Contrast is the difference in luminance
that makes an object (or its representation in an image or display) distinguishable. In visual perception
of the real world, contrast is determined by the difference in the colour
of the object and other objects within the same field of view
. The human visual system is more sensitive to contrast than absolute luminance
; we can perceive the world similarly regardless of the huge changes in illumination over the day or from place to place. The maximum ''contrast'' of an image is the contrast ratio
or dynamic range
Biological contrast sensitivity
According to Campbell and Robson (1968), the human contrast sensitivity function
shows a typical band-pass filter
shape peaking at around 4 cycles per degree, with sensitivity dropping off either side of the peak. That finding has led many to claim that the human visual system
is most sensitive in detecting contrast differences occurring at 4 cycles per degree. However, the claim of frequency sensitivity is problematic given, for example, that changes of distance do not seem to affect the relevant perceptual patterns (as noted, for example, in the figure caption to Solomon and Pelli (1994) While the latter authors are referring specifically to letters, they make no objective distinction between these and other shapes. The relative insensitivity of contrast effects to distance (and thus spatial frequency) may also be observed by casual inspection of a paradigmantic sweep grating, as may be observehere
The high-frequency cut-off represents the optical
limitations of the visual system's ability to resolve
detail and is typically about 60 cycles per degree. The high-frequency cut-off is related to the packing density of the retina
l photoreceptor cell
s: a finer matrix can resolve finer gratings.
The low frequency drop-off is due to lateral inhibition
within the retinal ganglion cell
s. A typical retinal ganglion cell presents a centre region with either excitation or inhibition and a surround region with the opposite sign. By using coarse gratings, the bright bands fall on the inhibitory as well as the excitatory region of the ganglion cell resulting in lateral inhibition and account for the low-frequency drop-off of the human contrast sensitivity function.
One experimental phenomenon is the inhibition of blue in the periphery if blue light is displayed against white, leading to a yellow surrounding. The yellow is derived from the inhibition of blue on the surroundings by the center. Since white minus blue is red and green, this mixes to become yellow.
For example, in the case of graphical computer displays, contrast depends on the properties of the picture source or file and the properties of the computer display, including its variable settings. For some screens the angle between the screen surface and the observer's line of sight is also important.
) increased through unsharp masking
There are many possible definitions of contrast. Some include color; others do not. Travnikova laments, "Such a multiplicity of notions of contrast is extremely inconvenient. It complicates the solution of many applied problems and makes it difficult to compare the results published by different authors."
Various definitions of contrast are used in different situations. Here, luminance
contrast is used as an example, but the formulas can also be applied to other physical quantities. In many cases, the definitions of contrast represent a ratio of the type
The rationale behind this is that a small difference is negligible if the average luminance is high, while the same small difference matters if the average luminance is low (see Weber–Fechner law
). Below, some common definitions are given.
Weber contrast is defined as
representing the luminance of the features and the background, respectively. The measure is also referred to as ''Weber fraction'', since it is the term that is constant in Weber's Law
. Weber contrast is commonly used in cases where small features are present on a large uniform background, i.e., where the average luminance is approximately equal to the background luminance.
Michelson contrast (also known as the ''visibility'') is commonly used for patterns where both bright and dark features are equivalent and take up similar fractions of the area (e.g. sine-wave gratings
). The Michelson contrast is defined as
representing the highest and lowest luminance. The denominator represents twice the average of the maximum and minimum luminances.
This form of contrast is an effective way to quantify contrast for periodic functions ''f''(''x'') and is also known as the modulation ''mf
'' of a periodic signal ''f''. Modulation quantifies the relative amount by which the amplitude (or difference) (''f''max
)/2 of ''f'' stands out from the average value (or background) (''f''max
)/2. In general, ''mf
'' refers to the contrast of the periodic signal ''f'' relative to its average value. If ''mf
'' = 0, then ''f'' has no contrast. If two periodic functions ''f'' and ''g'' have the same average value, then ''f'' has more contrast than ''g'' if ''mf
'' > ''mg
Root mean square
(RMS) contrast does not depend on the angular frequency content or the spatial distribution of contrast in the image. RMS contrast is defined as the standard deviation of the pixel
-th element of the two-dimensional image of size
is the average intensity of all pixel values in the image. The image
is assumed to have its pixel intensities normalized in the range