Summary
Highlights
This video is the third in a series on color vision, focusing on the CIE chromaticity diagram. It builds upon previous videos that covered basic color concepts and color matching, which led to the creation of the color map. The aim is to provide a deeper understanding of color relationships and color vision studies through this diagram.
The video briefly reviews the origin of the color map, starting from the color matching lab where any spectral color could be matched by combining three primary colors (red, green, blue). The initial color matching graph showed negative values in the red line, indicating certain colors couldn't be matched directly, necessitating the addition of a primary to the test color.
To overcome the inconvenience of negative values, a transformation was made using imaginary primary colors X, Y, and Z. This ensured all values remained positive. These XYZ values were then used to create a widely usable, standard color map to describe specific colors.
The mapping process involved transforming a three-dimensional color space into a two-dimensional plane using the XYZ primaries. Plotting vectors corresponding to spectral color matches traced out the spectral locus, forming the boundary of all visible colors. This two-dimensional map, known as the CIE chromaticity diagram, represents any color's position with two values (X and Y), with the third value representing luminance.
The outer edge of the diagram represents pure spectral colors, with wavelengths labeled. The bottom line connects various mixtures of blue and red, creating non-spectral colors like purple and magenta. The entire figure contains the full gamut of colors humans can see, specified by their X and Y coordinates. The center point, x=1/3 and y=1/3, represents equal energy white.
The diagram allows for precise color specification. For example, specific coordinates define a color in the cyan region or the exact red used in stoplights. Mixing two colors, A and B, results in a color C located at the midpoint of the line connecting A and B, when mixed in equal amounts. This illustrates additive color mixing, such as red plus green making yellow.
To find a color's dominant wavelength, a line is drawn from equal energy white (W) through the chosen color (D) to intersect the spectral locus. The intersection point indicates the dominant wavelength, representing a saturated spectral color mixed with white. Complementary colors are found by drawing a line from a spectral color through white to the opposite side of the spectrum; adding these two colors together makes white.
The video demonstrates that the triangle formed by connecting the three primary colors (red, green, blue) used in color matching defines the only colors that can be created by mixing them. A significant portion of the blue-green region lies outside this RGB triangle, meaning these colors cannot be matched by RGB mixing alone. This explains why some colors in early color matching required adding red to the test color.
Color televisions and computer monitors also operate on the RGB scheme, and their color gamut is limited by an RGB triangle on the chromaticity diagram. The video then discusses the concept of 'white,' noting that human vision perceives a range of colors as white. The CIE defines standard illuminants (A, B, C, D65) to standardize different types of white, such as tungsten light (A) or average daylight (D65), based on color temperature.
An example demonstrates how color temperature affects the appearance of an image, showing how a photo taken under tungsten light can be adjusted to appear as it would in sunlight or average daylight. The video concludes by reiterating that the chromaticity diagram provides a common language for describing color and reveals fundamental properties of human color perception. The next video will discuss color vision cones and opponent channels.