Edwin Land, "The Mondrian" Constancy Experiment, 1971PART I: Color Vision
Physical Causes of Color:
It makes sense to begin studying color by starting with physics in two ways: it moves us from the reality of physical stimuli to the nature of human response following a chronological order of discovery. This allows us to see a trajectory of understanding color first objectively-as in the world, and then subjectively-in terms of human reaction. It mimics the recent shift in studying color as an objective phenomenon toward a new emphasis on role of the brain and the particulars of its operation. In the past forty years, there has been much research that emphasizes the significance of the subject perceiving. This follows not only in discoveries about how the brain constructs color, but also in anthropological and linguistic studies of color perception and discrimination across cultures and the role of language. Our course will explore these disciplinary approaches and the understanding that they shed, but we will begin with the physical world of light and color. Kurt Nassau (“The Causes of Color,” Byrne/Hilbert, p. 4) describes perceived color as “merely the eyes’ measure and the brain’s interpretation of the dominant wavelength or frequency or energy of a wavelength.” The study of the physics of color is a study of the stimuli prior to or without regard to response. Since a component of color is light traveling at varying speeds, it makes sense to begin by defining light. Visible light is a small part of the spectrum of electromagnetic radiation. The receptors in our eyes are responsive to only a portion of this spectrum measured in wavelengths incrementally called nanometers (nm). The human eye can see between 370 and 730 nm. Some animals have a larger range of vision and can see into the infrared and/or ultraviolet ranges. Isaac Newton played a large role in initial discoveries about the nature of light and color. He discovered that pure light or white light is made up of all the colors of the visible spectrum and he was able to separate the wavelengths into different colors. The subdivisions of the spectrum are essentially arbitrary. Even though the color gradates without abrupt change throughout its length, it is customary to discuss six colors: violet, blue, green, yellow, orange, and red. Pure light (white light) is emitted or transmitted onto objects or material that absorbs some wavelengths and reflects others. The texture and other properties of the material will affect appearance in terms of level of reflection, refraction, diffraction or interference. Reflection is the return of light waves from a surface. Refraction is caused by changes in direction of a wave due to a change in speed. Diffraction is caused by the bending of the wave around small obstacles. Interference is addition of two or more waves that results in a new wave pattern.
Physical Response:
Color vision is the ability to discriminate among wavelengths of light, but color sensation derives from processing by the nervous system. Color processing in the human brain begins in the retina. The retina contains two different classes of photoreceptors known as rods and cones. Rods are of one type; they are less sensitive and play a role in achromatic night-vision. Cones are more sensitive and play a role in chromatic experience. They absorb light at different wavelengths and recombine to transmit to the brain. The cones are subdivided into three types based on their differing sensitivities to wavelength. The three types are morphologically identical, but have different spectral sensitivities. The three cone types are each maximally responsive to short, medium, and long wavelengths and are therefore referred to as L-cones, M-cones, and S-cones., These cones are sometimes also referred to as R, G, and B receptors because of the colors of the wavelengths they approximate. This nomenclature can be confusing as it is not precise and the named colors do not exactly represent where the peak-sensitivities of each cone. In 1801 Thomas Young proposed that human color vision is based on the peak sensitivities of the cone photoreceptors in the eye and Hermann von Helmholtz shortly thereafter expanded upon Young’s ideas by proving that human subjects needed the three wavelengths of red, green, and blue to create a full range of colors. Both Young and von Helmholtz inferred that the existence of three cone types affirmed the trichromacy of human vision. In other words, human color experience is referred to as trichromatic because it depends on three distinct color receptors. Animals or abnormal human subjects with one or two receptors are referred to as monochromatic, and dichromatic respectively. Shortly thereafter, Johann Wolfgang von Goethe observed and wrote extensively about the phenomenon of afterimage in his Theory of Colors of 1810. Afterimage is an optical illusion where a semblance of one color is seen after the eye is heavily stimulated by another color. Goethe was the first to write about these as oppositional pairs of colors. In 1875, German Physiologist Ewald Hering first proposed Opponent-Process Theory. Hering noted in his research that the colors red, green, yellow, and blue appeared to be distinct because all other colors can be made from them and that they appeared to exist in opposing pairs: red and green or yellow and blue. The facts that we see strong afterimages of certain hues after being overexposed to or retinally fatigued by certain hues; that we cannot imagine reddish-green or yellowish-blue were fodder for his ideas. These two main theories Trichromatic and Opponent-Process theories were seen as competitive until relatively recently. They are now seen as representing separate processes that occur at different stages in human physiology. Augmenting our knowledge of trichromacy, opponency is based on the idea that three independent neural processes generate color appearances that operate in a bipolar fashion: red-green; yellow-blue, and white-black. In other words, each cone type outputs to the brain along three axes. Color information is transmitted to the brain via the optic nerve. Optic tracts enter the thalamus and synapse at the lateral geniculate nucleus (LGN). Within the LGN, there are six separate cell layers. Within what are termed P-Cell layers, there are two chromatic opponent types: red/green and blue/red-green. After this synapse, the visual tract continues into the primary visual cortex.
The Weber–Fechner Law attempts to describe the relationship between the physical magnitudes of stimuli and the perceived intensity of the stimuli. Ernst Heinrich Weber (1795–1878) was one of the first people to approach the study of the human response to a physical stimulus quantitatively. Gustav Theodor Fechner (1801–1887) later expanded Weber's findings, and the law’s name became a hyphenate. In 1957,
Leo Hurvich and Dorethea Jameson provided quantitative data for Hering’s theory by recognizing that the hue cancellation could be used as the foundation of a measurement method. Hue cancellation refers to the fact that certain colors cancel each other when mixed together. Red plus green equals yellow, canceling both red and green. Yellow plus blue equals white, canceling both yellow and blue. Hurvich and Jameson reasoned that it should be theoretically possible to cancel any color out. Hurvich and Jameson’s work was instrumental in the computational study of color.
Psychology and Perceptual Phenomena:
Color appearance is not only determined by the spectral power distribution (SPD) and a uniform human response, but variance occurs from the following factors: the color of the surround or environment, and the condition of the perceiver. There also are many incongruities in the perceptual system. Here follows a list of phenomena and effects that pertain to human response.
Retinal Balance and Retinal Fatigue: With sustained exposure, retinal receptors become fatigued. An illusion of an opponent color will appear until the tired receptors have regenerated and retinal balance is restored. This happens not only with colors that correspond to the receptor-types. Theoretically, This should happen with any color and it’s perceptual opposite (see Byrne/Hilbert Philosophy of Color).
Color Constancy: This adaptive feature of human vision is important to our functioning and stability and ability to identify objects. Color constancy allows the perceived colors of objects to remain relatively constant under varying illumination conditions. Color constancy is believed to involve specialized neurons called double-opponent cells and takes place in the primary visual cortex. There, information from cone activity is computed and illumination information is discounted, to maintain a “normal” color perception for the object. Edwin Land’s Retinex Theory was formulated to explain the operation of color constancy. The term Retinex is a combination of ‘retina’ and cortex’, indicating Land’s interest in these locations as instrumental in color processing. Retinex theory may also imply that Trichromatic and Opponent-Process Theories are only partial explanations of color processing. In a 1959 article in Scientific American (“Experiments in Color Vision,”) Land reported that when two beams of yellow light, one representing the longest wavelength, and one the shortest, were projected and superimposed they can stimulate full-color perception (under specific conditions) (Riley)
Simultaneous Contrast: Refers to the manner in which two different areas of color affect each others’ appearance. First discovered by Michel Eugene Chevereul, and explored extensively by Josef Albers.
Positive Afterimage: If eyes are exposed to brightness immediately after dark-adaption, humans will continue to see delayed positive repetitions of the dark space for a brief period.
Negative Afterimage or Successive Contrast: The induced illusion proceeding retinal fatigue. If we look at a red shape on a white surround, we will see a blue-green replica of the shape if we immediately stare at a white space. If we immediately look at another red shape instead, we will the the shape replicated, but in black. This indicates that no receptors are active momentarily following such overexposure.
Bezold-Brucke Shift: A change in hue perception as intensity shifts. Spectral colors appear more blue at higher intensities, and more toward the red and green axis at lower intensities.
Cornsweet Illusion: (Tom Cornsweet) An optical Illusion If in the image of a single uniform color, a small slightly darker gradient is placed in the center, one entire half of the image-from the center over-will appear to be a darker color.
Spreading Effect: A field of color will appear to be darker in black lines intersect it, and lighter if white lines intersect.
Chubb Effect: (Chubb, 1989) The apparent contrast of an object depending on context. Low-contrast texture appears to be higher contrast when surrounded by another high contrast texture.
Mach Bands: An optical illusion consisting of an image of dark and light bands. The human eye will perceive narrow bands of brightness on both sides of the gradients that are not actually present.
Purkinje Effect: (Czech Anatomist Jan Evangelista Purkyne) The tendency for the peak sensitivity of the human eye to shift toward the blue end of the spectrum at low illumination levels.
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