Monthly Archives: June 2017

understanding CIE 1931 chromaticity

The International Telecommunications Union publishes Recommendation ITU-R BT.709 which specifies how image data is coded for transmission as video.  This includes a specification for the representation of color which applies to almost all modern video transmission including broadcast, cable/satellite, or over the internet (but not including emerging practices for  high dynamic range/wide color gamut).

We conventionally understand any color to be a combination of red, green, and blue primaries, but the photoreceptors in the human eye do not function on the basis of these primaries, and BT.709 does not include the first principles needed to understand color representation.  It only offers the following specification:

item parameter system value
1.3 Chromaticity coordinates
(CIE, 1931)
– Red (R)
– Green (G)
– Blue (B)
x y
1.4 Assumed chromaticity for
equal primary signals
(Reference white)
x y
0.3127 0.3290

In this post, we take a deeper look at Publication No. 15 of the Commission Internationale de l’Eclairage, CIE 15:2004, which includes the 1931 chromaticity specification, still the defacto standard for quantifying color in a camera and display device-invariant way.

Continue reading