What is the reason why channel A is copied to channel B in PS lab mode?

Lab mode is a color mode published by CIE in 1976.

RGB mode is the additive color mode of luminescent screen, and CMYK mode is the subtractive color mode of reflective color printing. Lab mode does not depend on light or pigment, and it is a color mode determined by CIE organization, which theoretically contains all colors that can be seen by human eyes. Lab mode makes up for the shortcomings of RGB and CMYK color modes.

The laboratory mode consists of three channels, but does not include R, G and B channels. One channel is brightness, that is, L, and the other two channels are color channels, denoted by A and B. Channel A includes colors from dark green (bottom brightness value) to gray (middle brightness value) to bright pink (high brightness value); Channel B is from bright blue (bottom brightness value) to gray (middle brightness value) and then to yellow (high brightness value). So this mixture of colors will produce bright colors.

Lab mode has the most colors defined, which is independent of light and equipment. Its processing speed is as fast as RGB mode and much faster than CMYK mode. Therefore, we can safely and boldly use Lab mode in image editing. Moreover, the color of Lab mode will not be lost or replaced when converting to CMYK mode. Therefore, the best way to avoid color loss is to edit the image in Lab mode and then convert it into CMYK mode for printing.

When you convert RGB mode to CMYK mode, Photoshop will automatically convert RGB mode to Lab mode and then to CMYK mode.

In the range of color expression, the first is Lab mode, the second is RGB mode, and the third is CMYK mode.

It should be noted that many functions of LAB mode cannot be used in PHOTOSHOP.

The advantage of Lab mode is that it makes up for the deficiency of the first two color modes. RGB has too many transition colors between blue and green, and too few transition colors between green and red. CMYK mode loses more colors in the process of editing and processing pictures, while Lab mode compensates in these aspects.

Lab mode is similar to RGB mode, and the mixture of colors will produce brighter colors. Only the value of the brightness channel will affect the brightness change of the color. Lab mode can be regarded as two channels of RGB mode plus one brightness channel mode.

The laboratory mode is independent of the equipment. You can edit and process any picture (including grayscale pictures) in this mode, and it is as fast as RGB mode and several times faster than CMYK mode. Lab mode can ensure that the colors in the range of CMYK will not be lost during the color mode conversion.

If the RGB mode picture is converted to CMYK mode, an intermediate step should be added in the operation steps, that is, first convert to Lab mode. In the typesetting process of achromatic newspapers, Lab mode is often used to convert pictures into gray images.