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Monitors can show only RGB models, which has different values on different settings of the monitor. CEIL LAB was designed to be device independent, however I have a confusion that essentially the monitor is going to convert a given LAB value to RGB, then how can we say CEIL LAB is device independent ?

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No. CIELAB images are just as subject to different displays as RGB images. As you note CIELAB is converted to RGB in order to be displayed since displays do not use CIELAB. However, if displays are calibrated to the same white point and profiled then they will display images consistently so long as the images are within the display's gamut.

And CIELAB is not device independent any more than Adobe RGB (1998) or sRGB is. In fact it's more dependent. Technically the two RGB colorspaces require white to be D65 while ProPhoto RGB specifies D50, even though in practice people often set their monitor's white point to other values.

CIELAB, on the other hand doesn't have a specified white point. So a LAB value is technically meaningless unless accompanied by a white point (in CIE XYZ). So it is simply scaled to whatever the display is set to.

For purposes of printing, the ICC specifies that LAB is referenced to D50 white point and further restricts the a* and b* range to +- 127/128.

Even though CIELAB has a much larger gamut than most RGB colorspaces there are surprises. For instance Adobe RGB (0,255,0) is slightly outside this ICC restricted LAB gamut. It has an a* of -129.1 and is clipped to -128.

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  • Saying "Technically the two RGB color spaces require white to be D65 while ProPhoto RGB specifies D50... " without also revealing that all three require the ambient light to match the target white points can be a bit misleading. If one's ambient lighting is closer to D50 than D65, then one should set the target white point to D50. Setting white point to D65 in a warmer ambient light setting will make on screen neutral colors appear blueish to the viewer. – Michael C Feb 26 at 22:31
  • @MichaelC Point taken but it's more complicated still. There is a psychological component that strongly influences visual perception. For instance if one is operating in a home environment with ambient at 2800K or office environment at 4000K CCT and one sets the monitor white point at 4000K it will look very yellowish while papers scattered on the desk will look white. While I prefer a display white point of D50, most people find it rather yellow and that's in my office which runs with pretty high quality 4000K lighting. There's some fun experiments exploring this. – doug Feb 27 at 4:03
  • @MichaelC Ambient chromaticity coords were not specified in the original sRGB. However a more current version specifies it at D50 though the display is D65. Adobe RGB does specify the ambient chromaticity as D65 though in my experience very few that use it actually run ambient at anywhere near D65. color.org/chardata/rgb/oprgb.xalter – doug Feb 27 at 4:26
  • If you're looking at a D65 screen in D50 lighting correctly color managed prints will not match your screen. The ambient lighting in the room where I edit measures 5600K. I have the monitor set to 5500K. When I get prints back from a national printing house they match the colors on my monitor. Most paper on my desk looks white, though papers with certain coating have a slight blue tint to them. Those same papers measure slightly blue when photographed next to a proper gray card and the image is balanced for the card to be neutral. – Michael C Feb 27 at 5:32
  • @MichaelC Apples and oranges. Viewing prints should be done with the same chromaticity lighting as the monitor and with a far higher lux level than is recommended for ambient lighting (well under 100 lux for all standard RGB spaces which is insufficient for prints) when using a monitor. I illuminate prints with 500 lux and close to D50 and the monitor at 160 cd/m^2 also at D50. Room ambient is about 40 lux. I don't compare prints to the monitor with room ambient. As for the blue paper. Soft proofing should match any blue tint. – doug Feb 27 at 6:25

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