The modulation transfer function of a display oscilloscope: measurements and comments

It is pointed out that "spot size" of high resolution oscilloscope displays typically refers to the width of the spot at its half peak amplitude point, not to the width of the whole light distribution. The modulation transfer function of oscilloscope displays can have a more pronounced effect upon stimulus contrast than is generally realized, particularly when short viewing distances are used. We present measures of the spot profiles of a typical display, and use these to derive the MTF of the instrument. A simple correction is described to derive the actual contrast of a sine wave grating, given the manufacturer's specification of spot size.