When conducting a visible dye penetrant inspection, the contrast sensitivity of the eye is important. Contrast sensitivity is a measure of how faded or washed out an image can be before it become indistinguishable from a uniform field.
Contrast sensitivity is a function of the size or spatial frequency of the features in the image.
However, this is not a direct relationship as larger objects are not always easier to see than smaller objects as contrast is reduced as demonstrated by the image below. In the image below, the luminance of pixels is varied sinusoidally in the horizontal direction. The spatial frequency increases exponential from left to right. The luminance of peaks and troughs remains constant along a given horizontal path through the image. If the detection of contrast was dictated solely by image contrast, the alternating bright and dark bars should appear to have equal height everywhere in the image.