Various conventions have been used to characterize the difference between the image and its background. These have included the contrast ratio, modulation and various formulas to define contrast and percent contrast.
Contrast ratio is the simplest, that is, the higher luminance divided by the lower luminance.
Contrast ratios and percent contrast values, however, do not result in scales that reasonably represent the response of the visual system. Modulation contrast, however, does have the attribute. For example, over a range where added contrast will make the item easier to see, an increase in modulation of equal steps will produce about equal improvements in performance. Log scales are used in acoustical work for the same reason.