What is CIE 1931 & CIE 1960 Chromaticity Diagrams?
Photometry does not take into account the wavelength combination of the light. Thus it is possible for two surfaces to have the same luminance but the reflected light to be made up of totally different combinations of wavelengths. It should be understood as the two surfaces will look different in colour. The CIE colourimetry system provides a means to quantify colour.
The CIE 1931 Standard Observer is used for colours occupying visual fields up to 4° of angular subtense. The CIE 1964 Standard Observer is
used for colours covering visual fields greater than 4° in angular subtense. The values of the colour matching functions at different wavelengths are known as the spectral tristimulus values.
The outer curved boundary of the CIE 1931 chromaticity diagram is called the spectrum locus. The CIE 1931 chromaticity diagram is perceptually non-uniform. Green colours cover a large area while red colours are compressed in the bottom right corner. This perceptual non-uniformity makes any attempt to quantify large colour differences using the CIE 1931 chromaticity diagram futile. In an attempt to improve this situation, the CIE first introduced the CIE 1960 Uniform Chromaticity Scale (UCS) diagram and then, in 1976, recommended the use of the CIE 1976 UCS diagram. Both diagrams are simply linear transformations of the CIE 1931 chromaticity diagram.
Understanding Correlated Color Temperature ( CCT)
To understand the CCT (correlated Color Temperature) its needed to express what is Black Body Radiator. The exact properties of the radiation from a heated body depend to some extent on the type of body (type of material) being heated. So, for the reliable analysis of these properties an unambiguous definition of the type of heated body is needed. For this purpose an idealized body, that is a perfect light absorber, is used: the so-called “black-body radiator”. At temperatures too low for emission of visible radiation, it will look perfectly black, hence the description black-body radiator. Blackbody gets hotter, the light it emits progresses through a sequence of colors, from red to orange to yellow to white to blue. This is very similar to what happens to a piece of iron heated in a blacksmith’s forge. The sequence of colors describes a curve within a color space. The diagram below shows the CIE 1931 color space, created by the International Commission on Illumination (CIE) to define the entire range of colors visible to the average viewer, with the black-body curve superimposed on it.
CIE 1931 chromaticity diagram
An incandescent lamp emits light with a color of roughly 2700 K, which is toward the warm or reddish end of the scale. Because an incandescent bulb uses a filament that is heated until it emits light, the temperature of the filament is also the color temperature of the light.
Spectral analysis of visible light makes it possible to define color temperatures for non-incandescent white light sources, such as fluorescent tubes and LEDs. The actual temperature of a 2700 K LED source is typically around 80° C, even though the LED source emits light of the same color as a filament heated to a temperature of 2700 K.
The American National Standards Institute (ANSI) has produced a series of standards that define the chromaticity of different types of light sources. For LED light sources, chromaticity standard C78.377A, published in 2008, defines 8 nominal CCTs ranging from 2700 K (warm) to 6500 K (daylight). For consistency across light source types, six of these CCTs correspond to the chromaticity specifications for compact fluorescent lamps (defined in the ANSI C78.376 standard, published in 2001). Since LED sources can be manufactured to produce any color temperature within the range, ANSI C78.277 defines two additional CCTs (4500 K and 5700 K) to fill in gaps along the black-body curve which are not accounted for by the CFL chromaticity standard.
CIE 1960 Uniform Chromaticity Scale
The size of each quadrangle is determined by the CCT and Duv ranges for each color temperature, as specified in the ANSI standard. For example, the quadrangle for nominal CCT of 3000 K is centered on 3045 K, with a tolerance of plus or minus 175. The 3000 K quadrangle, therefore, covers from 2870 K to 3220 K along the black-body curve. In the other dimension, the quadrangle extends 0.006 Duv above and below the curve.
Here is the graph shared below:
In practice, this means that the measured CCT and Duv of LED light sources can vary considerably and still be described as having a nominal CCT of 3000 K.
The threshold at which a color difference becomes perceptible is defined by a MacAdam ellipse. A MacAdam ellipse is drawn over the color space in such a way that the color at its center point deviates by a certain amount from colors at any point along its edge. The scale of a MacAdam ellipse is determined by the standard deviation of color matching (SDCM). A color difference of 1 SDCM “step” is not visible; 2 to 4 steps is barely visible; and 5 or more steps is readily noticeable.
As the illustration below shows, the size and orientation of MacAdam ellipses differ depending on their position within the CIE 1931 color space, even when each ellipse defines the same degree of deviation between the color at its center and any color along its edge.
The quadrangles that define the color ranges of the eight nominal CCTs described in the ANSI C78.377A standard are roughly equivalent in size to 7-step MacAdam ellipses. Any LED light source whose measured color point falls within one of these quadrangles is considered to have that nominal CCT. But since color differences of 7 SDCM steps are readily visible, light sources with the same nominal CCT can display fairly large — and noticeable — differences in hue.
For example, the chart to the right shows three hypothetical LED light sources, A, B, and C. Even though A and B are on either side of the black-body curve, the color difference between them is negligible, The color variation between A and C, on the other hand, is four times as great as the color variation between A and B. Nevertheless, all three light sources conform to the ANSI specification for nominal CCT of 3000 K.
One important goal for lighting fixture manufacturers is to ensure that color differences between fixtures are small, if not imperceptible. Since nominal CCT does not ensure this degree of color uniformity, LED fixture manufacturers devise various binning schemes to tightly manage color variations in the LED sources that they purchase and use in their fixtures.
NEMA / ANSI ANSLG C78.377-2008: Specifications for the Chromaticity of Solid State Lighting Products for Electric Lamps. American National Standards Institute, 2008-www.nema.org
Colorkinetics Optibin Technology Overview Publication: “www.colorkinetics.com”.
The Society of Light and Lighting : The SLL Lighting Handbook publication |www.cibse.org