WHAT IS THE TRUE FILM-SCREEN SENSITIVITY

A system for measuring the film-screen sensitivity has been described using an X ray source. A single film was used to assess the variation in speed when exposed to four different accelerating potentials: 60 kV(p), 80 kV(p), 90 kV, and 110 kV(p). These were chosen to cover the range of applied potentials used for chest radiography in the West Midlands. Two sets of measurements were carried out. Firstly a standard him-screen cassette combination was used in order to investigate variations in processor performance. Measurements were then carried out on thirteen different film-screen processor combinations to assess differences between the nominal speed quoted by the manufacturer and the speed actually obtained in each hospital. For the standard system of nominal speed 400, the minimum speed was 282 and the maximum was calculated to be 689, a range factor of 2.44. The median speed increased by 33% as the kV(p) was increased - 387 at 60 kV(p) to 512 at 110 kV(p). The measurement of film-screen processing systems using the hospital's own film-screen system produced speeds closer to the nominal values than those variations when the standard film-screen combination was used. The maximum variation was calculated to be 33%. The measured values in speeds at nominal 300 and 400 were very close in value, with four of the five speeds at 400 being within the measured range of values at nominal 300. There was only an overall difference of 15% in the average speed between the two groups rather than the 33% expected.