Development of accurate constitutive relations for the mechanical behavior of materials at high strain requires continuously interactive testing. Tensile testing at constant true strain rate illustrates this process. In such a test a servo-controlled testing machine is driven by a control signal produced by the difference between a feedback and a command signal. Response of the specimen to the testing environment produces the feedback signal. This paper describes an optical sensor which produces a signal suitable for controlling a servohydraulic test system. The device employs a projection system and a scanning galvanometer mirror to translate a silhouette of a test specimen past a fixed slit covering a photodiode. The photodiode produces an ac waveform, which subsequent processing converts to a dc signal proportional to the minimum dimension of the silhouette scanned. For many materials this provides sufficient information to measure the instantaneous true strain rate in the deforming region. The paper concludes with a discussion of limitations of the device.