# The meaning of Y in sRGB color space

In the definition of sRGB color space, https://en.wikipedia.org/wiki/SRGB

Is the Y relative luminance in the above table？ What does the value of Y equal to 1 for white point mean?
The chromaticity coordinates of black, gray and white are all the same. How can my LCD monitor display them? Is there any definition of absolute luminance for white point in sRGB color space?
It confuses me for a long time and maybe I miss some thing in understanding sRGB.

The table shows the CIE 1931 xyY color space equivalents for the 3 primary colors of sRGB and for white (sRGB 1 1 1).

So all the table says is:

``````sRGB(1,0,0) = xyY(0.6400, 0.3300, 0.2126)
sRGB(0,1,0) = xyY(0.3000, 0.6000, 0.7152)
sRGB(0,0,1) = xyY(0.1500, 0.0600, 0.0722)
sRGB(1,1,1) = xyY(0.3127, 0.3290, 1.0000)
``````

RGB color spaces are useful because of their close relationships with display technologies (think RGB pixels), while xyY and XYZ spaces are one step closer to human perception, which gives us the nice property of the Y component directly reflecting the perceived luminance of a color (***), so for example we can immediately see that primary green having Y=0.7152 is about 10x brighter than primary blue (0.0722).

Y value of 1 is simply a conventional maximum value of the luminance but it cannot mean any absolute brightness because, obviously, sRGB screens are used in various lighting conditions and enforcing constant brightness would not be practical.

I see wikipedia lists some "standard conditions" of the viewing environment (e.g. "Screen luminance level: 80 cd/m²") but it guess this is for definition purposes and not a technical requirement for being sRGB compliant. Human perception of colors slightly changes with brightness so sRGB defined for much darker or much brighter "standard brighness" would result in slightly different colors.

The chromaticity coordinates of black, gray and white are all the same. How can my LCD monitor display them?

If you mean, how the monitor can display them as different shades despite having the same chromaticity, then of course it's because they have different luminance. Colorspace diagrams pretend the luminance does not exist (and by throwing out one dimension they can conveniently show the 3D color space as a 2D diagram) but it does not mean the display devices ignore the luminance. One can imagine there is a series of similar looking chromaticity diagrams for every luminance, from dark to bright and the monitor can display all those colors.

***) ...and 'xy' of the 'xyY' are the coordinates of points in all those rainbow-colored horseshoe diagrams, compare the above values with red, green, blue and white locations in the wikipedia diagram.

• You reply is very helpful. Who is responsible for the definition of luminance standards of different shades? RGB color space is called device-dependent color space. Which part of the RGB standard is related to device? May 12 at 9:20

Government regulators that control TV broadcast transmissions were faced with the problem of allowing color TV to be broadcasted without hampering the majority of TV sets that were monochromatic (black & white. As you know, color TV followed by computer monitors, used a cathode ray vacuum tube. As you know, such color broadcasting systems require three color signals, we call R, G, B. Transmitting these three signals superposed on a monochromatic signal requires too much bandwidth, this was simply not available.

The solution adopted was to covert the RGB signals from the TV camera into YUV signals. The Y signal transmitted the lightness and darkness (luminance) of the color picture and the monochrome picture. The color images shown on a black & white TV set are rendered as shades of gray that most observers acknowledged were good renderings of colored objects (how light or dark these colored objects should be presented).

The U and V signals are color differences signals. The U is the difference between a B signal and a Y signal know as B minus Y (B-Y), and the V signal is the difference between R signal and the Y signal known as R minus Y (R-Y).

The U signal expresses “purplish-blue” or its complement “yellowish – green” and the V signal “purplish-red” or its complement “greenish-cyan”. This scheme has the advantage that the U and V signals will be zero should the broadcasted image have no color content. Further, since the human eye is more sensitive to details of luminance than to color content, the U and V signals transmitted lossy thus making best use of a limited band width. These receivers used demodulator circuits to extract color as R-Y, B-Y, and G-Y.

• That's some great background information, but I don't see how it answers the question. May 11 at 23:58
• Though the answer doesn't solve my problem, the information is very useful. The luminance is weighted average of R, G and B components. How is the weight determined? Do any weighted average can be called luminance? May 16 at 2:08