Color Theory and Color Management (I)

How the human eye receives color: additive color mixing

The colors we see, such as the apple red, are actually colors that appear under certain conditions. These conditions can be mainly summed up in three terms: 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 an object reflects light into our eyes. Scientists have discovered long ago that the change in the color of light can be described by data, which is called wavelength. The wavelength of light we can see ranges from 380 to 780 millimeters. From wavelengths ranging from short to long, the color that appears is from violet to red. The intensity of light reflected by different wavelengths is different. Therefore, by measuring the wavelength distribution reflected by an object, it is possible to determine what color the object is. For example, if an object has more reflections in the wavelength range of 700 to 760, then the The object tends to be red. If there is more reflection in the wavelength range from 500 to 700, the object tends to be green. By measuring the amount of light reflected by an object, scientists can accurately estimate whether the colors of two objects are the same.

The method of measuring the reflectance of light is accurate, but it is not useful because the eye does not recognize color by wavelength. Two kinds of cells are distributed in the omentum of the human eye. The rod cells are vertebral cells. These cells respond to light and form a perception of color. The rod cell is a highly sensitive receiving system that can distinguish the tiny difference in brightness and help us to identify the level of the object, but it cannot distinguish the color. Vertebral cells are less sensitive but have the ability to distinguish colors. Therefore, in the case of very low brightness, the objects appear to be gray and gray, because the vertebral cells can no longer function at this time, only the rod cells are working.

The vertebral cells respond differently to the amount of light. When a beam of light hits the omentum of the eye, the most sensitive values ​​of the vertebral cells are located in the three regions of wavelengths red, green, and blue. That is to say, the eye can only produce a combination of red, green and blue colors with different intensities and proportions to produce a perception of any color. Thus, red, green and blue can be said to be the three primary colors of the human eye. By using additive superposition of three primary colors, we can basically simulate various colors that appear in nature. This is the famous principle of optical trichromatics. Producing color in this way is also called additive color mixing. Screen imaging and photography are specific applications of this color mixing method.

Four-color printing: subtractive coloration

The printing principle of coloration is different from additive color mixing. Printing is based on a few fine dots, and the transparent ink is distributed on the paper according to certain rules to present colors. The dots with more dots are more concentrated, and the dots with less dots are lighter. The choice of clear ink is not arbitrary, but is based on the amount of light that can best absorb green and blue light. Therefore, Mafenta, Cyan, and Yellow become the three primary colors of printing. The reason is that magenta absorbs most of the green, blue absorbs most of the red, and yellow absorbs most of the blue. The combination of magenta and green, cyan and red, and yellow and blue is called a complementary relationship, or a complementary relationship. Dots printed on paper, if not in contact with other outlets, the color to be seen is the printing of three primary colors. If two base dots overlap, such as cyan and yellow, because yellow ink absorbs the blue in the light, blue absorbs the red in the light, and only the green light in the light reflects inside the eye, so we will see the green. If the three color dots all overlap together, we see black because all the light is absorbed. Printing is the use of this method of diminishing color to produce thousands of colors, so it is also called the method of coloring. Inkjet printing, sublimation printing, and watercolor painting are all examples of this application.

Theoretically, the same amount of magenta, cyan, and yellow are printed together to produce grayish black, but because ink production is not perfect, the purity of cyan ink is less than the purity of magenta, so the gray color is always reddish. In order to make up for the insufficiency of the ink process, black ink was introduced to enhance the effect of gray and prints could perform better. This is why we use four colors for printing. On this basis, some people even used black ink to completely replace the place where the same magenta, cyan, and yellow inks appeared. This technique, called color separation (GCR), was an early FreeHand software that converted RGB images to CMYK uses this technology. Replace spot color inks with less than ideal colors, in addition to gray, but also apply to other colors. Pantone's HexChome is starting in this direction, adding green and orange to the traditional four colors, to enhance the green and orange parts of the print less than ideal.

Coordinated screens and methods for printing colors

Although printing can reproduce tens of thousands of colors, due to the use of subtractive coloration, the brightness of the colors is reduced, and some of the brighter colors are difficult to express in print. On the other hand, due to the technique of adding color to the screen, the screen is indeed richer than printing in terms of color expression. This is why the colors that look beautiful on the screen cannot be copied with the print, resulting in differences in the colors of the screen and the print. The solution is either to improve the ink and paper composition so that it can reproduce the fresher, more pure colors, but this is not an overnight event. Another method is to narrow the color gamut of the screen to cater for printing so that what the screen sees is printed.

The so-called color gamut is the maximum range that a device can record or copy colors. The color gamut of the human eye is all visible light. Within the wavelength range of 380 to 780, the color gamut of printing consists of paper and ink. Different paper and ink combinations have different printing color gamuts. The domain is different from book paper, Pantone's color gamut is also different from DIC. Others such as screens, scanners, printers, etc. each have their own color gamut, and it is of practical significance to grasp the color gamut of a device because a device cannot record or copy colors outside the color gamut. For example, under normal circumstances, the human eye can not see the colors in the infrared or X-ray, and some people can easily distinguish the colors, like various metallic colors, but it is not easy to record the book on the scanner. The highest quality we can get is the gamut of one device and the domain of another device. How to make people feel in the simulation process that the color gamut of the two devices is similar is the important theme of color production.