Lab is a color space like RGB or CMYK but has a number of useful properties that make it essential for color management and color tolerancing. This article provides:
- A definition of Lab Color
- Images to help visualize Lab color space
- Uses of Lab color for both tolerancing and image sharpening
What Is Lab Color?
Lab, more precisely referred to as CIELAB or L*a*b*, is a color space, like RGB or CMYK, but different in some important and useful respects. It was developed by the Commission internationale de l’eclairage (also known as International Commission on Illumination or simply CIE) in 1976 based upon perception experiments on human subjects.
L* – describes Lightness, with a maximum value of 100 (white) and a minimum value of 0 (absolute black).
a* – quantifies the opposable Green-Magenta color axis, with a maximum value of 128 (magenta) and a minimum value of -128 (green)
b* – quantifies the opposable Yellow-Blue color axis, with a maximum value of 128 (yellow) and a minimum value of -128 (blue)
Note that a* and b* values of 0 indicate no saturation or chroma, or in plain english “gray”.
Visualizing Lab Color
CIELAB is a three-dimensional color space that incorporates all colors that can be perceived by humans. For simplicity, CIELAB is often shown as a 2-dimensional chromaticity diagram (shaped a bit like a horseshoe), ignoring the L* component. The diagram below shows sRGB interposed upon the CIELAB “horseshoe” diagram. At a glance, you can tell that sRGB, a color space commonly used for communicating color on the internet, incorporates less than half of the colors humans can see.
For more of a 3D view, below is screen shot of sRGB (outside red) with a CMYK printer profile, with the graph coordinates in CIELAB. There are a few things worth noting. First, while sRGB is substantially larger than this printer profile, there are still areas of the printer gamut that exceed sRGB – in yellow and cyan (not visible). Secondly, sRGB was designed to have a maximum L* value of 100 and minimum of 0 – absolute white and black. No real world printer can achieve these values. This printer/paper combination has a max L* value of 95 (paper white) and minimum of 15.
What Is Lab Color Used for?
So what makes CIELAB useful? It’s certainly not intuitive, so you’d have to be a masochist to design in Lab, though that is an option in Photoshop.
One of the more useful features of Lab is that it is “device independent”, meaning the same Lab value on two displays (or on a printer for that matter), will appear the same to your eyes. By contrast, the same RGB values on two non-calibrated displays are not likely to appear the same.
Another very useful feature of CIELAB is that it is (mostly) perceptually uniform. This means that if two greenish colors are a distance of 3 from each other, and two reddish colors that are also a distance of 3, then the difference between the two sets of colors will look about the same to you.
This perceptual uniformity allows CIELAB to be used for color tolerancing. If a designer incorporates a particular Pantone color (which all have corresponding CIELAB values) into their design, she could, for instance, specify that the delivered product must not vary more than 3 CIELAB units (also known as Delta-E) from specified Pantone color. This helps remove subjectivity from the color approval process and makes a tricky topic a bit more objective.
Color tolerancing based upon Delta-E is widely used in commercial print as well as apparel creation.
What Is Delta E?
There are various color tolerancing methods in use that all make use of CIELAB, and the distance in CIELAB space is called “Delta-E” (dE). Two colors that have a difference of 1 dE are difficult to tell apart except by the most discerning colorist. By contrast, almost everyone can perceive a difference of 3 dE.
The grandaddy of tolerancing systems is Delta-E 1976 (dE76), which came into existence at the same time as CIELAB. Over time, some perceptual non-uniformities were discovered in CIELAB, so more sophisticated tolerancing systems were developed. The apparel world adopted a tolerancing system known as CMC. In the world of print, dE 2000 (dE00), has been widely adopted in recent years.
Where does Lab fit into the hierarchy of color spaces?
Who knew there was a hierarchy of color spaces? There actually is. The most accurate way to specify and measure color is with a spectral curve. Ever notice how the color of walls in your kitchen look different in the morning than in the afternoon? That’s because our perception is dependent upon the light under which an object is being viewed. A spectrophotometer provides its own light source and thus doesn’t face that limitation. It is light source independent.
CIELAB can be mathematically derived from spectral data, but two parameters need to be specified – reference illuminant and observer angle. Like our eyes, CIELAB is illuminant dependent. This all gets a bit geeky, but the take away is the Spectral-to-CIELAB highway is a one way street. Once you’ve converted spectral to CIELAB, you’ve lost some information and can’t go back.
It is also possible to mathematically derive RGB or CMYK from CIELAB. But if you are provided a random RGB value, there’s no way to derive a CIELAB value. As stated previously, two uncalibrated monitors are likely to display the same RGB values differently. Which is “right”? These two different colors you’re seeing cannot have the same CIELAB values.
| The Heirarchy of Color Spaces | |
|---|---|
| Spectral | Illuminant Independent |
| CIELAB | Device independent |
| RGB / CMYK | Device dependent |
How to Use Lab Color in Photoshop
Lab Color is included with Photoshop, along with RGB and CMYK. However, Lab is not a particularly intuitive design space. Nevertheless, there are instances where is can be useful. Specifically, sharpening of images Lab color can reduce unwanted “haloes”. The process involves a number of steps and is outlined in this YouTube video:
For the record, though CIELAB can use either a D65 or D50 reference illuminant and a 2º or 10º observer angle, Photoshop has baked in D50 / 2º observer angle for its Lab calculations.
Summing Up
Lab is a color space that incorporates all of the human visual range and has the handy feature of being perceptually uniform. This makes it essential to color management and color tolerancing. It can also offer advantages for image sharpening.
So… if you’ve gotten this far you are probably a bit of a color geek and care whether colors are displayed accurately on your monitor. If this is the case, check out our TruHu monitor calibrator app – it’s easy to use, inexpensive, and fast.
