Contrast is the difference in luminance and/or color that makes an object distinguishable.
In visual perception of the real world, contrast is determined by the difference in the color
and brightness of the object and other objects within the same field of view. Because 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.
Contrast is also the difference between the color or shading of the printed material on
a document and the background on which it is printed, for example in optical character
recognition.
Biological contrast sensitivity 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. This tells us that the human visual system is most
sensitive in detecting contrast differences occurring at 4 cycles per degree, i.e. at
this spatial frequency humans can detect lower contrast differences than at any other spatial
frequency. 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 retinal photoreceptor cells: a finer matrix can resolve finer gratings.
The low frequency drop-off is due to lateral inhibition within the retinal ganglion cells.
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. Formula
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. Below, some
common definitions are given. Weber contrast
Weber contrast is defined as
with and representing the luminance of the features and the background, respectively.
The measure is also referred to as Weber fraction since it is the term which 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
Michelson contrast is commonly used for patterns where both bright and dark features are equivalent
and take up similar fractions of the area. The Michelson contrast is defined as
with and representing the highest and lowest luminance. The denominator represents twice
the average of the luminance. RMS contrast
Root mean square contrast does not depend on the spatial frequency content or the spatial
distribution of contrast in the image. RMS contrast is defined as the standard deviation
of the pixel intensities:
where intensities are the -th -th element of the two dimensional image of size by . is
the average intensity of all pixel values in the image. The image is assumed to have
its pixel intensities normalized in the range .
Contrast sensitivity Contrast sensitivity is a measure of the ability
to discern between luminances of different levels in a static image. Contrast sensitivity
varies between individuals, reaching a maximum at approximately 20 years of age, and at spatial
frequencies of about 2–5 cycles/degree. In addition it can decline with age and also
due to other factors such as cataracts and diabetic retinopathy.
Contrast sensitivity and visual acuity Visual acuity is a parameter that is frequently
used to assess overall vision. However, diminished contrast sensitivity may cause decreased visual
function in spite of normal visual acuity. For example, some individuals with glaucoma
may achieve 20/20 vision on acuity exams, yet struggle with activities of daily living,
such as driving at night. As mentioned above, contrast sensitivity describes
the ability of the visual system to distinguish bright and dim components of a static image.
Visual acuity can be defined as the angle with which one can resolve two points as being
separate, given that the image is shown with 100% contrast and is projected onto the fovea
of the retina. Thus, when an optometrist or ophthalmologist assesses a patient’s visual
acuity using a Snellen chart or some other acuity chart, the target image is displayed
at high contrast. A subsequent contrast sensitivity exam may demonstrate difficulty with decreased
contrast. To assess a patient’s contrast sensitivity,
one of several diagnostic exams may be used. Most charts in an ophthalmologist’s or optometrist's
office will show images of varying contrast and spatial frequency. Parallel bars of varying
width and contrast, known as sine-wave gratings, are sequentially viewed by the patient. The
width of the bars and their distance apart represent spatial frequency, measured in cycles
per degree. Studies have demonstrated that medium-level
spatial frequency, approximately 5-7 cycles per degree, is optimally detected by most
individuals, compared with low- or high-level spatial frequencies. The contrast threshold
can be defined as the minimum contrast that can be resolved by the patient. The contrast
sensitivity is equal to 1/contrast-threshold. Using the results of a contrast sensitivity
exam, a contrast sensitivity curve can be plotted, with spatial frequency on the horizontal,
and contrast threshold on the vertical axis. Also known as contrast sensitivity function,
the plot demonstrates the normal range of contrast sensitivity, and will indicate diminished
contrast sensitivity in patients who fall below the normal curve. Some graphs contain
“contrast sensitivity acuity equivalents,” with lower acuity values falling in the area
under the curve. In patients with normal visual acuity and concomitant reduced contrast sensitivity,
the area under the curve serves as a graphical representation of the visual deficit. It can
be because of this impairment in contrast sensitivity that patients have difficulty
driving at night, climbing stairs and other activities of daily living in which contrast
is reduced.
Recent studies have demonstrated that intermediate-frequency sinusoidal patterns are optimally-detected
by the retina due to the center-surround arrangement of neuronal receptive fields. In an intermediate
spatial frequency, the peak of the pattern is detected by the center of the receptive
field, while the troughs are detected by the inhibitory periphery of the receptive field.
For this reason, low- and high-spatial frequencies elicit excitatory and inhibitory impulses
by overlapping frequency peaks and troughs in the center and periphery of the neuronal
receptive field. Other environmental, physiologic and anatomical factors influence the neuronal
transmission of sinusoidal patterns, including adaptation.
Decreased contrast sensitivity arises from multiple etiologies, including retinal disorders
such as Age-Related Macular Degeneration, amblyopia, lens abnormalities, such as cataract,
and by higher-order neural dysfunction, including stroke and Alzheimer’s disease. In light
of the multitude of etiologies leading to decreased contrast sensitivity, contrast sensitivity
tests are useful in the characterization and monitoring of dysfunction, and less helpful
in detection of disease. Improving contrast sensitivity
It was once thought that contrast sensitivity was relatively fixed and could only get worse
with age. However new research has shown that playing videogames can slightly improve contrast
sensitivity. See also
Acutance Radiocontrast
Contrast ratio References
External links Details on luminance contrast