Pictorial data from a thermal imager is applied on a line to a RAM at which a histogram representing the numerical distribution of pixel values is produced. This is applied to a microprocessor which uses the histogram to produce a transform to control the gain and offset of the data output of the processor. The microprocessor sets a look-up table to apply the transform to the received data, the output of the table being converted to analogue form at an analogue-to-digital converter.