Images are filtered by the various stages of visual processing and undergo a perceptual transformation. To begin to quantify this process, the appearance of sinewave gratings, images with a sinusoidal luminance profile, was studied. The most salient features of these images is that at high contrast they do not in general appear sinusoidal. A logarithmic transform, based on Fechner's law, is suggested as a first approximation to the perceptual transform in the luminance domain, and shown to characterize the apparent luminance profiles of sinewave gratings. Furthermore, the sinusoidal image profile can be pre-processed (inverse transformed) to create a new image which, when viewed, reverses to a large extent the perceptual distortions discussed above. This `corrected' image would appear as if the combined effects of the perceptual transform had not acted, making the now non-sinusoidal profile conform much more closely to our symmetrical expectations of a sinusoidal appearance. The principle of inversion used here is suggested as a general way of testing the validity of some human vision models proposed to achieve better image processing algorithms based on visual perception.