| 摘要 |
Analog video signals representative of an X-ray image are gated to a first integrator during times in each horizontal line scan in a raster field when a window signal exists and the video signal amplitude is also above a threshold reference level for the integrator to produce a signal representative of the average brightness of the image in areas that are not excluded by their video signal being below threshold level. A second integrator integrates a constant reference voltage signal only during the time that the video signal is being integrated. Since the video scanning rate is constant, the simultaneously produced second integrator signal corresponds to the total continuous or discontinuous area or areas over which the video signal is integrated. The integrated signal representing average image brightness is divided by the signal representing area to yield a signal representing brightness per unit area. This signal is compared with a reference signal corresponding to desired image brightness and the resulting error signal so produced is used to vary the X-ray tube current regulator until the error is nulled. The brightness determination is independent of the X-ray field size set by the collimator and high X-ray attenuating objects in the X-ray beam are excluded.
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