LAB colour, also written as Lab* or CIELAB, is a device independent colour space developed by the International Commission on Illumination (CIE) that is designed to describe colour in a way that is consistent and uniform regardless of the specific device used to capture, display, or output an image. Unlike device dependent colour spaces such as RGB or CMYK - whose actual colour output varies depending on the specific characteristics of the monitor, printer, scanner, or camera being used - LAB colour is based on a mathematical model of human colour perception, making it a reliable and stable reference for colour management across different devices and workflows.
The LAB colour space consists of three separate components or channels. The L channel represents luminance - the lightness or brightness of a colour - on a scale from 0, which represents absolute black, to 100, which represents absolute white. This separation of luminance from colour information is one of the most powerful and distinctive characteristics of the LAB colour space, as it allows adjustments to be made independently to the tonal values and colour content of an image without the two influencing each other, as they inevitably do in RGB based editing.
The remaining two channels, A and B, carry the chromatic colour information of the image. The A channel describes colours along a axis running from green at the negative end to red and magenta at the positive end, while the B channel describes colours along an axis running from blue at the negative end to yellow at the positive end. Together, the A and B channels can represent the full gamut of colours visible to the human eye, and the combination of all three channels - L, A, and B - can describe any perceivable colour with precision and consistency.
One of the most significant practical advantages of the LAB colour space is that its gamut - the range of colours it can represent - is considerably larger than that of either RGB or CMYK, encompassing all colours that can be captured, displayed, or printed by any known device. This makes LAB an ideal intermediate colour space for colour management systems, which use it as a universal reference when converting colours between different device dependent colour spaces, ensuring that colours are translated as accurately as possible from one device to another.
In professional image editing software such as Adobe Photoshop, working in LAB colour mode offers photographers and retouchers a number of powerful creative and technical advantages. Sharpening applied to the L channel alone enhances detail and edge contrast without introducing colour noise or artefacts. Colour corrections and adjustments made to the A and B channels can alter hues dramatically and selectively without affecting the luminosity of the image. LAB is also widely used for certain specialised retouching techniques, luminosity based selections, and converting colour images to black and white with precise tonal control.