Hi Debra,
You are asking many good question in this forum recently which tells me you are interested in learning some of the finer points of media properties. So, with respect to the LAB color scale, here goes:
Understanding the CIELAB color model and more importantly having a “feel for the numbers” is, IMHO, one of the fundamental pieces of knowledge that serious digital photographers and printmakers should know, yet it’s rarely taught even in major photographic workshops and degree programs at anything more than a cursory level. Part of the reason is that most of the texts on Color theory are geared towards color scientists and engineers rather than photographers, artists, and printmakers. That said, One of the classic text on understanding color principles is :Principles of Color Technology by Billmeyer and Saltsman, eloquently updated by Roy S. Burns. Not for the faint of heart and expensive as well, but also as good as it gets in terms of combining all the math and science along with really useful diagrams and visual examples that illustrate all the key concepts.
http://www.amazon.com/Billmeyer-Saltzmans-Principles-Color-Technology/dp/047119459X Bruce Lindbloom’s site that Ernst mentioned earlier is also a great resource, and LULA regulars like Jeff Schewe and Andrew Rodney have also written extensively on color management principles geared in more practical terms for photographers and printmakers. Just search on their names on Amazon.com, for example, to find their publications. Because CIELAB and CIEXYZ form the backbone of the PCS (profile connection space) it's impossible to learn color management practices without picking up an understanding of the CIE LAB, XYZ, and/or LCH color models.
All that said, we can narrow the discussion of LAB in a very practical way to a smaller subset of useful LAB values in order to get a visual feel for media white properties in terms of overall lightness and color appearance(i.e., does the nominally "white" paper look warm of cool?). L* is lightness with the brightest (more technically correct to say lightest) papers reaching about L*=98 while most common RC photo papers have L values down around 93-95. The new group of Metallic papers will drop to about L*= 85-87, but the spectrophotometers used to measure L* for these metallic papers don’t really factor in the “metallic sheen” which gives those media their desirable traits. The a* value in the LAB color model represents the magenta-green axis on a visual color scale (often stated as red-green, but in purest complementary color theory the a* axis is truly magenta-green). Increasingly Positive a* values denote increasing vivid magenta hue while negative values denote greenish hues. When both a* and b* = 0 under a given specified illuminant, the human observer sees a truly neutral tone which can vary between pure white to pure black as L* goes from 100 to 0. As for any nominally white paper color a* rarely gets outside a range between -2 and +2, -2 being very slightly greenish, and +2 being very slightly magenta in appearance. Note that in all L*, a*, and b* scales, a unit value increment of 1 is considered to be perceptually “just noticeable” by people with normal color vision. Media manufacturers rarely go outside the -2 to + 2 range for a* because most folks don’t find such strong media white greenish or magenta color tints all that appealing. b* get much more useful range. The b* axis on the color plot represents the yellow-blue scale, with positive b* moving yellow and -b* moving blue, The most bluish “cool-white” media I”ve measured can reach -10 on the b* scale while the most yellow “warm white” papers range up to about +7 (there are truly tan colored papers for other uses which will reach much higher positive b* values, but no one would call them a nominally "white" paper. Hence, you can use the initial b* value reported the AaI&A tests to determine if a paper is initially a cool white paper or a warm white paper in its visual appearance. When media yellowing/discoloration starts to occur the b* value starts to move in the positive direction. For a High OBA content paper it might start at -5 and end up at +3 or higher meaning the paper has lost all of it’s original cool white appearance and now gone over to the warm white category. When yellowing becomes more severe the b* value for the paper can go well beyond +10, hence well beyond what any observer would call a nominally “white” paper any more.
If you are following me so far, then I hope I’ve given you some sense of the the LAB numbers as they pertain to media white point color, both initially and as the product ages over time. Obtain a few sample media that are represented in the AaI&A database and compare the media colors side by side under a good light source like the Solux 4600K lamps. This will help you to train your eyes to pick up on the cool versus warm properties of “white papers” and get an even better feel for what the LAB numbers in the AaI&A test reports mean in practical terms to the artist. Once you've learned this LAB "vocabulary" you can then predict which papers you will probably like for certain application even before tracking down a sample, presuming that you can find the LAB values for that product before buying it
Stable media color values are critical in fine art prints to preserve the initial carefully chosen highlight colors of the artwork over time which is why I personally place a lot of emphasis on media whitepoint stability in my tests. If media white point color wasn't a particularly significant attribute to artists when they choose a fine art paper, the media manufacturers would never have felt so compelled to tinker with OBAs and other means of tuning white point properties as much as they do now.
cheers,
Mark
http://www.aardenburg-imaging.com
