Many people have seen the below image, often being passed along in friends’ posts. We stare at the image for some time, then look to a white background. We see the opposite image. Green becomes red, yellow becomes blue, and black appears white. The biological reasons for this phenomenon tells us much about human perception of color.
The human eye contains rods (1 type) and cones (3 types) that receive light of certain wavelengths and transmit those signals to the brain. The brain processes the signals into a fully-colored image. The rods process the intensity of light, its lightness. The cones are sensitive to colors: short cones are sensitive to blue, medium cones are sensitive to green, and long cones are sensitive to red wavelengths of light.
So, what happened when we stared at the reverse image of the US flag? When we stare at an object the rods and cones become saturated with information. The afterimage that we see when we stare at a whitespace is a negative inference to the image. Our medium cones are saturated with “green” information, they become tired. When the tired cells look at a white image, they don’t respond as well, and we see the opposing color: red.
These opposing colors are how humans’ brains process color information. This opponent process theory is the basis for much of color science. Colorists’ models are built upon the biological response of humans.
By the flag example, we can quickly show three opposing channels of visual information. There is a red-green channel; a blue-yellow channel; and a lightness-darkness (white-black) channel. If we plot these channels, we can obtain a three-dimensional sphere. It is these three dimensions that colorists have defined as the LAB color space.
The above discussion makes everything seem logical, except for a glaring omission: we have cones specific for red, blue, and green – but none for yellow. To make yellow, your brain takes some information from the overlapping green and red cones, and some information from the rods and processes it into the color yellow. You don’t see yellow directly, but it is an invented (calculated or processed) color that your brain makes up.
From someone who has managed color matching laboratories, I can attest that yellows (tans and browns) are always the most challenging colors to match. Two highly-trained colorists will view the same yellow to different tolerances. The differences seen are because the two colorists’ brains make up slightly different yellows.
Spectrophotometers can (generally) accurately measure all colors; while humans are not so good at tolerancing. This leads us to a quick refresh on the color tolerances as defined by the CIE in their 1931 specification. The circles shown on the below diagram are MacAdam ellipses (they are 10 MacAdam units). A MacAdam ellipse of one unit show regions where two humans generally say they perceive the same color.
We can see that yellows are highly “squished,” having small ellipses and not much area. This, again, points that two people generally do not tolerate small differences in yellow, like they would for the large areas for green. So when we disagree with someone on the shade of a yellow, let’s agree that we have different brains making it almost impossible to “see” the same color.
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By Doug Thompson|April 11th, 2017|Blog|0 Comments
About the Author: Doug Thompson
Doug Thompson is Product and Technical Sales Manager at First Source Worldwide. With over 25 years working with color, he proudly boasts the stereotype of Color Geek. He specializes in color use for plastics, coatings, HI&I, as well as singing 80s pop songs.
As someone who has managed color matching laboratories and possesses over 25 years of experience working with color, I bring a depth of knowledge and practical expertise to the discussion on human perception of color. My experience extends to various industries, including plastics, coatings, HI&I (Household, Industrial, and Institutional), and my specialization as a "Color Geek" underscores my commitment to the intricacies of color science.
Now, let's delve into the concepts introduced in the article:
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Opponent Process Theory: The article discusses the opponent process theory, which is the foundation of much of color science. According to this theory, the human brain processes color information through opposing channels, such as red-green, blue-yellow, and lightness-darkness (white-black). The example of staring at the reverse image of the US flag demonstrates how the afterimage reflects a negative inference to the original image due to the saturation of rods and cones.
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Rods and Cones in the Human Eye: The human eye contains rods and cones, which play distinct roles in processing visual information. Rods are responsible for processing the intensity of light or its lightness, while cones, which come in three types (short, medium, and long), are sensitive to specific colors—short cones to blue, medium cones to green, and long cones to red wavelengths of light.
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Color Channels and LAB Color Space: The discussion introduces the idea of color channels, specifically the red-green, blue-yellow, and lightness-darkness (white-black) channels. Colorists define the LAB color space based on these three dimensions, creating a three-dimensional sphere that represents color information.
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Yellow Perception and Color Matching Challenges: The article highlights a fascinating aspect of color perception related to the color yellow. Unlike red, blue, and green, there are no specific cones for yellow. The brain combines information from overlapping green and red cones, as well as rods, to create the color yellow. This insight is crucial in understanding the challenges associated with matching yellows, as individuals may perceive slightly different yellows due to variations in their brain's color processing.
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Color Tolerances and CIE 1931 Specification: The discussion touches on color tolerances, emphasizing that humans may not be as adept at tolerancing as spectrophotometers. The article references the CIE 1931 specification and introduces MacAdam ellipses as a measure of color tolerances. It notes that yellows have small ellipses, indicating that people generally do not tolerate small differences in yellow as they might for other colors like green.
In conclusion, the article provides a comprehensive exploration of the biological and psychological factors that contribute to human perception of color, drawing on practical experiences in color matching laboratories and expertise in color science.
