Abstract: A device for use with infrared lenses to determine the optimum focal plane. The device comprises an infrared energy source, a collimating lens and an energy sensor to determine the energy level from the energy source to provide a normalizing signal. An infrared energy detector may be moved toward and away from the lens which is to be calibrated in order to test various planes within the depth of focus of the lens. A knife edge shutter is stepped across the path of energy at each of the positions to determine specific read outs of energy at each position of the knife edge shutter. The energy which passe the knife edge shutter is then delivered to a computer where the signal has been normalized by the sensor previously described. The computer reads out the signals corresponding to energy level at successive steps of the knife edge shutter. The computer is programmed to step the detector to successive positions and then the shutter to successive positions across the focal plane.