The white points are individually definedfor the color spaces. The CIELab color space was intended for equal perceptual differences for equal chan-ges in the coordinates L.,a. and b. Color differences deltaE are definedas Euclidian distances in CIELab. This document shows color charts in CIELab for several RGB color spaces.
How Closely Does the Sample Match the Standard?Even if two colors look the same to one person, slight differences may be found when evaluated with a color measurement instrument. If the color of a sample does not match the standard, customer satisfaction is compromised and the amount of rework and costs increase.
Because of this, identifying color differences between a sample and the standard as early in the production process as possible is important.Color difference can be defined as the numerical comparison of a sample's color to the standard. It indicates the differences in absolute color coordinates and is referred to as Delta (Δ). These formulas calculate the difference between two colors to identify inconsistencies and help users control the color of their products more effectively.To begin, the sample color and the standard color should be measured and the values for each measurement saved. The color differences between the sample and standard are calculated using the resulting colorimetric values.Identifying Color Differences Using CIE L.a.b. CoordinatesDefined by the Commission Internationale de l'Eclairage (CIE), the L.a.b.
color space was modeled after a color-opponent theory stating that two colors cannot be red and green at the same time or yellow and blue at the same time. As shown below, L. indicates lightness, a.
is the red/green coordinate, and b. is the yellow/blue coordinate.
Deltas for L. (ΔL.), a. (Δa.) and b. (Δb.) may be positive (+) or negative ( -). The total difference, Delta E (ΔE.), however, is always positive.ΔL. (L.
sample minus L. standard) = difference in lightness and darkness (+ = lighter, - = darker)Δa. (a. sample minus a. standard) = difference in red and green (+ = redder, - = greener)Δb. (b.
sample minus b. standard) = difference in yellow and blue (+ = yellower, - = bluer)Let's compare Apple 1 to Apple 2 (see Figure 1).Figure 1Looking at the L.a.b. values for each apple in Figure 1, we can objectively determine that the apples don't match in color. These values tell us that Apple 2 (sample) is lighter, less red, and more yellow in color than Apple 1 (standard). If we put the values of ΔL.=+4.03, Δa.=-3.05, and Δb.=+1.04 into the color difference equation, it can be determined that the total color difference between the two apples is 5.16.5.16 = 4.03.2 + -3.05.2 + 1.04.21/2Identifying Color Differences Using CIE L.C.H. CoordinatesThe L.C.h color space is similar to L.a.b., but it describes color differently using cylindrical coordinates instead of rectangular coordinates. In this color space, L.
indicates lightness, C. represents chroma, and h is the hue angle. Chroma and hue are calculated from the a. and b. coordinates in L.a.b.
Deltas for lightness (ΔL.), chroma (ΔC.), and hue (ΔH.) may be positive (+) or negative ( -). These are expressed as:ΔL. (L. sample minus L. standard) = difference in lightness and darkness (+ = lighter, - = darker)ΔC. (C.
sample minus C. standard) = difference in chroma (+ = brighter, - = duller)ΔH. (H. sample minus H. standard) = difference in hueLet's compare Apple 1 to Apple 2 (see Figure 2).Figure 2Looking at the L.C.h values for each apple in Figure 2, we can objectively determine that the apples don't match in color. Like the L.a.b. values, these values tell us that Apple 2 (sample) is lighter and duller in appearance than Apple 1 (standard).
The positive ΔH. value of +1.92 indicates Apple 2 falls counterclockwise to Apple 1 in the L.C.h color space. This tells us that Apple 2 is less red than Apple 1., such as colorimeters and spectrophotometers, can detect differences indiscernible to the human eye and then instantly display these differences in numerical terms. After identifying color differences using L.a.b.
or L.C.h values, it should be decided whether the sample is acceptable or not using tolerance limits.Like this article? To stay updated on the element of color, best practices for controlling and evaluating the color of objects, educational seminars, and advanced technology for research and manufacturing environments.The technological leader in color and light measurement solutions, Konica Minolta Sensing Americas helps organizations formulate, evaluate, and control color to meet product quality and operational goals more efficiently.
Like geographic coordinates – longitude, latitude, and altitude – L.a.b. color values give us a way to locate and communicate colors. What’s the history of L.a.b.?In the 1940’s, Richard Hunter introduced a tri-stimulus model, Lab, which is scaled to achieve near uniform spacing of perceived color differences.
While Hunter’s Lab was adopted as the de facto model for plotting absolute color coordinates and differences between colors, it was never formally accepted as an international standard.Thirty-one years later, the CIE published an updated version of Hunter’s Lab: CIELab. The correct way to pronounce it is “see-lab”, or “L-star, a-star, b-star,” but some applications and instruments simply call it L, A, B, or Lab.
What does L.a.b. stand for?Regardless of the which color space name you call it, it’s important to know what L., a., and b.stand for. L.: Lightness. a.: Red/Green Value.
b.: Blue/Yellow ValueAs an example, see figures 17 and 18, showing the color-plotting diagrams for L.a.b. The a. axis runs from left to right.
A color measurement movement in the +a direction depicts a shift toward red. Along the b. axis, +b movement represents a shift toward yellow. The center L. axis shows L = 0 (black or total absorption) at the bottom.
At the center of this plane is neutral or gray.To further demonstrate how L.a.b. values represent specific colors, see flowers A and B, below. We’ve also plotted their values on the CIELAB Color Chart above.We hope these examples help you better understand L.a.b.
color space. If you have more questions about color spaces or methods of color communication, – we’re happy to help!