Knowledge application of chromatics

The way human eyes receive colors: additive color mixing

The colors people see, such as apple red, are actually colors that appear under certain conditions. These conditions can be mainly summarized into three items, namely, light, object reflection and eyes. Light and color coexist. Without light, there is no color. It can be said that color is the perception that objects reflect light into people's eyes. A long time ago, scientists have found that the color intensity change of light can be described by data, which is called wavelength. The wavelength of light that people can see is between 380 and 780 nanometers. As the wavelength changes from short to long, the color changes from purple to red. Different wavelengths reflect different light intensities. Therefore, by measuring the wavelength distribution reflected by an object, we can determine what color it is. For example, if an object has more reflections in the wavelength range of 700 to 760, it tends to be red, and if it has more reflections in the wavelength range of 500 to 700, it tends to be green. By measuring the amount of light reflected by objects, scientists can accurately infer whether the colors of two objects are the same. The method of measuring light reflection is very accurate, but it is not easy to use, because the eyes do not recognize colors by wavelength. There are two kinds of cells in the retina of human eyes. Rod cells are spinal cells, which respond to light and form color perception. Rod cell is a highly sensitive receiving system, which can distinguish tiny brightness differences and help people identify the level of objects, but it can't distinguish colors. Vertebrate cells are not sensitive, but they have the ability to distinguish colors. Therefore, in the case of weak brightness, the object looks gray, because at this time, the vertebral cells can no longer function, and only the rod cells are working.

Vertebral cells have different reactions to light. When a beam of light shines on the retina of the eye, the maximum sensitivity of spinal cells is located in three regions of red, green and blue wavelengths. In other words, the eyes can perceive any color only by combining three colors with different intensities and proportions, so red, green and blue can be said to be the three primary colors of the human eye. Using the superposition of three primary colors, people can basically simulate all kinds of colors that appear in nature, which is the famous principle of optical three primary colors. The colors produced in this way are also called additive mixing. Screen imaging and photography are the specific applications of this color mixing method.

Print four colors: subtractive color.

The coloring principle of printing is different from additive color mixing. Printing is to distribute transparent ink on paper according to certain rules, and use some tiny dots to present colors. The part with more dots is thicker, and the part with less dots is lighter. The choice of transparent ink is not arbitrary, but depends on the amount of red, green and blue light that can be absorbed most. Therefore, magenta, cyan and yellow become the three primary colors of printing. The reason is that magenta absorbs most of the green, cyan absorbs most of the red and yellow absorbs most of the blue. The combination of magenta and green, cyan and red, yellow and blue is called complementary color relationship, or complementary color relationship. If the dots printed on the paper don't touch other dots, the colors they see are the three printed colors. If two primary colors overlap, such as cyan and yellow, people will see green, because yellow ink absorbs the blue in the light, cyan absorbs the red in the light, and only the green in the light is reflected to their eyes. If all three colors overlap, people will see black because all the light is absorbed. Printing is to use this method of reducing color and light to produce thousands of colors, so it is also called subtractive coloring. Ink-jet printing, thermal sublimation printing and watercolor painting are all concrete applications of this principle. Theoretically, the same amount of magenta, cyan and yellow can be printed together to produce gray-black, but due to imperfect ink production, the purity of cyan ink is not as good as magenta, so the gray produced is always reddish. In order to make up for the shortage of ink technology, black ink is introduced to enhance the gray effect, so that printed matter can show a better sense of hierarchy, which is why people use four colors in printing now. On this basis, some people even completely replaced the same magenta, cyan and yellow ink with black ink. This technology is called achromatic structure (GCR) in color separation, and the early FreeHand software used this technology to convert RGB images into CMYK. Replacing unsatisfactory colors with spot color inks can be applied to other colors and gray. Pantone's HexChome started from this direction, adding special green and special orange in addition to the traditional four colors to strengthen the unsatisfactory part of green and orange in printing.