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2024-12-16
When measuring color, the colorimeter uses a specific color space as the basis, represents the color information with different symbols, and quantifies the color information digitally. CIE Lab and CIE LCh are the most commonly used color spaces in colorimeter, among which CIE LCh is obtained by deforming CIE Lab . So, what is the difference between colorimeter Lab and LCh ? How to convert between Lab and LCh ?
CIE Lab is a uniform color space developed by CIE (International Commission on Illumination), which consists of L (brightness) and three elements of color a and b. As shown in the figure below, L* represents brightness, a* represents the range from magenta to green, and b* represents the range from yellow to blue. The range of L* is 0 to 100, and the range of a* and b* is -100 to +100. a*: dark green → 50% gray (neutral gray) → magenta. In the grayscale diagram of this channel, dark represents green: gray less than 128 (ie 50%) is green, the closer the grayscale value is to 50% gray, the lower the saturation of green, and the farther the grayscale value is less than 50% gray, the higher the saturation of green. The bright end is magenta, and the one greater than 128 (ie 50% gray) is magenta. The brighter it is, the higher the saturation. Conversely, the closer the value is to 128 degrees gray, the lower the saturation is. The b* channel displays from cyan → 50% gray (neutral gray) → yellow. The bright area of the channel grayscale image is the yellow area. The higher the brightness, the higher the saturation, the closer it is to 50% neutral gray, and the lower the saturation. The dark area of the channel grayscale image is the blue area. The darker the display area, the higher the saturation, the closer it is to 50% neutral gray, and the lower the blue saturation.
The CIE LCh color space uses the same color space model as CIE Lab , and Lab is represented by polar coordinates in the LCh color space. As shown in the figure below, L* represents the brightness value of the color, C* represents the saturation value of the color, and H* represents the hue value of the color. The vertical coordinate L* value also corresponds to the lightness of the three elements of color, with a value of 0-100; C* represents chroma, that is, the distance between the coordinate position of a certain color and the origin, with a value of 0-181. The saturation C* value of the color at the center of the circle is 0, and the farther away from the center of the circle, the greater the C* value; H* is the hue, with a value of 0°-360°, and h* represents the hue angle, which is the angle between the projection point of a certain color coordinate in the chromaticity plane and the line connecting the coordinate origin and the a-axis. It is stipulated that h* starts from the positive a-axis (red) as 0°, counterclockwise as positive, the positive b-axis (yellow) as 90°, the negative a-axis (green) as 180°, and the negative b-axis (blue) as 270°. The LCh value is more consistent with people's habit of describing colors, but for color measurement, Lab is generally used to describe colors.
The colorimeter CIE Lab color space and CIE LCh color space are two color spaces commonly used in color measurement. In actual application, these two color spaces have differences in different places. The following article takes the printing industry as an example to introduce the differences between these two color spaces.
1. Professionalism
In the printing industry, CIE Lab color space is usually used to describe colors. In the Lab uniform color space system formulated by CIE, the color distribution is more uniform, and CIE LCh also uses the same color space model of CIE Lab for conversion, so when using the color space CIE Lab for mixing, it is more professional when mixing spot color inks. As for the amount of green ink and blue ink that needs to be adjusted, this requires operators to improve the sensitivity of the human eye to color through a lot of practice, and form their own experience value to reach a professional level.
2. Popularity
Since CIE LCh is converted from the same color space model as CIE Lab , in the understanding of CIE LCh , LCh is consistent with the three attributes of color brightness, saturation, and hue, making it easier to infer color attributes. Adjusting the hue angle clockwise and counterclockwise, and adding the corresponding primary color ink according to the positive and negative values of the brightness saturation difference, is more directional and easier to adjust the formula. Compared with Lab , LCh uses the three attributes of color to make judgments, and the description of color deviation is more understandable, and it is also more convenient to use for spot color inks.
It can be seen that an ink can be mixed using the color space CIE Lab and CIE LCh in spot color ink mixing. The difference between the two is reflected in the different methods and techniques of ink mixing. The ultimate goal is to achieve spot color ink that is suitable for printing.
The CIE Lab color space has no direct correspondence with vision, that is, when a set of Lab values is given, the specific color cannot be imagined vividly. For this reason, we often use the cylindrical color space LCh obtained by deforming the Lab space to represent it. This color space is represented by L, C, and H, where L represents lightness, with a value of 0-100; C represents chroma, with a value of 0-181; H (Hie) represents hue, with a value of 0°-360°. The LCh color space is derived from the CIE Lab color space. A color can be represented by Lab or LCh . Using LCh to correct the image is in line with the habit of describing colors in daily life. Color correction is easier to understand, more intuitive, and easier to control. The conversion relationship between the CIE Lab color space and the CIE LCh color space is as follows:
