For those interested in how LAB works, and also in gamut issues, I’ve posted a downloadable LAB grid for experimentation. It looks, more or less, like this:
The vertical axis, from top to bottom, goes from positive to negative in the A channel, that is, from magenta to teal. The horizontal axis, from left to right, goes from negative to positive in the B, or from blue to yellow. The maximum value is +75 and the minimum -75; the grid is in 15-point increments. The very center point of the grid is therefore 0a0b, a neutral.
When you open the file, the L value is a constant 70L, where 100L is pure lightness and 0L pure darkness. However, a curves adjustment layer allows you to change the L to whatever you like. Interesting things happen when you do.
For example, in the right-hand column of the center row, the current value is 70L0a75b. LAB theory says that this is a brilliant yellow, but as we can see, it’s more of a golden shade.
This happens because there’s no such thing as a yellow this dark. If we lighten the grid to, say, 85L, then the yellowness becomes apparent. But if we do that, we will no longer recognize the upper right corner as being red. In fact, even as it stands the upper right is more of an orange than a red. We’d have to crank up the L channel to say, 50L before we’d see what could be called deep reds.
The other major use of this tool is to evaluate gamut. Inexperienced users sometimes try to force more color into an LAB file than the output device can take. When this happens, usually detail gets lost and the color doesn’t improve.
LAB is supposed to be “perceptually uniform”, which for present purposes means that each square in the grid should be just as distinct against its neighbors as any square elsewhere on the grid would be. As you can see, that certainly isn’t true in the lower left corner, the rich cyans; definitely the squares are more distinct higher up in the grid.
What you are looking at now isn’t an LAB file, because the site can’t display one. Instead, I converted it to sRGB for posting. The lower left calls for colors that are out of the sRGB gamut, so everything kind of got smooshed together. That’s how I know that you can’t see the distinctions: they’re actually missing from the file you’re viewing. Here’s a graphic that illustrates the differences between the sRGB file and what the LAB file calls for. Where the background is light gray, there is no difference. Elsewhere, the bigger the difference, the more intensely it shows up in the graphic, and clearly the biggest differences are in the lower left.
In the downloadable file, though, the distinctions are there. The question is whether you will be able to see them on your screen. The more important question is whether you will be able to see them if you attempt to print the file. And then you may need to consider what happens if you change the grid’s L value, which will cause other squares to become less distinct even if, perhaps, it restores the distinctions in the lower left.
If you like to push the envelope on colors, these questions can become important. All systems of calibration have trouble at the edges of the gamut. Now matter how well calibrated you think your system is, there’s no guarantee that all your devices will agree in these tricky areas. A member of the appliedcolortheory list tried an interesting experiment: he viewed the LAB file on several different monitors, including some that are designed for high-end production, a laptop, and a monitor not ordinarily used to view pictures. To his surprise, he reported that the squares were more distinct on the inferior monitors.
I hope you find the download interesting.