Understanding CIE L*a*b Color Space TB 130A

CIE *L*A*B COLOUR SPACE

Because one person’s view of colour may differ from others, visual colour matching is extremely subjective. Xrite (1997), a manufacturer of spectrophotometers explains “This can lead to confusion and frustration between customers, suppliers,vendors, production, and management.” CIE Lab Colour Space can be used to measure colour quantitatively.

Since Isaac Newton discovered that white light contained all the colours of the spectrum in 1666 (Beretta, G. pg. 17), people have been trying to adequately describe colour in a quantifiable and reproducible way. Light moves as a wave, with wavelengths from 360-750 nanometers visible to the naked eye. When we see something as a certain colour, what we are actually seeing are the reflected colours that are not absorbed by the object.

Figure 1:

How does this work? Let us take, for example, three common colours: John Deere Green, Candy Apple Red, and Kodak Yellow. You have likely seen these colours before and can picture them in your mind. When we shine light on something, it reflects some of it back. The mixture of wavelengths it reflects and the strength of that reflection is plotted as a spectral curve. Figure 1 shows the spectral curve (as a reflected percentage) of these three colours under cool white fluorescent light.

The two major factors in how we see colour are the lighting source and the quality of the viewer’s colour vision. The lighting source can radically affect the appearance of a colour sample. Although two samples may appear to be the same colour under sunlight, putting them under fluorescent light may make them look significantly different. This difference between samples under different lighting sources is called metamerism, which can lead to problems in colour matching. If we make a colour match in the *L*a*b under fluorescent lighting and the customer compares those colours by an office window it may look like a poor match.

In this case, we lack both a standard observer and a standard light source, and we are comparing apples to oranges. To eliminate such discrepancies CIE *L*a*b Colour Space was developed to standardize and quantify objectively how we measure a colour.

CIE *L*A*B COLOUR SPACE

With standardization of the lighting source the most subjective of part of the equation is still left open that of the standard observer. In the CIE *L*a*b Colour Space colour is measured electronically, not with the human eye. The machine, called a spectrophotometer, reads the colour and records the spectral data. Since this machine is calibrated with a standard, all spectrophotometer readings can be compared regardless of who took them or when. Using this data, the machine mathematically derives the colour’s position in the CIE *L*a*b colour space.

Figure 2:

INTERPRETATION OF THE CIE *L*A*B COLOUR SPACE

CIE *L*a*b envisions a three dimensional space, with three axes, represented in Figure 2 (Eye-Colour. 2002)

*L is the light and dark axis. It can be thought of as the average value of a spectral curve, with 0 representing no reflection (black) and 100 representing high reflection (white).

*a is the red and green axis. A positive *a value indicates a colour is red. A negative value indicates green.

*b is the yellow to blue axis. A positive *b value indicates a colour is yellow. A negative value indicates blue.

Taken together, these three numbers represent a particular colour that can be compared anywhere in the world. Our three common colours under CWF2 using the CIE *L*a*b Colour Space yield the results shown below.

After establishing standards for a given colour, we can then begin to compare them. On page

INTERPRETATION OF THE CIE *L*A*B COLOUR SPACE

After establishing standards for a given colour, we can then begin to compare them. Below, in Figure 3 are spectral data from a production lot compared to a standard.

Figure 3:

USING CIE *L*A*B

Using the CIE*L*a*b system we can look at the *L*a*b values and calculate the differences as seen below. Since we can quantify the standard, the production lot, and the difference, we have a reference to communicate, and ensure the quality of the colour. In this case, the Production Lot is slightly lighter, less red, and more yellow than our standard. These differences are called deltas. Derived from these deltas is another measure called Delta E, which signifies the overall colour difference.

REFERENCES

Xrite. (1997). A guide to Understanding Color Tolerancing, L10-024.pdf retrieved online February 24, 2003

Beretta, Giordano. (2000). Understanding Color, UnderstandColor.pdf retrieved online February 24, 2003

Eye-Color. (2002). Color Spaces, CIELab_65percent.jpg retrieved online February 24, 2003