the output of a test light of a densitometer is divided into a test beam and a reference beam . the test beam is caused to traverse film samples , and impinge on a detector that produces a signal responsive to the intensity of the received light . a second detector is arranged to receive the reference beam , which does not traverse the film sample . a measuring system is responsive to the output of the first detector , and utilizes the output of the second detector to compensate for variations in the intensity of the test light .