The wavelength and intensity of electromagnetic waves can be very different. Within the range of wavelengths that people can feel (about 380 nanometers to 780 nanometers), it is called visible light, sometimes referred to as light. If we list the intensity of each wavelength of a light source together, we can get the spectrum of this light source. The spectrum of an object determines the optical properties of the object, including its color. Different spectra can be received as the same color. Although we can define a color as the sum of all these spectra, the colors seen by different animals are different, and the colors felt by different people are also different, so this definition is quite subjective.
A surface that diffusely reflects all wavelengths of light is white, while a surface that absorbs all wavelengths of light is black.
Each color represented by a rainbow contains only one wavelength of light. We call such colors monochromatic. The spectrum of the rainbow is actually continuous, but people generally divide it into seven colors: red, orange, yellow, green, cyan, blue, and purple, but each person’s division is always slightly different. The intensity of monochromatic light also affects people's perception of the color of light of one wavelength, such as dark orange and yellow are perceived as brown, and dark yellow and green are perceived as olive green, and so on.
There are also many colors that cannot be monochromatic, because there is no such monochromatic color. Black, gray, and white are such colors, as are pink or magenta.
The wave equation is an equation used to describe light, so we should be able to get color information by solving the wave equation. The wave equation of light in vacuum is as follows:
utt = c2(uxx + uyy + uzz)
Here c is the speed of light, x, y, and z are the coordinates of space, t is the coordinates of time, u(x,y,z) is the function of describing light, and the subscript indicates the partial derivative. At a fixed point in space (x, y, and z are fixed), u becomes a function of time. Through Fourier transform we can obtain the amplitude of each wavelength. From this we can get the intensity of this light at each wavelength. In this way we can obtain a spectrum from the wave equation.
But in fact, to describe what color a set of spectra produces, we still need to understand the physiological functions of the retina.
Aristotle had already discussed the relationship between light and color, but it was Isaac Newton who really clarified the relationship between the two. John Wolfgang Goethe also studied the causes of color. Thomas Young first proposed the theory of ternary colors in 1801, and later Hermann von Helmholtz perfected it. In the 1960s, people discovered the pigments that perceive colors in the human eye, which confirmed the validity of this theory.
Both cone-shaped cells and rod-shaped cells in the human eye can perceive color. Generally, there are three different types of cone-shaped cells in the human eye: The most sensitive point is around 535 nanometers; the third type mainly feels blue, and its most sensitive point is around 445 nanometers. There is only one type of rod-shaped cell, and its most sensitive color wavelength is between blue and green.
The sensitivity curve of each cone cell is roughly bell-shaped. Therefore, the light entering the eye is generally divided into 4 signals of different intensities corresponding to the three types of cone cells and rod cells.
Because each cell type also reflects other wavelengths, not all spectra can be distinguished. For example, green light can not only be received by green cone cells, but other cone cells can also produce signals of a certain intensity. The combination of all these signals is the sum of the colors that the human eye can distinguish.
If our eyes look at a color for a long time, we turn our eyes away and see the complementary color of this color in other places. This is called the complementary principle of color. Simply put, when a cell is stimulated by light of a certain color, it will simultaneously release two signals: stimulate yellow, and at the same time simulate yellow complementary color purple.
In fact, the signal generated by the cells on the retina from the light of a certain scene is not 100% equal to how people feel about the scene. The human brain processes these signals and analyzes and compares surrounding signals. For example, a photo of the White House taken with a green filter-the image of the White House is actually green. But because of the human brain’s inherent impression of the White House, coupled with the green hue of the surrounding environment, the human brain will remove the green barriers-many times still feel the White House as white. This is called a phenomenon called "Retinex" in English-a synthesis of two words, retina (retina) and cerebral cortex (cortex). Van Gogh once used this phenomenon to paint.
The human eye can distinguish 10 million colors in total, but this is only an estimate, because the structure of each human eye is different, and the color seen by each person is also slightly different, so the distinction between colors is quite subjective. If one or more cone cells of a person cannot normally reflect the incident light, then the person can distinguish fewer colors. Such a person is called color weak. Sometimes this is also called color blindness, but in fact this term is incorrect, because there are very few people who can only distinguish black and white.
Rod cells. Although rod-shaped cells are generally considered to be able to distinguish black and white, their sensitivity to different colors is slightly different. Therefore, when the light is dimmed, the photosensitive characteristics of rod-shaped cells become more and more important, and it can change us The feeling of color.
From an evolutionary point of view, people should have the same perception of basic colors.
Some animals have fewer types of cells that perceive color than humans, such as most other mammals. Some animals can feel colors that are invisible to humans. For example, bees can feel ultraviolet light.
If we use the x, y, and z axes in Euclidean space to represent the intensity of the most sensitive wavelengths of the three cone cells of humans, then we can obtain a three-dimensional color space. The origin of this space represents black. The further away from the origin, the stronger the light intensity. White does not have a fixed point in this space. According to the difference in color temperature and surrounding light, we may regard different points in this picture as white. The color that people can feel in this picture is a cone shaped like a horse kick at the bottom. In theory, this cone has no stopping point, but too strong light can damage human eyes. In the case of low light intensity, people's perception of color will change, but in general, people are sensitive to the part depicted by the black line on the right.
To be precise, there are no such colors as brown or gray in this picture. These colors are actually orange and white that are darker than the surrounding colors. We can easily prove this: when we look at the image of a projector projected on a white cloth, we will see the black characters projected on the white cloth, but in fact the color of the black characters and the white cloth have not been projected. The colors are the same. After projection, the white cloth around these black characters is illuminated, so we feel that they are darker.
We can also see from the picture that people cannot see pure red, green or blue. This is because our cone cells also react to other colors. When we look at pure blue, our red and green cone cells also produce signals, as if red and green are interspersed with blue.
Different spectrums can produce the same color perception in the human eye. For example, the white light of a fluorescent lamp is composed of several fairly narrow spectral lines, while the sunlight is composed of a continuous spectrum. In terms of light, the human eye cannot distinguish between the two. Only when they are reflected on objects of different colors, we can see that one is the light of a fluorescent lamp and the other is the sunlight.
In most cases, the colors that people can see can be made up of primary colors. Photos, printing, television, etc. use this method to reflect colors.
Even so, the matched colors are often not exactly the same as pure monochromatic colors, especially the colors matched in the middle of the visible spectrum can only be very close to monochromatic light, but it cannot fully achieve its effect. For example, green light (530 nanometers) and blue light (460 nanometers) together can produce cyan light. But this blue light always makes people feel impure. This is because human red cone-shaped cells can also feel green and blue at the same time, and their reflection of the matched color is stronger than that of pure cyan (485 nm), so we will feel that the matched color is a bit "red" ", a bit impure.
In addition, the primary colors generally used in technology are not pure themselves, so in general they cannot completely express pure monochromatic light. However, there is very little true pure monochromatic light in nature, so in general, colors composed of primary colors can reflect the original colors well. The sum of the colors that a technical system can produce is called the color gamut.
Errors can also occur when recording colors through a camera or scanner. Generally, the photosensitive characteristics of the photosensitive elements in these instruments are far from the photosensitive characteristics of the human eye. Therefore, the color produced by these instruments under special light may be very different from what the human eye feels.
Animals that have different color perception from human eyes (such as birds can perceive four different colors) can distinguish the same color for humans, so images suitable for humans can sometimes be very incomprehensible to them.
Luminous media (such as TV sets) use the triadic colors of red, green, and blue, and each type of light stimulates only the cone-shaped cells directed against them as much as possible and does not stimulate other cone-shaped cells. The color gamut of this system occupies most of the color space that people can feel, so televisions and computer screens use this system.
In theory, we can also use other colors as primary colors, but using red, green and blue we can maximize the human color space. Unfortunately, there is no fixed wavelength definition for red, green, and blue, so different technical instruments may use different wavelengths to produce slightly different colors on the fluorescent screen.
Applying transparent pigments of cyan, magenta and yellow on a white base we can get a larger color gamut. These three colors are subtractive ternary colors. Sometimes we also add black to produce a darker color.
Diffraction, a certain color of light will be reflected to a certain angle. The surface of this object will produce a special rainbow-like flash. Peacock feathers, wings of many butterflies, fritillaria, etc. will produce such structural colors. Recently, some car manufacturers have also used special paints to achieve such a fluorescent effect.
Comments