We present a way to measure the contrast of helmet- mounted displays. Standards exist for direct view flat panel displays and projection displays, but head-mounted displays do not seem to be covered by the standards yet. We start by discussing a basic setup (4-mm aperture, lens, detector) with the aperture residing in the exit pupil of the display. A 4 x4 checkerboard forms a contrast test image that is measured in all 16 rectangles by rotating the 4-mm aperture around a point 10.5 mm behind the aperture (to simulate the natural rotation of the eye). Subsequently, the relation between the measured contrast and the measurement field size is investigated. The data presented indicates that a measurement field size smaller than 1/12 the width and 1/12 the height of the image gives an error in the contrast measurement smaller than 5%. Next, we discuss that, for most cases, the detector can be placed in a fixed position even though this means that the image might be out of focus in the detector plane. Finally, a method is discussed to compensate for the error in the contrast measurement caused by stray light arising in the measurement device.