensity of the fluorescent light depends considerably on the quality and intensity of the incident X-rays, and the proper conditions for exposure of the screens had not been determined. In the present experiment it was found that satisfactory values of intensifying factors were obtained if the X-rays were filtered by a sufficient thickness of aluminum to give the same intensity and quality of the X-rays as is transmitted by a I5-cm. thickness of water under the standard conditions of X-ray tube operation; a correction factor must be applied to take account of the difference in the sensitivity of the screens to direct and scattered radiation.
The exposure conditions determined upon were as follows: 60 kilovolts on the X-ray tube; 4 milliamperes; I cm. aluminum filter; and 5 Β° cm. target film distance; the ratio of the exposure with and without screens for a density of one by standard development must be multiplied by 0.83 for single screens and 0.78 for double screens to obtain the correct intensifying factor.
Cassettes were tested for contact between film and screen by the use of a screen ruled with cross-lines; cassettes with rigid back lined with thin felt gave poor contact in central area, owing to the bulge in the face of the cassette. The back of a cassette should be slightly flexible and should be lined with thick soft felt.