A new effect of acousto-optic refraction has been detected. The effect is observed in optical images of sound fields and has the appearance of bright fringes in the images. The fringes are shown to identify contours of equal refraction angle. Since the angle of optical refraction induced by a sound field is—under certain conditions—proportional to the spatial gradient or slope of the acoustic pressure, the name "iso-slope fringes" is applied. This new effect can be used to make optical whole field measurements of sound fields with significantly better spatial resolution than is possible with current measurement methods. The new effect may be of particular interest to investigators analyzing non-linear acoustic propagation phenomena.