In known film processing techniques, the amount by which a film is processed at each stage is determined chiefly by time. If a film is either over- or underexposed, it may be incorrectly processed producing unsatisfactory results. The present invention utilises an arrangement which measures changes in the infrared density of the film during processing to ensure that satisfactory results are produced. The arrangement comprises an infrared light emitting diode (22) and an infrared sensitive photodiode de-tector (26) which are both mounted in a support (20). Film (34) passes between the diode (22) and the detector (26) so that the amount of infrared radiation being transmitted through the film can be determined to provide a measure of the infrared density of the film at each stage during its processing.