Patent US 4,091,287

In a radiological apparatus, arranged to produce a representation of the variation of absorption of penetrating radiation over a planar, cross-sectional slice of a body under examination, the radiation emergent from the body along a plurality of paths in the plane of the slice is detected by detector means including one or more photomultipliers. The output signals from the photomultiplier or photomultipliers are monitored and compared with upper and lower threshold levels which represent the extremes of substantially linear operation of the photomultiplier or photomultipliers. In the event of the output signals departing from the range defined by said threshold levels a correction signal is generated and utilized to tend to restore the output signals within said range.