When individual detectors in scanned focal plane arrays are sampled at a rate of once per instantaneous field of view (IFOV), a problem arises in measuring detector optical modulation functions, (SWR, MTF). Ideally, one would like to pass a slit illumination or knife edge illumination across an IFOV, generating respectively, a line spread function or knife edge response curve from which optical modulation could be calculated versus spatial frequency. Use of sampling phase delays is a possible solution, but is subject to phase errors resulting from scan rate variations. The method described here is a "phased knife edge" approach. It employs a multi-bar reticle image, designed so that successive bar edges are slightly shifted in phase with respect to an IFOV area as the IFOV signal is sampled during a single scan. The resulting data, after conversion to a knife edge response function, is convolved with a sliding phase, computer-generated square wave image to find SWR versus spatial frequency. MTF is obtained by converting the knife edge response to a line spread function and applying standard algorithms for convolution of the LSF with sinusoidal bar patterns of desired spatial frequency.