The purpose of this paper is to communicate some observations on a some-what subtle aspect of the performance of x-ray phototimers: viz, the way in which their response depends on the x-ray energy spectrum and on the presence of scattered radiation. We specifically avoid the more basic question of sta-bility, reliability, and speed of response. Much of the phenomena which we have studied would, in the jargon of the radiological engineer, fall under the heading of "KVP dependence." However, from the point of view of the physicist, it is the effective energy, n, of the beam which emerges from the patient, ra-ther than the KVP, which is the relevant variable. The point is that E depends on both KVP and the amount and nature of the absorbing substance in the beam. For example, we would expect the beam emerging from a skull study at 80 KVP to have higher R than that of a chest film at the same KVP. If this difference were significant to the phototimer, then it would be meaningless to speak of a "KVP correction."
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