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Kolb H, Fernandez E, Nelson R, editors. Webvision: The Organization of the Retina and Visual System [Internet]. Salt Lake City (UT): University of Utah Health Sciences Center; 1995-.

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Webvision: The Organization of the Retina and Visual System [Internet].

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The Perception of Color

1 and 2.

Department of Optometry and Vision Sciences University of Melbourne, Australia

Created: ; Last Update: July 9, 2007.

Overview

Color vision processing in the primate visual system is initiated by absorption of light by three different spectral classes of cones. Consequently, color vision is described as being trivariant or trichromatic, and initial psychophysical studies demonstrated that colors could be matched by the use of three different primaries. In 1802, Thomas Young (1) proposed a model that perception of color can be coded by three principal color receptors rather than thousands of color receptors coding for individual colors.

Spectral sensitivity of cones can be determined through several methods. Two of these methods include isolating receptoral responses (2) using calculations from color matching function of normals and dichromats (a dichromat is a subject whose retina has one cone photopigment missing) (3), microspectrometry (4), or reflection densitometry (5, 6). The microspectrometer technique involves isolating a single cone and passing light through it. The change in transmission of different wavelengths can be used to calculate the spectral absorption of the cone or determine the change in electrical response. Reflection densitometry involves directing light in the retina and determining the change in absorption as a function of wavelength. These results are subsequently used to calculate spectral absorption.

Three classes of cones in the human retina have been isolated from the above techniques. These three classes of cones are the short-wavelength-sensitive (S-cones), middle-wavelength-sensitive (M-cones), and long-wavelength-sensitive (L-cones), and all have different but overlapping spectral sensitivities. The spectral sensitivity of S-cones peaks at approximately 440 nm, M-cones peak at 545 nm, and L-cones peak at 565 nm after correction for pre-retinal light loss, although the various measuring techniques result in slightly different maximum sensitivity values (Fig. 1).

Figure 1. Spectral sensitivity of the S-cone, M-cone, and L-cone.

Figure 1

Spectral sensitivity of the S-cone, M-cone, and L-cone. Combined results from various authors using different methods including retinal densitometry from Rushton (▾ and ▿), microspectrometry from Brown and Wald (▪ and □) (more...)

Color Matches

The trichromatic nature of color vision will enable almost any color to be matched by a mixture of three colors. This trichromacy of vision is also linear. This means that colorimetric equations have properties of an ordinary equation.

The color matches of quantity C s of a stimulus S can be expressed as:

Image kallcolore1.jpg

where li are the three primary colors (not necessarily monochromatic spectral colors) and Ci are tristimulus values.

Tristimulus values represent the quantities of each of the three primaries necessary to achieve a match for color and luminosity (Fig. 2). They can be expressed in units of luminous flux or radiant flux, or even an arbitrary scale provided that the arbitrary scale conveys the relative proportions of the three quantities.

Figure 2. Subjects are asked to adjust three colors on the left-hand side of the bipartite field to match a standard color presented on the right-hand side of the bipartite field.

Figure 2

Subjects are asked to adjust three colors on the left-hand side of the bipartite field to match a standard color presented on the right-hand side of the bipartite field.

Additive and Subtractive Color Mixtures

Two or more colors can be added together to produce a new color composed of the mixture of the initial colors used. This can be demonstrated on a white screen with three colors: blue, green, and red. Cyan, magenta, yellow, and white are formed from the mixture of these colors (Fig. 3). One of the requirements in choosing colors to be used in color mixing experiments is that two of them cannot be mixed to produce the third.

Figure 3. Additive color mixtures of blue, green, and red to produce cyan, magenta, yellow, and white.

Figure 3

Additive color mixtures of blue, green, and red to produce cyan, magenta, yellow, and white.

Subtractive color mixtures involve the selective absorption of wavelengths. Cyan, magenta, and yellow are subtractive primaries. If white light was shone through a yellow filter, the yellow filter will absorb blue and transmit red and green (which makes yellow). Therefore, yellow can be considered as a -B filter. A magenta filter subtracts or absorbs green (-G filter) from white light, and cyan subtracts or absorbs red (-R filter) from white light (Fig. 4).

Figure 4. Subtractive color mixtures of cyan, magenta, and yellow to produce blue, green, and red.

Figure 4

Subtractive color mixtures of cyan, magenta, and yellow to produce blue, green, and red.

Grassmann's Laws provide a quantitative description of color matching data (7). They hold up well within a prescribed set of conditions (with respect to brightness, adaptation of the observer, size of the field, etc). Grassmann's Laws are useful in quantifying color matching data, but as a rule, color matching data are affected by the following:

  • macular pigment (xanthophyll) variations in subject's central 4-5 degrees of vision
  • chromatic aberrations
  • rod intrusion, especially when large fields and low photopic light levels are involved
  • failure of Abney's Law of luminance additivity including the Helmholtz-Kohlrausch effect
  • Bezold-Brücke hue shift at bright intensities

Color Specification

The Munsell Color System

All colors can be fully specified in terms of their hue, lightness, and saturation. The Munsell system has three dimensions: hue, value, and chroma. These three dimensions correspond to the three perceptual attributes of human color vision.

The three dimensions of the Munsell color system are:

1.

Hue: Related to wavelength or dominant wavelength. Hue is denoted by a combination of letters and numbers making up a 100-step scale (Fig. 5). There are 10 letter categories used to denote hue, with each of these further subdivided (by the use of numerals 1 to 10) into 10 subgroups. If the numeral denoting the hue subgroup is 5, then it can be omitted (e.g., 5R is the same hue as R).

2.

Value: Specified on a numerical scale from 1 (black) to 10 (white), and this attribute is related to reflectance and luminosity (or lightness).

3.

Chroma: The Munsell term corresponding to saturation. It is indicated numerically on a scale of 0 to the various maxima, dependent on the saturation obtainable with available pigments.

Figure 5. The Munsell top showing the location of the different colors.

Figure 5

The Munsell top showing the location of the different colors.

For example, a color may have a notation 2GY 6/10. This means it is a green/yellow that is quite close to being a yellow; it has a value of 6 (i.e., almost midway in the black/white scale) and a chroma of 10 (i.e., it is saturated).

The scaling used in the Munsell system is designed to be perceptually uniform. In other words, the color samples are arranged in equal, visual steps. For example, the perceived difference between a chroma of 3 and a chroma of 4 is (nominally) the same as the perceived difference between chroma 4 and 5. This scaling is the same for all three dimensions, although step sizes along different dimensions are not comparable (that is, a single step difference in hue does not have the same perceptual difference as a single step in saturation).

A schematic representation of the Munsell system is shown in Fig. 6. The value scale is on the vertical axis, the hue scale is on the perimeter of the cylinder, and chroma is on a radial scale. The Munsell top, shown in Fig. 5, is a more precise representation of this concept.

Figure 6. Munsell color system, illustrating hue, value, and chroma.

Figure 6

Munsell color system, illustrating hue, value, and chroma.

1931 Commision Internationale d'Elairage (CIE) Chromaticity System

Trichromatic color matches using three colors can be illustrated on Newton's color circle (Fig. 7). Newton's color circle consists of the following components:

Figure 7. Newton's color circle.

Figure 7

Newton's color circle.

  • a circle representing the spectral colors (although not shown here, mixtures of blue and red (purple locus) are not spectral colors, and hence, a straight line between R and B is more appropriate)
  • a triangle whose vertices represent the three primary colors used to make color matches (R, G, and B)
  • the center of the circle representing white (W)

Newton's color circle provides a qualitative description of color matches and can be used to explain why two colors may not be sufficient to make color matches and to also explain the use of "negative" colors. For example, if 500 nm is required to be matched (spectral color located on the circle), blue and green will be required. However, blue and green primaries alone will produce a desaturated 500 nm. Therefore, red must be added to the spectral color to desaturate it and make the match (Fig. 8). When the third primary is added to desaturate the color mixture, negative tristimulus values result (Fig. 5). This can be demonstrated by the following equation:

Figure 8. The use of Newton's color circle to illustrate matching a spectral color of 500 nm using the three primaries.

Figure 8

The use of Newton's color circle to illustrate matching a spectral color of 500 nm using the three primaries.

Image kallcolore2.jpg

or

Image kallcolore3.jpg

To deal with the "negative" colors, the CIE devised the XYZ system that uses unreal (imaginary) primaries to describe color space. The 1931 CIE chromaticity system chose three imaginary primaries (reference stimuli) X, Y, and Z, so that all spectral loci lying inside this triangle will be positive. The alychne are locus of colors with no luminosity, and this was chosen to lie along the X to Z on the XYZ chromaticity system. All luminosity is expressed in Y. The reference loci of Y was chosen to just enclose the domain of real colors. Equi-energy white was chosen to have equal chromaticity co-ordinates, that is, 0.33, 0.33 (Fig. 9 and Fig. 10). Chromaticity co-ordinates represents the relative contribution of the three primaries, and the sum of the co-ordinates equals 1.0. Therefore, z can be calculated, by knowing the co-ordinates x and y because x + y + z = 1.

Figure 9. 1931 CIE chromaticity diagram.

Figure 9

1931 CIE chromaticity diagram. The triangle represents the three primaries used in the RGB system. From Le Grand (27).

Figure 10. 1931 CIE chromaticity diagram with approximate color representation.

Figure 10

1931 CIE chromaticity diagram with approximate color representation. From Benjamin (28).

Dominant wavelength, complementary wavelength, and excitation purity can be easily located for a sample. The dominant wavelength represents the principle wavelength of the color. The complementary wavelength is the wavelength that produces white when mixed in appropriate portions with the dominant wavelength. Spectral complementaries can be found when a line joined by a line passes through the achromatic point shown as C (Fig. 11). The dominant wavelength of A is given by the spectral wavelength at DA, and the complementary is given by the wavelength at CA (Fig. 11). Point C identifies the location of the white point and B identifies another wavelength that, when mixed at the appropriate proportions, will produce white.

Figure 11. Complementary and dominant spectral wavelengths of color A.

Figure 11

Complementary and dominant spectral wavelengths of color A. Color B is also complementary to color A, because an appropriate mix of the two wavelengths will produce white.

The achromatic point varies, depending upon the standard illuminant that is used (Fig. 12). A shift in the x and y co-ordinates occurs as color temperature increases. For standard illuminant C, there is no complimentary wavelengths for green (between wavelengths 492 nm to 567 nm). However, white light can be formed with a suitably chosen purple light (Fig. 12).

Figure 12. Variation position of the achromatic point according to color temperature.

Figure 12

Variation position of the achromatic point according to color temperature. From Benjamin (28).

Color Discrimination Functions

The three variables in color vision—hue, saturation, and brightness—all depend on wavelength. Color discrimination experiments allow us to know how much change in wavelength is required to detect a difference in hue, saturation, and brightness.

Hue discrimination describes the amount of change in wavelength (l + ∆l) that is required to be able detect a change in hue. For blue and red light, a large change in wavelength is required to detect a change in hue, whereas a less than 2-nm change in wavelength is needed for most of the spectrum for a person with normal color vision (Fig. 13).

Figure 13. Mean wavelength discrimination curve.

Figure 13

Mean wavelength discrimination curve. From Davson (29). See also Marriott (30).

Saturation discrimination describes the degree of paleness of the color. Saturation is related to colorimetric purity (P), which is also defined as:

Image kallcolore4.jpg

where L is the luminance of the spectral color and Lw is the luminance of the white that is mixed with the spectral color. The colorimetric purity of a color quantifies the amount of white mixed with the spectral color. If the spectral color is pure (no white added), then the colorimetric purity is 1.

In saturation discrimination experiments, the luminance is kept constant. A bipartate field is used with white (Lw) on one side and white mixed with the spectral color on the other side (Lw + ΔL). It can be seen in Fig. 14 that more 570-nm color is required to make the white patch appear colored. Therefore, yellow has low saturating power, whereas blue and red have high saturation power.

Figure 14. Saturation discrimination of Priest and Brickwide in 1938.

Figure 14

Saturation discrimination of Priest and Brickwide in 1938. From Graham (31).

The V(λ) function closely matches the sensation of brightness and hence is commonly considered to reflect brightness discrimination. The wavelength 555 nm is perceived as the brightest in the color spectrum (see the earlier section on the photopic and scotopic luminosity function).

Theories of Color Vision

Any theory of color vision must predict all the perceptual attributes noted earlier. We present here a simplistic view of the trichromatic and color opponent theories. See the Color Vision chapter by Peter Gouras for a discussion on the physiological correlate of color opponency.

The trichromatic theory was first proposed by Thomas Young in 1802 (1) and was explored further by Helmholtz (8) in 1866. This theory is primarily based on color mixing experiments and suggests that a combination of three channels explain color discrimination functions.

Evidence for the trichromatic theory includes:

  • identification of the spectral sensitivities of two cone pigments by Rushton's retinal densitometry (5)
  • identification of three cone pigments by microspectrometry (9)
  • identification of the genetic code for L, M, and S cones (10, 11)
  • color matching functions
  • isolating photoreceptors and measuring their physiological responses as a function of wavelength (2)
  • Spectral sensitivity measurements (Wald-Marre spectral sensitivity functions and Stiles' π-mechanisms)

However, the trichomatic theory fails to account for the four unique colors—red, green, yellow and blue—and also fails to explain why dichromats can perceive white and yellow. It also fails to fully explain color discrimination functions and opponent color percepts.

The opponent color theory was first proposed by Hering (12) in 1872. At the time, this theory rivaled the well-accepted trichromatic theory, which explains the trichromasy of vision and predicts color matches. Hering's opponent color theory suggests that there are three channels, red-green, blue-yellow, and black-white, with each responding in an antagonist way. That is, either red or green is perceived and never greenish-red. Hering, however, never challenged the initial stages of processing expressed by the trichromatic theory. He simply argued that any color vision theory should explain our perception, that is, color opponency as revealed by colored after images.

Hurvich and Jameson (13) provided quantitative data for color opponency. Using hue cancellation paradigms, the psychophysical color opponent channels were isolated. The Vλ function was used for brightness discrimination to describe the perception of blackness and whiteness. Therefore, by adjusting the amount of blue or yellow AND red or green, any sample wavelength can be matched (Fig. 15). Complementary wavelengths can be used to cancel each other for all wavelengths except the four unique hues (blue, green, yellow, and red).

Figure 15. Hurvich and Jameson experiment using blue or yellow AND red or green to match all wavelengths of the visible spectrum (13).

Figure 15

Hurvich and Jameson experiment using blue or yellow AND red or green to match all wavelengths of the visible spectrum (13). From Benjamin (28).

Other evidence supporting the opponent color theory include:

  • Electrical recordings of horizontal cells from fish retina show blue-yellow opponent process and red-green opponent (14).
  • Electrical recordings from the lateral geniculate nucleus show opponent color processes (15).
  • Electrical recordings of ganglion cells from primate retinas show opponent color processes (16-18).

Stage Theory

This has led to the modern model of normal color vision, which incorporates both the trichromatic theory and the opponent color theory into two stages (Fig. 16). The first stage can be considered as the receptor stage, which consists of the three photopigments (blue, green, and red cones). The second is the neural processing stage, where the color opponency occurs. The second stage is at a post-receptoral level and occurs as early as the horizontal cell level.

Figure 16. Model for normal human color vision.

Figure 16

Model for normal human color vision.

Color Vision Deficiencies

Color vision deficiencies (CVDs) can be congenital or acquired. Congenital CVD means that the CVD is present at birth and is inherited, whereas acquired CVD occurs secondary to eye disease. Congenital CVD affects ~8% of males and ~0.5% of females.

CVDs are classified into three groups. These are monochromasy, dichromasy, and anomalous trichromasy. People with normal color vision are called trichromats. Monochromats are typically totally color blind and may have one cone pathway in addition to the rod pathway. Dichromats have a cone photopigment missing; therefore, they only have two cone channels. Anomalous trichromats have all three cone photopigments; however, one cone photopigment is anomalous, having a shifted peak sensitivity The types and prevalence of CVDs are listed in Table 1.

Table 1. Prevalence of congenital color deficiencies.

Table 1

Prevalence of congenital color deficiencies.

Dichromasy and anomalous trichromasy can be classified according to the affected cone photopigment. Three terms that are used also used to describe CVD are protan, deutan, and tritan (from the Greek protos, first; deuteros, second; and tritos, third; the order that the CVDs were described). A protan has the longer wavelength cone photopigment missing or anomalous, a deutan has the middle wavelength cone photopigment missing or anomalous, and the tritan has the shorter wavelength cone photopigment missing or anomalous (Fig. 17).

Figure 17. Classification according to the cone photopigment affected.

Figure 17

Classification according to the cone photopigment affected.

The pattern of inheritance for deutan (red-green) CVD is sex-linked recessive, whereras a tritan CVD has an autosomal dominant inheritance. The genetics of CVDs is particularly important in the clinic because patients are often keen to understand why they are CVD and whether they will pass their CVD to their children. For some time, it was thought that congenital inherited tritanopia did not exist because so few cases had been reported. Also, tritan-like CVDs are associated with disease, making it essential to discriminate between acquired and congenital tritan defects. The existence of congenital inherited tritanopia was originally established by family studies (19-21) and subsequently confirmed by molecular genetics (10, 11).

Using the CIE Diagram to Develop Diagnostic Color Vision Tests

Discrimination of color by dichromats is limited due to one photopigment being absent. Therefore, when it comes to color matching, certain colors are confused with another. Confusion lines are lines joining points on the chromaticity diagram that appear the same in color for dichromats. The number of confusion lines also provide information about the amount of change in wavelength (Δl) before another color is discriminated (when the next confusion line is met). All confusion lines converge to a point called the copuntal point (Fig. 18).

Figure 18. Schematic of two confusion lines showing the copunctal point and Δλ for a protanope.

Figure 18

Schematic of two confusion lines showing the copunctal point and Δλ for a protanope.

Fig. 19 shows the confusion lines for a protanope and a deuteranope. There are 17 confusion lines for a protanope and 27 confusion lines for a deuteranope. Therefore, deuteranopes can discriminate more colors because smaller changes in wavelength (Δl steps) can be discerned. Fig. 20 shows the confusion lines for the tritanope.

Figure 19. Confusion lines for a protanope (a) and a deuteranope (b).

Figure 19

Confusion lines for a protanope (a) and a deuteranope (b). Pitts' data in 1935 from Le Grand (27).

Figure 20. Confusion lines for a tritanope.

Figure 20

Confusion lines for a tritanope. From Benjamin (28).

Color Vision Tests

Confusion lines form the basis of many color vision tests such as the Farnsworth Panel D-15 and Ishihara Pseudoisochromatic plates. Pseudoisochromatic plate tests are also commonly used in the clinic to screen for color vision deficiency. Colors are carefully chosen based on the confusion lines. The most commonly used pseudoisochromatic plate in the clinic would be the Ishihara Isochromatic plates (for screening red-green color vision deficiency) and the Tritan (F-2) plate.

Pseudoisochromatic plates are designed in four ways:

1.

Transformation plates: where a person with normal color vision sees one figure and a CVD person sees another (Fig. 21a).

2.

Vanishing plates: where a person with normal color vision sees the figure whereas a CVD person will not (Fig. 21b).

3.

Hidden-digit plates: where a person with normal color vision does not see a figure whereas a CVD will see the figure (Fig. 22a).

4.

Diagnostic plates: designed to be seen by normal subjects and persons with CVDs see one number more easily than another (Fig. 22b).

Figure 21. a, the transformation plate of the Ishihara.

Figure 21

a, the transformation plate of the Ishihara. Normal should see 3, whereas a CVD person should see 5. b, the vanishing plate of the Ishihara. Normal should see 73, whereas a CVD person will not read the figures correctly.

Figure 22. a, the hidden-digit plate of the Ishihara.

Figure 22

a, the hidden-digit plate of the Ishihara. Normal should not see anything, whereas a CVD person should see 5. b, the diagnostic plate of the Ishihara. Normal should see both the 2 and the 6. Deutan-type CVD should see 2 more easily, whereas a protan-type (more...)

The Ishihara color vision test is a screening test, and the fail criterion for the Ishihara is typically four or more plates. Further color vision testing will be required to confidently diagnose the type of color vision defect. Another useful screening plate test is the Farnsworth F-2 plate (22). There are many color vision tests available for screening and for diagnosis, of which only a few will be discussed here. The colors of the Panel D-15 are also carefully chosen from the CIE diagram so that they are all isoluminant (that is, have the same value), as seen in Fig. 23.

Figure 23. Chromaticity co-ordinates of the colors of the Farnsworth Panel D-15.

Figure 23

Chromaticity co-ordinates of the colors of the Farnsworth Panel D-15. From Benjamin (28).

Patients are asked to arrange 15 colored caps in sequential order based on the similarity from the pilot color cap (Fig. 24). The type of color vision defect can be detected from their arrangement of the caps. These color caps are arranged in a particular fashion because of the confusion of colors that lie on the confusion lines (Fig. 25). The criterion for failure in the Panel D-15 test is two or more major crossings (i.e., greater than a two-cap error). Deutan, protan, and tritan will produce characteristic errors (crossings) according to their confusion lines. Rod monochromats are color blind, and their Vl peaks at about 507 nm. They arrange the D-15 caps according to the scotopic reflectance of the D-15 caps.

Figure 24. Farnsworth Panel D-15.

Figure 24

Farnsworth Panel D-15.

Figure 25. The Farnsworth Panel D-15 results from patients with various CVDs.

Figure 25

The Farnsworth Panel D-15 results from patients with various CVDs. The rod monochromatic results are idealised to illustrate the scotopic axis along 5-14. As a rule, rod monochromats give variable results, with a tendency of crossing errors to fall along (more...)

Other arrangement color tests include the L'Anthony's Desaturated Panel D-15, saturated H-16, and the Farnsworth 100-Hue Test. The desaturated panel D-15 is particularly useful in the early diagnosis of acquired diseases and mild congenital deficiencies. The sequence of test administration includes screening for CVDs with a pseudo-isochromatic plate test such as the Ishihara and F-2 plate. Failure of either of these tests implies proceeding to the panel D-15. A patient who fails the Panel D-15 is said to be a moderate to severe anomalous dichromat, or a dichromat. A patient who then fails the H-16 is a dichromat or very severe anomalous trichromat, whereas a moderate anomalous trichromat will pass the H-16. A subject who passes the D-15 can proceed to the desaturated D-15, which can pick up the mild anomalous trichomats. The very mild anomalous trichromats who may or may not fail pseudoisochromatic plates can be diagnosed with the Nagel anomaloscope.

The Farnsworth 100-Hue is another arrangement test (Fig. 26). Unlike the tests mentioned above where the colors are specifically chosen to lie close to the confusion lines, the Farnsworth100-Hue is a discrimination test. Fig. 27 shows where the colors of the 100-Hue lie on the chromaticity diagram. The colors are chosen to have the same Munsell value and chroma. Originally, there were 100 hues, but Farnsworth removed 15 to make the series more uniform. Performance on the Farnsworth 100-Hue is rated by calculating the total error score.

Figure 26. Farnsworth 100-Hue.

Figure 26

Farnsworth 100-Hue.

Figure 27. Colors of the 100-Hue on the chromaticity diagram.

Figure 27

Colors of the 100-Hue on the chromaticity diagram. Point C represents testing conditions using standard illuminant C and point W is the equal energy white. From Hart (32).

Lantern tests have been used since the19th century as a means of assessing color vision, especially for occupational reasons. Lantern tests simulate colored signal lights. They usually present pairs of red, white, and green lights, because these are the signal colors used at sea and in air navigation, and the subject is required to name the colors. There are a great number of different lantern tests that vary quite widely in the level of difficulty they present. The level of difficulty depends on the size of the stimulus and its intensity (see Cole and Vingrys (23, 24) for a review). The lanterns in current use are the Farnsworth lantern, now superseded by the Optec 900, the Holmes Wright Type A and B lanterns, and the Beyne lantern (Fig. 28).

Figure 28. The different lanterns used in occupational testing of color vision.

Figure 28

The different lanterns used in occupational testing of color vision.

Lantern tests are usually failed by dichromats and anomalous trichromats whose defect is severe enough to cause them to fail the Farnsworth D15 test, but the ability to recognize small colored signal lights can vary quite widely among those with mild anomalous trichromasy. A pass at the D15 test or a small range at the anomaloscope does not necessarily mean that signal light colors can be recognized (25).

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1

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Michael Kalloniatis was born in Athens Greece in 1958. He received his optometry degree and Master's degree from the University of Melbourne. His PhD was awarded from the University of Houston, College of Optometry, for studies investigating colour vision processing in the monkey visual system. Post-doctoral training continued at the University of Texas in Houston with Dr Robert Marc. It was during this period that he developed a keen interest in retinal neurochemistry, but he also maintains an active research laboratory in visual psychophysics focussing on colour vision and visual adaptation. He was a faculty member of the Department of Optometry and Vision Sciences at the University of Melbourne until his recent move to New Zealand. Dr. Kalloniatis is now the Robert G. Leitl Professor of Optometry, Department of Optometry and Vision Science, University of Auckland. e-mail: ua.ude.wsnu@sitainollak.m

2

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Charles Luu was born in Can Tho, Vietnam in 1974. He was educated in Melbourne and received his optometry degree from the University of Melbourne in 1996 and proceeded to undertake a clinical residency within the Victorian College of Optometry. During this period, he completed post-graduate training and was awarded the post-graduate diploma in clinical optometry. His areas of expertise include low vision and contact lenses. During his tenure as a staff optometrist, he undertook teaching of optometry students as well as putting together the "Cyclopean Eye", in collaboration with Dr Michael Kalloniatis. The Cyclopean Eye is a Web based interactive unit used in undergraduate teaching of vision science to optometry students. He is currently in private optometric practice as well as a visiting clinician within the Department of Optometry and Vision Science, University of Melbourne.

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