Maths & me @CIE1931 vs CIE1976
In 1931, the Commission internationale de l’éclairage or CIE (International Commission on Illumination in English) defined the most commonly used color space ...
While there are a number of different types of color spaces, we are specifically interested in chromaticity diagrams, which only measure color quality, independent of other factors like luminance. A color space is a uniform representation of visible light. It maps the all of the colors visible to the human eye onto an x-y grid and assigns them measureable values. This allows us to make uniform measurements and comparisons between colors, and offers certainty that images look the same from display to display when used to create color gamut standards.
The CIE 1931 colour spaces made the first clearly defined links between wavelengths in the visible spectrum and colour vision in humans. The development of this colour space became the standard tool for colour management in printing, digital displays and cameras. The two colour spaces – CIE 1931 RGB and CIE 1931 XYZ – were developed from a series of experiments in the late 1920s by William David Wright and John Guild.
Here’s a look at the anatomy of the CIE 1931 color space:
What makes a good color space?
An effective color space should map with reasonable accuracy and consistancy to the human perception of color. Content creators want to be sure that the color they see on their display is the same color you see on your display.
This is where the CIE 1931 standard falls apart. Based on the work of David MacAdam in the 1940’s, we learn that the variance in percieved color, when mapped in the CIE 1931 color space, is not linear from color to color. In other words, if you show a group of people the same green, then map what they see against the CIE 1931 color space, they will report seeing a wide decprepancy of different hues of green. However, if you show the same group a blue image, there will be much more agreement on what color blue they are seeing. This uneveness creates problems when trying to make uniform measurements with CIE 1931.
The result of MacAdam’s work is visualized by the MacAdam Elipses. Each elipse represents the range of colors respondents reported seeing when shown a single color, which was the dot in the center of each elipse.
Image CIE 1931 vs CIE 1976
A better standard
It was not until 1976 that the CIE was able to settle on a significantly more linear color space. If we reproduce MacAdam’s work using the new standard, variations in percieve color are minimalized and the MacAdam’s Elipses mapped on a 1976 CIE diagram appear much more evenly sized and circular, as opposed to oblong. This makes color comparisons using CIE 1976 significantly more meaningful.
The difference of the CIE 1976 color space, particularly in blue and green, is immediately apparent. For this reason the CIE developed a new colour space diagram in 1976. Notice that the McAdam ellipses are now of almost identical size right across the diagram. This is a “colour map” with a consistent scale that can usefully be used to measure the distance, in colour terms, that a light source has shifted over time
“…we strongly encourage people to abandon the use of the 1931 CIE color diagram for determining the color gamut… The 1976 CIE (u',v') color diagram should be used instead. Unfortunately, many continue to use the (x,y) chromaticity values and the 1931 diagram for gamut areas.”
Calculating CIEx and CIEy is a fundamental starting point for more advanced colorimetric calculations. It is actually a simple process if you have access to an SPD (spectral power distribution) and the tabulated tristimulus functions.
The calculation is as follows ;
After calculating X, Y, and Z, the coordinates CIEx and CIEy are simply
CIExyY convertion to CIELuv formula;
Using CIEXYZ data value derived from measured CIELAB is converted to xy - u'v' coordination step by step process and plot this two different chromaticity diagram. Below chart shows IFRA standard value and our measured value for comparison, same CIELAB in two different model - CIE1976 give more detail on red area than CIE1931.
Two gamut (IFRA, Our gamut) actual position in chromaticity diagram and same in enlarged view :
CIELAB and CIELCh color space with our sample and IFRA target :
Furthermore, the CIELab color space has the characteristics of an Euclidean space. Each point can be described by:
its perpendicularly plotted coordinates L*, a* and b*, where
L* represents the lightness
a* represents the red / green color specification
b* represents the yellow/blue color specification
Note: Position of the color points in perpendicularly plotted coordinates L*, a* and b* of the CIELab system
We can calculate Hue angle and Chroma of every colour from CIELAB values. This give clear picture of hue shifting from target and strength of chroma deviation from target both together in polar graph shown here.
Cylindrically plotted coordinates L*, C* and h, where
L* still represents the lightness
C* represents the colorfulness or the chroma
h represents the hue angle or the shade of the color.
Note: Position of the color points in cylindrically plotted coordinates L*, C* and h of the CIELab system
As a result of the transformation, there is no chromaticity diagram for the CIELab color space. In the color planes defined by the values a*, b* or C*, h, the colors cannot be added any more.
The CIELab color space is roughly (not absolutely) structured according to perception; statistically, it corresponds to human visual color perception. Therefore, it is not strictly uniform for color evaluation in terms of the psychology of perception, but it simplifies the interpretation of a color point and of colorimetric deviations.
By calculating CIELUV values we can clearly place / spot points of measured paper white in graph and analyse our sample Newsprint shade with target, it is also easy to calculate distance between reference and sample. White paper shade is more important that decide the gamut size and shape, so this CIE76 chart give clear picture and help for Newsprint selection process.