'LAB' Colour Space
The 'Lab' colour space is one of the least used image formats as no print production, picture library, or website would ever consider using it. And yet, ironically, everyone does. 'Lab' covers all potential colours, every colour that the human eye is able to see. As such it is an absolute and therefore is not colour managed as there are no degrees of 'Lab' for a colour profile to describe. Because of this Photoshop uses it as its native colour space. So every time an image is converted from RGB to CMYK, or back again, it is passed through 'Lab' in the process.
FIGURE 1 An RGB image converted to 'Lab' will look exactly the same with no loss of colour
Converting an image to 'Lab' is the easiest conversion to do. No matter what colour space you start with it will always fit within the extreme space of 'Lab'. There are no settings that the conversion depends on, as with RGB and CMYK, and there are no decisions to be made. You just convert by selecting Image > Mode > Lab Color from the menu bar.
FIGURE 2 When the individual channels are viewed the 'Lab' format looks very different
NOTE: The channels have been shown in colour in the figures above and below. This is just for illustration purposes to show which colours the 'a' and 'b' represent. Normally you would view these as Greyscales when using Photoshop, just as you would when viewing the individual channels in an RGB image.
FIGURE 3 The 'A' and 'B' channels are coloured here for illustration purposes
When any image is converted to 'Lab' it will look exactly the same. Because the 'Lab' contains all possible colours there can be no deterioration in the image quality as all colours translate unaltered. But when the separate 'Lab' channels are looked at they can seem a little odd at first. An RGB image is easily understood as there are three logical colours. But 'Lab' has a mix of one channel with no colour (L), plus two channels with a dual colour combination that have no contrast (a+b).
The 'L' channel, or Lightness, is the easiest to understand as it is a Greyscale. It has no colour value at all; it just contains the contrast between the lightest and darkest points in the image. On the other hand the 'a' and 'b' channels need a little explanation. The 'a' is the colour balance between Green and Magenta, and the 'b' is the colour balance between Blue and Yellow. In the example above the red shirt is more Magenta than Green and the grass is more Green than Magenta. Similarly, both the red shirt and the grass are more Yellow than Blue but the sky is more Blue than Yellow. Wherever the channel is mid grey there is a balance between the dual colours.
NOTE: While the 'L' stands for Lightness, the 'a' and 'b' are arbitrary names; they could just as easily have been labelled 'α' and 'β', or '1' and '2' instead.