Color constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main illumination is white sunlight, and also at sunset, when the main illumination is red. This helps us identify objects.
Ibn al-Haytham gave an early explanation of color constancy by observing that the light reflected from an object is modified by the object's color. He explained that the quality of the light and the color of the object are mixed, and the visual system separates light and color. He writes:
Again the light does not travel from the colored object to the eye unaccompanied by the color, nor does the form of the color pass from the colored object to the eye unaccompanied by the light. Neither the form of the light nor that of the color existing in the colored object can pass except as mingled together and the last sentient can only perceive them as mingled together. Nevertheless, the sentient perceives that the visible object is luminous and that the light seen in the object is other than the color and that these are two properties. [1]
Monge (1789), Young (1807), von Helmholtz (1867), Hering (1920), and von Kries (1902, 1905), as well as subsequent researchers Helson and Jeffers (1940), Judd (1940), and Land and McCann (1971), have all made significant contributions to the investigation of colour constancy. The idea that the occurrence of colour constancy was the consequence of unconscious inference (Judd, 1940; von Helmholtz, 1867) and the idea that it was the result of sensory adaptation (Helson, 1943; Hering, 1920) coexisted for a significant portion of this time. To clarify the nature of observers' color-constancy judgements, Arend and Reeves (1986) conducted the first systematic behavioural experiments. Subsequently, new colour constancy models, physiological information on cortical mechanisms, and photographic colorimetric measurements of natural scenes all appeared. [2]
Color vision is how we perceive the objective color, which people, animals and machines are able to distinguish objects based on the different wavelengths of light reflected, transmitted, or emitted by the object. In humans, light is detected by the eye using two types of photoreceptors, cones and rods, which send signals to the visual cortex, which in turn processes those colors into a subjective perception. Color constancy is a process that allows the brain to recognize a familiar object as being a consistent color regardless of the amount or wavelengths of light reflecting from it at a given moment. [3] [4]
The phenomenon of color constancy occurs when the source of illumination is not directly known. [5] It is for this reason that color constancy takes a greater effect on days with sun and clear sky as opposed to days that are overcast. [5] Even when the sun is visible, color constancy may affect color perception. This is due to an ignorance of all possible sources of illumination. Although an object may reflect multiple sources of light into the eye, color constancy causes objective identities to remain constant. [6]
D. H. Foster (2011) states, "in the natural environment, the source itself may not be well defined in that the illumination at a particular point in a scene is usually a complex mixture of direct and indirect [light] distributed over a range of incident angles, in turn modified by local occlusion and mutual reflection, all of which may vary with time and position." [5] The wide spectrum of possible illuminances in the natural environment and the limited ability of the human eye to perceive color means that color constancy plays a functional role in daily perception. Color constancy allows for humans to interact with the world in a consistent or veridical manner [7] and it allows for one to more effectively make judgements on the time of day. [6] [8]
The physiological basis for color constancy is thought to involve specialized neurons in the primary visual cortex that compute local ratios of cone activity, which is the same calculation that Land's retinex algorithm uses to achieve color constancy. These specialized cells are called double-opponent cells because they compute both color opponency and spatial opponency. Double-opponent cells were first described by Nigel Daw in the goldfish retina. [9] [10] There was considerable debate about the existence of these cells in the primate visual system; their existence was eventually proven using reverse-correlation receptive field mapping and special stimuli that selectively activate single cone classes at a time, so-called "cone-isolating" stimuli. [11] [12] Human brain imaging evidence strongly suggests that a critical cortical locus for generating color constancy is located in cortical area V4, [13] damage in which leads to the syndrome of cerebral achromatopsia.
Color constancy works only if the incident illumination contains a range of wavelengths. The different cone cells of the eye register different but overlapping ranges of wavelengths of the light reflected by every object in the scene. From this information, the visual system attempts to determine the approximate composition of the illuminating light. This illumination is then discounted [14] in order to obtain the object's "true color" or reflectance: the wavelengths of light the object reflects. This reflectance then largely determines the perceived color.
There are two possible mechanisms for color constancy. The first mechanism is unconscious inference. [15] The second view holds this phenomenon to be caused by sensory adaptation. [16] [17] Research suggests color constancy to be related changes in retinal cells as well as cortical areas related to vision. [18] [19] [20] This phenomenon is most likely attributed to changes in various levels of the visual system. [5]
Cones, specialized cells within the retina, will adjust relative to light levels within the local environment. [20] This occurs at the level of individual neurons. [21] However, this adaptation is incomplete. [5] Chromatic adaptation is also regulated by processes within the brain. Research in monkeys suggest that changes in chromatic sensitivity is correlated to activity in parvocellular lateral geniculate neurons. [22] [23] Color constancy may be both attributed to localized changes in individual retinal cells or to higher level neural processes within the brain. [21]
Metamerism, the perceiving of colors within two separate scenes, can help to inform research regarding color constancy. [24] [25] Research suggests that when competing chromatic stimuli are presented, spatial comparisons must be completed early in the visual system. For example, when subjects are presented stimuli in a dichoptic fashion, an array of colors and a void color, such as grey, and are told to focus on a specific color of the array, the void color appears different than when perceived in a binocular fashion. [26] This means that color judgements, as they relate to spatial comparisons, must be completed at or prior to the V1 monocular neurons. [26] [27] [28] If spatial comparisons occur later in the visual system such as in cortical area V4, the brain would be able to perceive both the color and void color as though they were seen in a binocular fashion.
The "Land effect" is the capacity to see full color (if muted) images solely by looking at a photo with red and gray wavelengths. The effect was discovered by Edwin H. Land, who was attempting to reconstruct James Clerk Maxwell's early experiments in full-colored images. Land realized that, even when there were no green or blue wavelengths present in an image, the visual system would still perceive them as green or blue by discounting the red illumination. Land described this effect in a 1959 article in Scientific American. [29] In 1977, Land wrote another Scientific American article that formulated his "retinex theory" to explain the Land effect. The word "retinex" is a blend of "retina" and "cortex", suggesting that both the eye and the brain are involved in the processing. Land, with John McCann, also developed a computer program designed to imitate the retinex processes taking place in human physiology. [30]
The effect can be experimentally demonstrated as follows. A display called a "Mondrian" (after Piet Mondrian whose paintings are similar) consisting of numerous colored patches is shown to a person. The display is illuminated by three white lights, one projected through a red filter, one projected through a green filter, and one projected through a blue filter. The person is asked to adjust the intensity of the lights so that a particular patch in the display appears white. The experimenter then measures the intensities of red, green, and blue light reflected from this white-appearing patch. Then the experimenter asks the person to identify the color of a neighboring patch, which, for example, appears green. Then the experimenter adjusts the lights so that the intensities of red, blue, and green light reflected from the green patch are the same as were originally measured from the white patch. The person shows color constancy in that the green patch continues to appear green, the white patch continues to appear white, and all the remaining patches continue to have their original colors.
Color constancy is a desirable feature of computer vision, and many algorithms have been developed for this purpose. These include several retinex algorithms. [31] [32] [33] [34] These algorithms receive as input the red/green/blue values of each pixel of the image and attempt to estimate the reflectances of each point. One such algorithm operates as follows: the maximal red value rmax of all pixels is determined, and also the maximal green value gmax and the maximal blue value bmax. Assuming that the scene contains objects which reflect all red light, and (other) objects which reflect all green light and still others which reflect all blue light, one can then deduce that the illuminating light source is described by (rmax, gmax, bmax). For each pixel with values (r, g, b) its reflectance is estimated as (r/rmax, g/gmax, b/bmax). The original retinex algorithm proposed by Land and McCann uses a localized version of this principle. [35] [36]
Although retinex models are still widely used in computer vision, actual human color perception has been shown to be more complex. [37]
Color or colour is the visual perception based on the electromagnetic spectrum. Though color is not an inherent property of matter, color perception is related to an object's light absorption, reflection, emission spectra and interference. For most humans, colors are perceived in the visible light spectrum with three types of cone cells (trichromacy). Other animals may have a different number of cone cell types or have eyes sensitive to different wavelength, such as bees that can distinguish ultraviolet, and thus have a different color sensitivity range. Animal perception of color originates from different light wavelength or spectral sensitivity in cone cell types, which is then processed by the brain.
Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different frequencies independently of light intensity. Color perception is a part of the larger visual system and is mediated by a complex process between neurons that begins with differential stimulation of different types of photoreceptors by light entering the eye. Those photoreceptors then emit outputs that are propagated through many layers of neurons and then ultimately to the brain. Color vision is found in many animals and is mediated by similar underlying mechanisms with common types of biological molecules and a complex history of evolution in different animal taxa. In primates, color vision may have evolved under selective pressure for a variety of visual tasks including the foraging for nutritious young leaves, ripe fruit, and flowers, as well as detecting predator camouflage and emotional states in other primates.
The visual system comprises the sensory organ and parts of the central nervous system which gives organisms the sense of sight as well as enabling the formation of several non-image photo response functions. It detects and interprets information from the optical spectrum perceptible to that species to "build a representation" of the surrounding environment. The visual system carries out a number of complex tasks, including the reception of light and the formation of monocular neural representations, colour vision, the neural mechanisms underlying stereopsis and assessment of distances to and between objects, the identification of a particular object of interest, motion perception, the analysis and integration of visual information, pattern recognition, accurate motor coordination under visual guidance, and more. The neuropsychological side of visual information processing is known as visual perception, an abnormality of which is called visual impairment, and a complete absence of which is called blindness. Non-image forming visual functions, independent of visual perception, include the pupillary light reflex and circadian photoentrainment.
Chromatic adaptation is the human visual system’s ability to adjust to changes in illumination in order to preserve the appearance of object colors. It is responsible for the stable appearance of object colors despite the wide variation of light which might be reflected from an object and observed by our eyes. A chromatic adaptation transform (CAT) function emulates this important aspect of color perception in color appearance models.
Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cell in the eye. Organisms with tetrachromacy are called tetrachromats.
An afterimage is an image that continues to appear in the eyes after a period of exposure to the original image. An afterimage may be a normal phenomenon or may be pathological (palinopsia). Illusory palinopsia may be a pathological exaggeration of physiological afterimages. Afterimages occur because photochemical activity in the retina continues even when the eyes are no longer experiencing the original stimulus.
Trichromacy or trichromatism is the possessing of three independent channels for conveying color information, derived from the three different types of cone cells in the eye. Organisms with trichromacy are called trichromats.
Cone cells, or cones, are photoreceptor cells in the retinas of vertebrates' eyes, including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cones function best in relatively bright light, called the photopic region, as opposed to rod cells, which work better in dim light, or the scotopic region. Cone cells are densely packed in the fovea centralis, a 0.3 mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina. Conversely, they are absent from the optic disc, contributing to the blind spot. There are about six to seven million cones in a human eye, with the highest concentration being towards the macula.
The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from photoreceptor cells in an antagonistic manner. The opponent-process theory suggests that there are three opponent channels, each comprising an opposing color pair: red versus green, blue versus yellow, and black versus white (luminance). The theory was first proposed in 1892 by the German physiologist Ewald Hering.
In the study of human visual perception, scotopic vision is the vision of the eye under low-light conditions. The term comes from Greek skotos, meaning "darkness", and -opia, meaning "a condition of sight". In the human eye, cone cells are nonfunctional in low visible light. Scotopic vision is produced exclusively through rod cells, which are most sensitive to wavelengths of around 498 nm (blue-green) and are insensitive to wavelengths longer than about 640 nm (red-orange). This condition is called the Purkinje effect.
The Young–Helmholtz theory, also known as the trichromatic theory, is a theory of trichromatic color vision – the manner in which the visual system gives rise to the phenomenological experience of color. In 1802, Young postulated the existence of three types of photoreceptors in the eye, with different but overlapping response to different wavelengths of visible light.
The colour centre is a region in the brain primarily responsible for visual perception and cortical processing of colour signals received by the eye, which ultimately results in colour vision. The colour centre in humans is thought to be located in the ventral occipital lobe as part of the visual system, in addition to other areas responsible for recognizing and processing specific visual stimuli, such as faces, words, and objects. Many functional magnetic resonance imaging (fMRI) studies in both humans and macaque monkeys have shown colour stimuli to activate multiple areas in the brain, including the fusiform gyrus and the lingual gyrus. These areas, as well as others identified as having a role in colour vision processing, are collectively labelled visual area 4 (V4). The exact mechanisms, location, and function of V4 are still being investigated.
OPN1LW is a gene on the X chromosome that encodes for long wave sensitive (LWS) opsin, or red cone photopigment. It is responsible for perception of visible light in the yellow-green range on the visible spectrum. The gene contains 6 exons with variability that induces shifts in the spectral range. OPN1LW is subject to homologous recombination with OPN1MW, as the two have very similar sequences. These recombinations can lead to various vision problems, such as red-green colourblindness and blue monochromacy. The protein encoded is a G-protein coupled receptor with embedded 11-cis-retinal, whose light excitation causes a cis-trans conformational change that begins the process of chemical signalling to the brain.
Chromostereopsis is a visual illusion whereby the impression of depth is conveyed in two-dimensional color images, usually of red–blue or red–green colors, but can also be perceived with red–grey or blue–grey images. Such illusions have been reported for over a century and have generally been attributed to some form of chromatic aberration.
Visual perception is the ability to interpret the surrounding environment through photopic vision, color vision, scotopic vision, and mesopic vision, using light in the visible spectrum reflected by objects in the environment. This is different from visual acuity, which refers to how clearly a person sees. A person can have problems with visual perceptual processing even if they have 20/20 vision.
Impossible colors are colors that do not appear in ordinary visual functioning. Different color theories suggest different hypothetical colors that humans are incapable of perceiving for one reason or another, and fictional colors are routinely created in popular culture. While some such colors have no basis in reality, phenomena such as cone cell fatigue enable colors to be perceived in certain circumstances that would not be otherwise.
A parasol cell, sometimes called an M cell or M ganglion cell, is one type of retinal ganglion cell (RGC) located in the ganglion cell layer of the retina. These cells project to magnocellular cells in the lateral geniculate nucleus (LGN) as part of the magnocellular pathway in the visual system. They have large cell bodies as well as extensive branching dendrite networks and as such have large receptive fields. Relative to other RGCs, they have fast conduction velocities. While they do show clear center-surround antagonism, they receive no information about color. Parasol ganglion cells contribute information about the motion and depth of objects to the visual system.
Vision is an important sensory system for most species of fish. Fish eyes are similar to the eyes of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Birds and mammals normally adjust focus by changing the shape of their lens, but fish normally adjust focus by moving the lens closer to or further from the retina. Fish retinas generally have both rod cells and cone cells, and most species have colour vision. Some fish can see ultraviolet and some are sensitive to polarised light.
In astronomy, a green star is a white or blueish star that appears greenish in some viewing conditions. Under typical viewing conditions, there are no greenish stars, because the color of a star is more or less given by a black-body spectrum. However, a few stars appear greenish to some observers in certain viewing conditions—for example, the optical illusion that a red object can make nearby objects look greenish. Some multiple star systems, such as Antares, have a bright reddish or yellowish star where this contrast makes other stars in the system seem greenish.
Memory color is the canonical hue of a type of object that allistic human observers acquire through their experiences with instances of that type. For example, most allistic human observers know that an apple typically has a reddish hue; this knowledge about the canonical color which is represented in memory constitutes a memory color.
Here "Reprinted in McCann" refers to McCann, M., ed. 1993. Edwin H. Land's Essays. Springfield, Va.: Society for Imaging Science and Technology.
