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− | [[File:lighterstill.jpg]] | + | [[File:lighterstill.jpg]][[File:Contrast.jpg|right|frame]] |
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| '''Contrast''' is the [[difference]] in [[visual]] properties that makes an object (or its [[representation]] in an image) distinguishable from other objects and the background. 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. | | '''Contrast''' is the [[difference]] in [[visual]] properties that makes an object (or its [[representation]] in an image) distinguishable from other objects and the background. 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. |
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− | The human contrast [[sensitivity]] [[function]] shows a typical band-pass 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 [http://en.wikipedia.org/wiki/Spatial_frequency spatial frequency]. | + | The human contrast [[sensitivity]] [[function]] shows a typical band-pass 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 [https://en.wikipedia.org/wiki/Spatial_frequency spatial frequency]. |
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| 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 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. |
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− | 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 [http://en.wikipedia.org/wiki/Lateral_inhibition lateral inhibition] and account for the low-frequency drop-off of the [http://en.wikipedia.org/wiki/Contrast_(vision)#Contrast_sensitivity human contrast sensitivity function]. | + | <center>For lessons on the [[topic]] of '''''Contrast''''', follow [https://nordan.daynal.org/wiki/index.php?title=Category:Contrast '''''this link'''''].</center> |
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| + | 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 [https://en.wikipedia.org/wiki/Lateral_inhibition lateral inhibition] and account for the low-frequency drop-off of the [https://en.wikipedia.org/wiki/Contrast_(vision)#Contrast_sensitivity human contrast sensitivity function]. |
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| 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. | | 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. |
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| 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. | | 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. |
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− | 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 [http://en.wikipedia.org/wiki/Optical_character_recognition optical character recognition].[http://en.wikipedia.org/wiki/Contrast_%28vision%29] | + | 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 [https://en.wikipedia.org/wiki/Optical_character_recognition optical character recognition].[https://en.wikipedia.org/wiki/Contrast_%28vision%29] |
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| [[Category: General Reference]] | | [[Category: General Reference]] |
| [[Category: The Arts]] | | [[Category: The Arts]] |