A grating having holographic properties is employed in a pixel clock for a raster-output scanner. A pilot beam, off-axis from the writing beam creating a latent image on the photoreceptor, is focused on the grating. The grating is adapted to direct a portion of the pilot beam to a photodetector in a fixed point location. The holographic properties of the grating ensure that a wave associated with a reflected interference pattern is always directed at the photodetector throughout the scan path of the pilot beam.