An imaging device has been developed to acquire images during external photon-beam radiotherapy treatments. It consists of a linear array of 128 zinc tungstate (ZnWO4) scintillation crystals each of which is individually optically coupled to a photodiode and associated electronics. The image is formed by scanning the linear array across the radiation field using a stepping motor under the control of a microcomputer. Image archive, display, and analysis are performed using a microVAX II computer. Results from a general theoretical analysis are presented before a detailed description of the particular detector construction. The mechanical design of the detector is such that the detector is automatically positioned to within a millimeter relative to the treatment source. This simplifies procedures for analyzing setup variations when comparing a treatment image to any other treatment, or planning, images. Image acquisition takes under 4 s with a contrast resolution of better than 1% at a spatial resolution of 2.5 mm in the object plane. The primary dose used to form these images is 0.55 cGy although the dose received by the patient will be closer to 25 cGy due to the linear scanning geometry and 3.8-s scan time that is used.