Understanding NTSC 72 CIE1931 and Its Relation to sRGB

Introduction:

The terms NTSC 72 and CIE1931 often come up in discussions about color gamuts, but what do they really mean for display accuracy and standardization? In this article, we will explore the meanings of these terms and their relationship to the commonly used sRGB color space.

Understanding the Terms:

NTSC 72

The term NTSC 72 signifies that a display can reproduce 72% of the colors defined by the NTSC standard. Historically, NTSC 72 has been used as a rough equivalent to the sRGB color space, which is frequently utilized for standard displays and web content.

CIE 1931

CIE 1931 refers to the CIE (International Commission on Illumination) 1931 color space, which is a scientific model of color perception. This standardized model is based on human vision and provides a mathematical representation of colors. When color gamuts are described in terms of CIE 1931, they are grounded in this standardized model of color perception.

Conclusion:

While NTSC 72 is broadly equivalent to sRGB100, the addition of CIE 1931 indicates that the measurement or representation of the color gamut is being referenced within the context of the CIE 1931 color space.

In practical terms, a display that can reproduce NTSC 72 CIE1931 colors is generally expected to match the coverage of sRGB. However, the exact match may depend on how colors are measured or represented in relation to the CIE standards.

So while NTSC 72 and sRGB100 are closely related, the inclusion of CIE1931 emphasizes the scientific basis of the color representation. For most practical purposes, you can consider NTSC 72 CIE1931 to be approximately equivalent to sRGB100.

Additional Insights:

Expressing color gamuts in terms of percentage area relative to a reference gamut can be misleading. It tells you nothing about the specific primaries and their alignment in terms of hue and saturation.

CIE 1931 specifically refers to the CIE Yxy color space established in 1931, based on the color-matching functions X, Y, and Z established that same year. In the Yxy space, the sRGB standard gamut is about 72% of the gamut area of the 1953 NTSC primary set. However, the NTSC set is an inferior standard for reference - while it is a wider-gamut standard than many CRT-focused specs, it was never widely used in practice. The NTSC-specified green could not be achieved with practical phosphors.

Simply stating that a gamut covers 72% of another gamut’s area does not indicate how the primaries of the two actually align in terms of hue and saturation. So while “72 NTSC” is a fair description of the sRGB gamut, there can be many other gamuts that are also 72% of the area of NTSC and do not align at all with the sRGB gamut.