However, diminished contrast sensitivity may cause decreased visual function in spite of normal visual acuity. Visual acuity is a parameter that is frequently used to assess overall vision. Log-log plot of spatial contrast sensitivity functions for luminance and chromatic contrast In addition it can decline with age and also due to other factors such as cataracts and diabetic retinopathy. 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. Luminance difference Average luminance.Ĭontrast sensitivity is a measure of the ability to discern between luminances of different levels in a static image. In many cases, the definitions of contrast represent a ratio of the type Here, luminance contrast is used as an example, but the formulas can also be applied to other physical quantities. Various definitions of contrast are used in different situations. It complicates the solution of many applied problems and makes it difficult to compare the results published by different authors." Travnikova laments, "Such a multiplicity of notions of contrast is extremely inconvenient. There are many possible definitions of contrast. For some screens the angle between the screen surface and the observer's line of sight is also important.Ī photograph of a leaf with several colors-the bottom image has an 11% saturation boost and around 10% increase in contrast. įor 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. Since white minus blue is red and green, this mixes to become yellow. The yellow is derived from the inhibition of blue on the surroundings by the center. One experimental phenomenon is the inhibition of blue in the periphery if blue light is displayed against white, leading to a yellow surrounding. 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. A typical retinal ganglion cell presents a centre region with either excitation or inhibition and a surround region with the opposite sign. The low frequency drop-off is due to lateral inhibition within the retinal ganglion cells. The high-frequency cut-off is related to the packing density of the retinal photoreceptor cells: a finer matrix can resolve finer gratings. 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.
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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 observed here 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. 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. 3.1 Contrast sensitivity and visual acuityĪccording 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.