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2024-12-11
The CIELab and CIELCh modes of colorimeter are two commonly used colorimetric models in colorimeter. The CIELab model is the most commonly used color measurement model of colorimeter, while the CIELCh mode is a model derived from CIELab. There is a certain relationship between the two. This article introduces the meaning of the color difference values of the CIELab and CIELCh modes of colorimeter.
CIELab Mode
CIELab mode is a uniform color mode announced by the International Commission on Illumination (CIE) in 1976. It is also written as L*a*b* to distinguish it from the Lab mode established in 1931. CIELab mode is also divided into three channels, namely a lightness channel (L* channel) that expresses light and dark and two channels that express color (a* and b* channels). The L* value represents the light and dark channel, with a value range of 0 to 100 (0 is the darkest and 100 is the brightest); the a* value represents the red and green channels, with a value range of -128 to 127 (-128 is dark green, 0 is gray, and 127 is bright pink); the b* value represents the yellow and blue channels, with a value range of -128 to 127 (-128 is dark blue, 0 is gray, and 127 is yellow). CIELab mode is a mode that does not rely on equipment and separates brightness and color information. The color gamut it expresses is larger than the color gamut of the display or even human vision. This mode is currently the most widely used color mode in the field of scientific research. Its main advantages are the largest color gamut and separated brightness, which can make color expression unaffected by light.
The CIELab color difference measurement method refers to the quantitative measurement of sample color using L, a, and b values. L represents the lightness value; a represents the red/green value; and b represents the yellow/blue value. The figure below is a CIELab three-dimensional color space, in which the positions and (L, a, b) coordinates of all colors can be seen.
The CIELab color difference formula is centered on the standard and measures the deviations of L, a, and b values to find out the causes of color difference.
△L=L (actual measurement)-L (standard plate): indicates the brightness difference. When the △L deviation is positive, the measured brightness is lighter; when the △L deviation is negative, the measured brightness is darker.
△a=a (actual measurement)-b (standard plate): indicates the difference between red and green. When the △a deviation is positive, the actual measurement is redder; when the △a deviation is negative, the actual measurement is greener.
△b=b (actual measurement) -b (standard plate): indicates the difference between yellow and blue. When the △b deviation is positive, the actual measurement is yellowish; when the △b deviation is negative, the actual measurement is blue.
CIELab color difference formula: △E=[（△L）2+（△a）2+（△b）2]1/2, △ represents the difference, △E is the total color difference value.
From the CIELab color difference formula, we can see that its measurement method is determined by three values: lightness, red-green hue, and yellow-blue hue.
CIELCh mode
The LCh mode (written as LCH in some literature) is an expression derived from the CIELab mode, so it can be converted to and from the CIELab mode. L represents the lightness value (Lightness), which is the same as L* in the CIELab mode, and the value range is 0 to 100 (0 is the darkest and 100 is the brightest). C represents the saturation value (Chroma), and the value range is 0% to 100% (0% is the darkest and 100% is the brightest). h represents the hue (Hue), also known as the hue angle value, with a value range of 0° to 360°, indicating the spectral position information. Red, green, and blue are 120° apart on the color wheel (red 0, green 120°, blue 240°). Its expression is more in line with practical applications in life, and the content of expression is richer, so it is selected as a color screening indicator.
The CIELCh color difference measurement method refers to the quantitative measurement of sample color using L, C, and H values. L (Lightness) represents the lightness value; C (Chroma) represents the chroma value; and H (Hue) represents the hue value. The lightness value is the same as the CIELab measurement method. The relationship between C, H and a, b values is: C=(a2+b2)1/2; H=arctan(b/a). See the figure below.
LCh color space
CIELCh takes the standard as the center and measures the deviations of L, a, b, C, and H respectively to find out the causes of color difference.
△L=L (measured)-L (standard): indicates the brightness difference. When the △L deviation is positive, the measured brightness is lighter; when the △L deviation is negative, the measured brightness is darker.
△C = C (measured) - C (standard): indicates the saturation difference. If the △C deviation is positive, the measured color is more fresh; if the △C deviation is negative, the measured color is more earthy.
△H=[（△Eab）2-（△L）2-（△C）]1/2 represents the hue difference. If the △H deviation is positive, the measured hue is counterclockwise; if the △H deviation is negative, the measured hue is clockwise.
From the CIELCh color difference formula, we can see that its measurement is determined by three values: lightness, chroma, and hue. The formula includes three factors: lightness, chroma, and hue. When the chroma of the color C>10 or L is around 27, and C>18, we think that the color is bright and generally use the CIELCh color difference formula.
