Section 36–1 The sensation of color

 (Land color effect / Benham’s Disk / Opponent-process theory)

 

In this section, Feynman discusses Land color effect, Benham’s Disk (Fechner colors), and opponent-process theory of color vision that are related to subjective sensation of colors. For example, Fechner colors could be defined as the creation of illusory (subjective) colors through a repeated pattern of black and white stimulus. Thus, the section could be specified as “the subjective sensation of color.”

 

1. Land color effect:

“In fact, Land has shown that if we mix that apparent blue and the red in various proportions, by using two photographic transparencies with absorption in front of the red and the white in different proportions, it can be made to represent a real scene, with real objects, rather faithfully. In this case we get a lot of intermediate apparent colors too, analogous to what we would get by mixing red and blue-green; it seems to be an almost complete set of colors, but if we look very hard at them, they are not so very good. Even so, it is surprising how much can be obtained from just red and white (Feynman et al., 1963, p. 36–1).”

 

Edwin Land’s two-color experiment involving white light and red light was a pioneering experiment in color vision. Specifically, Land was using two black-and-white transparencies of the same colored-scene: the first transparency was taken through a red filter and the second transparency was taken through a green filter (See figure below). In other words, one may emphasize the need of illuminating a photograph (long record) with longer wavelengths of light and the photograph (short record) with shorter wavelengths of light (Land, 1959). The experiment demonstrates that, through the mixing of red and white light, observers could perceive a variety of intermediate colors, creating the illusion of a broad color spectrum. Importantly, Land’s experiment could not be explained by classical color theory principles, which need three primary colors to form a wide gamut of colors.

 

Source: Land, 1959

Note: Edwin H. Land was the inventor of instant photography or Polaroid camera. According to some writers, e.g., Erik Calonius, Steve Jobs was inspired by Land. In his biography of Steve Jobs, Isaacson (2011) quotes Jobs as saying that Land was one of his heroes.

 

Edwin Land proposed the retinex theory of color vision, which requires the brain’s interpretation of relative differences in light reflected from surfaces instead of the intensity of light that reaches the eyes. The word “retinex” is a mix of “retina” and “cortex,” meaning that both the eye and the brain are involved in the sensing of color. The theory was also used to explain the phenomenon of color constancy under a wide range of lighting conditions. In actual scenes, the perceived color of each region depends on the colors of its neighboring points and the overall color of the scene. Essentially, Land’s experiment seek to understand whether color is inherent in the physical world or it is a subjective sensation of human eye.

 

2. Benham's Disk:

“Another example is the appearance of “colors” in a black-and-white rotating disc, whose black and white areas are as shown in Fig. 36–1. When the disc is rotated, the variations of light and dark at any one radius are exactly the same; it is only the background that is different for the two kinds of “stripes.” Yet one of the “rings” appears colored with one color and the other with another (Feynman et al., 1963, p. 36–1).”

 

At the end of the previous lecture, Feynman discussed the phenomenon of Fechner colors*. The color effect is also known as the Benham’s Top or Benham's Disk, which involves the sensation of colors when an object with black-and-white pattern is rotated. The sensation of colors depends on several factors, such as the speed of rotation, i.e., the brain interprets colors due to the integration and motion of the black-and-white pattern. For example, if the disk (as shown below) is rotated clockwise, the innermost arcs may form red rings, the inner arcs may form orange rings, the outer arcs may form green rings, and the outermost arcs may form blue rings. If it is rotated anticlockwise, the pattern is reversed, with the innermost arcs form blue rings and the outermost red rings. Brighter light conditions may result in more vivid or saturated colors, while dimmer light may lead to less distinct colors or grayscale appearance.

Source: Nishiyama, 2014.

 

*In the Audio Recordings of the previous lecture, Feynman says something like: “It is comparing what it sees in one region with another not in the conscious way, but already in retinal level and this is demonstrated by this crazy color phenomenon known as the Fechner colors.”

 

Source: The Feynman Lectures Audio Collection: https://www.feynmanlectures.caltech.edu/flptapes.html

 

“No one yet understands the reason for those colors, but it is clear that information is being put together at a very elementary level, in the eye itself, most likely (Feynman et al., 1963, p. 36–1).”

 

The Pattern Induced Flicker Colors (PIFC) model and Yutaka Nishiyama’s Dynamic Interference model, e.g., offer explanations for the phenomenon observed in Benham's disk, but they approach it from different perspectives. Firstly, the PIFC model suggests that the sensation of colors in Benham’s disk arises from interactions between different types of retinal cells and neural circuits. It proposes that the alternating black and white patterns on the disk stimulate different types of retinal cells differently, leading to the sensation of colors. On the other hand, the interference model attributes the perception of colors in Benham’s disk to interference patterns generated within the visual system. It suggests that the black and white patterns on the disk create interference patterns as they move across the retina. Both models offer insights into the complex processes underlying the sensation of colors in Benham’s disk.

 

3. Opponent process theory:

“The fact that there are three pigments does not mean that there must be three kinds of sensations. One of the other theories of color vision has it that there are really opposing color schemes. That is, one of the nerve fibers carries a lot of impulses if there is yellow being seen, and less than usual for blue. Another nerve fiber carries green and red information in the same way, and another, white and black (Feynman et al., 1963, p. 36–2).”

 

Perhaps Feynman could have used the term “opponent process theory of color vision” instead of opposing color schemes. Historically, Ewald Hering, a German physiologist, made significant contributions to this color theory in the late 19th century. Hering suggested that color vision involves three sets of receptors (red-green, blue-yellow, black-white) where these pairs of colors cannot be perceived simultaneously. (For example, a cell was excited by green light would be inhibited by red light, and vice versa.) It was proposed to understand the observation of color afterimages, where staring at one color for an extended period leads to the perception of its complementary color after looking away. It implies that we cannot sense greenish reds or yellowish-blues as colors. In a sense, the theory was based on Hering’s belief that Helmholtz was unable to give a good physiological explanation for color-contrasted phenomena (Finger, 2001). 

 

“In the modern literature all we find on the subject are repeats of the same statement, or of one by a German psychologist, who uses as one of his authorities Leonardo da Vinci, who, of course, we all know was a great artist. He says, ‘Leonardo thought there were five colors’ (Feynman et al., 1963, p. 36–2).”

 

Perhaps Feynman could have identified the German psychologist that says, “Leonardo thought there were five colors.” However, Leonardo da Vinci writes, “White is given by light, without which no color may be seen, yellow by earth, green by water, blue by air and red by fire, and black by darkness which stands above the element of fire, because there is no substance or dimension on which the rays are able to percuss and accordingly to illuminate it (cited in Hoeppe, 2007, p. 59).” In short, da Vinci thought there were six colors: white, yellow (earth), green (water), blue (air), red (fire) and black. However, da Vinci was inconsistent and sometimes did not include black and white as colors (Kuehni & Schwarz, 2008). 

 

Review Questions:

1. How would you explain the Land color effect?

2. How would you explain the Fechner colors?

3. How would you explain the opponent process theory of color vision?

 

The moral of the lesson: Color is not merely a property of light or objects—it is a construction of the mind. These phenomena reveal that our perception of color arises not from passive reception of wavelengths, but from the brain’s active interpretation of contrasts, context, timing, and neural opponency. Whether through Land’s two-color illuminations, Benham’s rotating patterns, or Hering’s opposing channels, we are reminded that what we “see” is a subjective reality—a careful negotiation between the physical world and the perceptual machinery of eye and brain. Thus, in both science and daily experience, we must remain humble: our senses do not simply mirror the world, but is an act of interpretation and meaning-making. 

References:

1. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

2. Finger, S. (2001). Origins of neuroscience: a history of explorations into brain function. Oxford University Press.

3. Hoeppe, G. (2007). Why the sky is blue: discovering the color of life. Princeton, New Jersey: Princeton University Press.

4. Kuehni, R. G., & Schwarz, A. (2008). Color ordered: a survey of color systems from antiquity to the present. New York: Oxford University Press.

5. Land, E. H. (1959). Experiments in color vision. Scientific American, 200(5), 84-99.

6. Isaacson, W. (2011). Steve Jobs. New York: Simon and Schuster.

7. Nishiyama, Y. (2014). The Mathematics of Benham’s Top. International Journal of Pure and Applied Mathematics, 93(3), 399.

35–6 Physiochemistry of color vision

 (Rod visual pigment / Cone visual pigment / Color sensation)

 

In this section, Feynman discusses the rod visual pigment, cone visual pigment, and ends the chapter by demonstrating the subjectivity of color sensation. The two demonstrations are sometimes known as the Land effect and Fechner color effect.

 

1. Rod visual pigment:

“The pigments that can be obtained from a retina consist mainly of a pigment called visual purple... This fact was discovered in 1877 (Feynman et al., 1963, p. 35–9).”

 

Feynman adopts the archaic term visual purple instead of rhodopsin and mentions that the fact was discovered in 1877. Historically, the discovery of rhodopsin are attributed to two German physiologists Franz Christian Boll and Wilhelm Kühne. In Jan 1876, Boll described the color of rods dissected from dark adapted frog retinas as “Sehrot” (Wade, 2008). Kühne coined the term rhodopsin, in which “rhodo” and “opsis” refer to rose and sight respectively. Furthermore, Boll’s “Sehrot” was translated as vision red and Kühne's “Sehpurpur” as vision purple. Instead of saying a retina consists mainly of a pigment called visual purple, one may clarify that the numerical difference between rods and cones in the retina is substantial: about 120 million rod cells and 6 to 7 million cone cells (having different visual pigments). Remarkably, Feynman suggests that this is due to the need of more visual purple to see at low lighting conditions.

 

Note: In section 36–3 The rod cells, Feynman clarifies that “[t]here are layer after layer of plane structures, shown magnified at the right, which contain the substance rhodopsin (visual purple), the dye, or pigment, which produces the effects of vision in the rods. The rhodopsin, which is the pigment, is a big protein which contains a special group called retinene, which can be taken off the protein, and which is, undoubtedly, the main cause of the absorption of light.”

 

“The most remarkable features of this are, first, that it is in the eye of almost every vertebrate animal, and second, that its response curve fits beautifully with the sensitivity of the eye, as seen in Fig. 35–9, in which are plotted on the same scale the absorption of visual purple and the sensitivity of the dark-adapted eye... (Feynman et al., 1963, p. 35–9).”

 

Feynman explains that the response curve fits beautifully with the sensitivity of the eye in Fig. 35–9, but it oversimplifies their relationship. However, most researchers in the field of color vision had produced widely differing sensitivity curves for the mechanisms of the trichromatic theory (Davson, 1962). The dark-adapted sensitivity curve is influenced by the combined responses of rod cells containing rhodopsin, and the relationship is more complex than the beautiful fit as shown in the graph. On the other hand, a remarkable feature of the rhodopsin is composed of a protein called opsin and a light-sensitive molecule known as retinal, which is derived from vitamin A. Ensuring an adequate intake of vitamin A through a balanced diet is crucial for preventing night blindness. George Wald, a New Yorker, was awarded the 1967 Nobel Prize in Physiology or Medicine for his discoveries about chemical and physiological visual processes in the eye (including vitamin A is found in rhodopsin).

 

Note: In the next chapter, Feynman mentions that if we do not eat enough of Vitamin A, we do not get a supply of retinene, and the eye becomes what we call night blind. (see section 36-3)

 

2. Cone visual pigments:

“The light goes right down into the sensitive point, bounces at the bottom and comes back out again, having traversed a considerable amount of the color-vision pigment; also, by looking at the fovea, where there are no rods, one is not confused by visual purple. But the color of the retina has been seen a long time ago: it is a sort of orangey pink; then there are all the blood vessels, and the color of the material at the back, and so on (Feynman et al., 1963, p. 35–9).”

 

Feynman says that the color of the retina is a sort of orangey pink, however, the light goes right down into the sensitive point, and it traverses a considerable amount of the color-vision pigment (or cone visual pigments). Furthermore, the description of the color of the retina as orangey pink oversimplifies the complex composition of the retina, i.e., the retina does not have a uniform color. When we look at the retina using an ophthalmoscope (see figure below), we can see an orange background, also known as the fundus, with a network of arteries (Valberg, 2007). The fundus refers to the interior surface of the eye, including the retina, optic disc, and blood vessels. In the context of an eye examination, it is more appropriate to describe the “color of the fundus” rather than the “color of the retina.” The orange color of the normal fundus is a result of complex interactions among the visual pigments, vascularization, and the optical properties of the eye.

 

Source: Fundus photography - Wikipedia

“How do we know when we are looking at the pigment? Answer: First we take a color-blind person, who has fewer pigments and for whom it is therefore easier to make the analysis. Second, the various pigments, like visual purple, have an intensity change when they are bleached by light; when we shine light on them they change their concentration (Feynman et al., 1963, p. 35–9).”

 

Bleaching occurs in both rod and cone visual pigments, although the process may differ slightly between them. Perhaps Feynman could have clarified that the term bleaching in this context whether it simply means a change in color to white, almost transparent, or other colors with a deeper meaning. In early 1876, Boll noted that the frog retina is paler after light exposure and can become completely colorless in direct sunlight. On the other hand, Kühne established the notion of “visual cycle”, i.e., visual purple in the rods is bleached by light to form visual yellow which is later transformed into visual white (Wade, 2008). Kühne also showed that the rate of bleaching was dependent not only on the intensity of light but also on its wavelength.

 

In his Nobel Lecture, Wald (1968) explains: “We have been in the habit of saying that light bleaches visual pigments. What it does however is to isomerize the chromophore. The end of this process, if it is allowed to go to completion, is a steady-state mixture of isomers of the chromophore, in proportions that depend upon the wavelength of irradiation and the relative quantum efficiencies of the photoreactions.” In short, bleaching refers to the photodecomposition of the pigment molecules triggered by light absorption.

 

3. Color sensation:

“Color is not a question of the physics of the light itself. Color is a sensation, and the sensation for different colors is different in different circumstances (Feynman et al., 1963, p. 35–10).”

Colors are a result of the complex interaction between light, objects, and the human visual system. The concept of color can be defined from the perspective of cone cells, visual spectrum, and human observer: (1) Cone cells: Color perception is subjective and varies from person to person, i.e., influenced by individual differences in the sensitivity of the eye's cones; (2) Color is the perceptual result of light incident upon the retina in the visible region of the spectrum, having wavelengths in the region of 400 nm to 700 nm; (3) Color is a product of human perception, and it doesn’t exist in the same way without an observer with the ability to perceive and interpret the visual stimuli. To show that color is a sensation and not simply the effect of physical light, Feynman ends the lecture by demonstrating the Land effect and Fechner color effect.

 

In the Audio Recordings* [57 min: 00 sec] of this lecture, Feynman says something like: “One of the possible explanations is this: that the three different color receptors have different timing of response and so because of the flashings, the red and green and so on information come to the head at different times and so you get different effective colors. But look if in this region here, the eye if it held in one spot would see black, black, black, and white and in this region black, black, black, and white, and so on the same proportion. In other words, in coming to the eye here and here are the same flashings exactly (three parts black and one part white) and nevertheless the colors are different. Why? The only difference in the two cases is the characteristics of the background and neighborhood of the bar and therefore there is an integration of the information at the background and the bar (not just the bar). It is very important to appreciate that the retina is already thinking about the light. It is comparing what it sees in one region with another not in the conscious way, but already in retinal level and this is demonstrated by this crazy color phenomenon known as the Fechner colors. Thank you very much.” After the lecture, one undergraduate approached Feynman and mentioned that he had read about Land’s theory some time ago, however, the Land effect is elaborated in the beginning of the next lecture.

*The Feynman Lectures Audio Collection: https://www.feynmanlectures.caltech.edu/flptapes.html

 

Review Questions:

1. How would you explain the most remarkable features of rod visual pigments?

2. What is the meaning of bleaching in the context of cone visual pigments?

3. How would you explain the sensation for different colors is different in different circumstances?

 

The moral of the lesson: The absorption curve of visual pigments in the eye aligns well with the sensitivity curve of the dark adapted eye, based on Rushton’s ophthalmoscope method to detect changes in pigment concentration, which provide insights on visual pigments and color perception.

 

References:

1. Davson, H. (Ed.). (2014). The Visual Process: The Eye. Academic Press.

2. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

3. Valberg, A. (2007). Light vision color. Hoboken, NJ: John Wiley & Sons.

4. Wade, N. J. (2008). Visual purple (Sehpurpur). Perception, 37(11), 1617-1620.

5. Wald, G. (1968). Molecular basis of visual excitation. Science, 162(3850), 230-239.

Section 35–5 The mechanism of color vision

(Young-Helmholtz theory / Dichromatic color blindness / Spectral sensitivity curves)

 

In this section, Feynman discusses Young-Helmholtz theory of color vision, dichromatic color blindness, and spectral sensitivity curves of a normal trichromat’s receptors. In a sense, the title of the section “the mechanism of color vision” may imply the interaction of light, photoreceptor cells in the retina, three types of cone cells, and complex processing in the eye-brain system. However, the trichromatic theory and the opponent-process theory (instead of Young-Helmholtz theory) help explain how the eye-brain system perceives and interprets a wide spectrum of colors and color blindness. Alternatively, the section could be titled as “Three types of cone visual pigments” that are closely related to dichromatic color blindness and spectral sensitivity curves.

 

1. Young-Helmholtz theory:

“The simplest theory, proposed by Young and Helmholtz, supposes that in the eye there are three different pigments which receive the light and that these have different absorption spectra, so that one pigment absorbs strongly, say, in the red, another absorbs strongly in the blue, another absorbs in the green (Feynman et al., 1963, p. 35–7).”

 

Historically, Young and Helmholtz did not propose that the three different cone-pigments are primarily sensitive to red, green, and blue. In 1802, Young initially thought the eye required receptors that were sensitive to three principal colors (red, yellow, and blue). In “Chromatics” (an entry in Encyclopaedia Britannica), Young (1817) proposed that the three primary colors are red, green, and violet. Building on Young’s theory, Helmholtz classified the cone photoreceptors as short (violet), middle (green), and long (red). In Handbuch der Physiologischen Optik, Helmholtz (1866) writes, “In the eye there are three types of nerve fibers. Stimulation of the first one excites the sensation of red, stimulation of the second the sensation of green, stimulation of the third the sensation of violet (Valberg, 2007, p. 278).”

 

“Now if we adjust the brightness or the intensity of one color against the other, there comes an intensity where the flicker at 16 cycles disappears… It is possible to match two colors for “equal brightness” by this flicker technique. The results are almost, but not exactly, the same as those obtained by measuring the threshold sensitivity of the eye for seeing weak light by the cones. Most workers use the flicker system as a definition of the brightness curve (Feynman et al., 1963, p. 35–8).”

 

We may use the term, flicker fusion, which refers to the phenomenon where the eye perceives a continuous image (or still image) when presented with a rapid succession of discrete images (or flickering image), typically above a certain frequency threshold. Feynman suggests that we can adjust the brightness of one color against the other such that the flicker disappears at 16 Hz, but the eye may perceive visual flicker artifacts at rates over 500 Hz when the image includes high frequency spatial edges (Davis, Hsieh, & Lee, 2015).  However, some opin that the frame rate of computer displays should be 72 Hz to avoid flicker completely (Barten, 1999). (Standard-definition television may operate at 25 or 30 frames per second, or sometimes at 50 or 60 half-frames per second.) In short, flicker fusion could be related to the Talbot-Plateau law, which describes the conditions under which the perceived brightness of a flickering image will appear to be equal to the brightness of a still image.

 

2. Dichromatic color blindness:

“By measuring all these types we can determine the three curves! It turns out that there are three types of dichromatic color blindness; there are two common types and a third very rare type, and from these three it has been possible to deduce the pigment absorption spectra (Feynman et al., 1963, p. 35–8).”

 

Feynman explains that there are three types of dichromatic color blindness. However, color blindness can be categorized as monochromatism, dichromatism, and anomalous trichromatism. Firstly, monochromatism (total color blindness) refers to the condition characterized by the total inability to perceive color. Secondly, dichromacy includes protanopia, deuteranopia, and tritanopia, where one type of cone is non-functional, leading to difficulties in perceiving or distinguishing certain colors. Lastly, anomalous trichromatism refers to conditions where there is an abnormality in two types of cones, often leading to a variation in color perception but not complete color blindness. In addition, there are variations within the sub-categories, such as protanomaly, deuteranomaly, or tritanomaly, which refer to a reduced sensitivity of the cone cells instead of a complete absence (see figure below).

Source: What Is Color Blindness? Condition and Types Explained (verywellhealth.com)

 

“Figure 35–6 shows the color mixing of a particular type of color-blind person called a deuteranope. For him, the loci of constant colors are not points, but certain lines, along each of which the color appears to him to be the same. If the theory that he is missing one of the three pieces of information is right, all these lines should intersect at a point (Feynman et al., 1963, p. 35–8).”

 

Feynman says that the loci of constant colors for a color-blind person are not points, but certain lines along each of which the color appears to him to be the same and all these lines should intersect at a point. Specifically, these lines of confusion* cannot be distinguished (or confused) by the protanope or deuteranope, are also known as pseudo-isochromatic lines. One may clarify that these apply not only to the colors on the confusion lines, but all the colors between any two closest lines, especially under certain lighting conditions. Furthermore, we may adopt the term copunctal point, which refers to the convergence point of these confusion lines outside the chromaticity diagram. This is a theoretical reference point where all the confusion lines meet or intersect.

Source: Color blindness - Wikipedia

 

In the Audio Recordings* [46 min: 00 sec] of this lecture, Feynman says: “lines of confusion” instead of “loci of constant colors.”

*The Feynman Lectures Audio Collection: https://www.feynmanlectures.caltech.edu/flptapes.html

 

“If we carefully measure on this graph, they do intersect perfectly. Obviously, therefore, this has been made by a mathematician and does not represent real data! (Feynman et al., 1963, p. 35–8).”

 

The co-punctal-point of the CIE diagram could be attributed to James Clerk Maxwell. In a letter dated Jan. 4, 1855 to G. Wilson, J. C. Maxwell writes, “If we find two combinations of colors which appear identical to a color-blind person, and mark their positions on the triangle of colors, then the straight line passing through these points will pass through all points corresponding to other colors, which, to such a person, appear identical with the first two. We may in the same way find other lines passing through the series of colors which appear alike to the color-blind. All these lines either pass through one point or are parallel, according to the standard colors which we have assumed, and the other arbitrary assumptions we may have made. Knowing this law of color-blind vision, we may predict any number of equations which will be true for eyes having this defect.” Maxwell was a Scottish physicist, but he was also known as a mathematician.

 

3. Spectral sensitivity curves:

“Yustova gets approximately the same position in this case. Using the three different kinds of color blindness, the three pigment response curves have finally been determined, and are shown in Fig. 35–8  (Feynman et al., 1963, p. 35–9).”

 

It could be confusing to some that Feynman mentions pigment response curves, but the caption of Fig. 35–8 is “The spectral sensitivity curves of a normal trichromat’s receptors.” However, the spectral sensitivity curves are essentially a representation of how the human visual system responds to light across the spectrum, and they are due to the responses of color-sensitive pigments in the cones of the retina. Interestingly, Feynman explains that the spectral sensitivity curves were obtained using an ophthalmoscope in the next section. This experiment relied on the subjective judgment of the observer to interpret the reflection of light from the retina and determine the spectral sensitivity. Thus, Feynman adds that “[e]ven today it can be said that the color pigments of the cones have never been obtained in a test tube (Feynman et al., 1963, Section 35–6).”

Spectral sensitivity curves represent the response of the eye's different types of cones to varying wavelengths of light. There are at least three aspects that make them somewhat arbitrary: (1) Individual differences: Spectral sensitivity curves are based on averages derived from studying groups of individuals with normal color vision. (2) Experimental limitations: Color matching experiments or testing with different monochromatic lights have their constraints, and the accuracy of the measurements might be influenced by the experimental setup used. (3) Environmental factors: Factors such as lighting conditions, adaptation to different light levels, and background colors can influence cone responses. In essence, the spectral sensitivity curves are not entirely arbitrary but are based on measurements obtained through the experiments.

 

Review Questions:

1. Did Young and Helmholtz propose that the three different cone-pigments are primarily sensitive to red, green, and blue (or violet)?

2. How would you explain the three types of dichromatic color blindness?

3. How would you explain the spectral sensitivity curves or pigment response curves?

 

The moral of the lesson: the three types of cone visual pigments are integral to normal color vision, and their properties help explain both dichromatic color blindness and the spectral sensitivity of the human visual system; variations in these pigments contribute to individual differences in color perception.

 

References:

1. Barten, P. G. (1999). Contrast sensitivity of the human eye and its effects on image quality. Bellingham: SPIE press.

2. Davis, J., Hsieh, Y. H., & Lee, H. C. (2015). Humans perceive flicker artifacts at 500 Hz. Scientific reports, 5(1), 7861.

3. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

4. Maxwell, J. C. (1855). On the theory of colours in relation to colour-blindness, Letter of Jan. 4, 1855 to G. Wilson. Researches on Colour-Blindness. Edinburgh: Sutherland-Knox.

5. Valberg, A. (2007). Light vision color. Hoboken, NJ: John Wiley & Sons.

6. Young, T. (1817). Chromatics. Supplement to the Encyclopaedia Britannica, 3, 141-63.