Contrast Sensitivity

Contrast is the contradiction in luminance or colour 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 and brightness 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. Images with a contrast ratio close to their medium's maximum possible contrast ratio experience a ''conservation of contrast'', wherein any increase in contrast in some parts of the image must necessarily result in a decrease in contrast elsewhere. Brightening an image will increase contrast in dark areas but decrease contrast in bright areas, while darkening the image will have the opposite effect. Bleach bypass destroys contrast in both the darkest and brightest parts of an image while enhancing luminance contrast in areas of intermediate brightness.

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 observe
here

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

:$\frac.$

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

Weber contrast is defined as

:$\frac,$

with $I$ and $I_\mathrm$ 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

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
:$\frac,$

with $I_\mathrm$ and $I_\mathrm$ 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 ''m_f'' of a periodic signal ''f''. Modulation quantifies the relative amount by which the amplitude (or difference) (''f''_max − ''f''_min)/2 of ''f'' stands out from the average value (or background) (''f''_max + ''f''_min)/2. In general, ''m_f'' refers to the contrast of the periodic signal ''f'' relative to its average value. If ''m_f'' = 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 ''m_f'' > ''m_g''.

RMS contrast

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 intensities:

:$\sqrt$

where intensities $I_$ are the $i$-th $j$-th element of the two-dimensional image of size $M$ by $N$. $\bar$ is the average intensity of all pixel values in the image. The image $I$ is assumed to have its pixel intensities normalized in the range ,1/math>.

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 angular frequencies of about 2–5 cycles per 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