Introduction
For 8 bit color, [0, 255] range is referred as Full Range, and [16, 235] range is referred as Narrow Range (aka Limited Range, Low RGB, Broadcast). For YUV signals the Narrow Range is [16, 235 ] for Y, and [16, 240] for UV. In broadcast environment levels from 1 to 254 (Legal Range) are available for video, levels 0 and 255 are used exclusively for SDI interface synchronization. In file based environment all levels from 0 to 255 are available for video.
In broadcast environment Y, R, G and B 8 bit values are conventionally shifted and scaled to the range [16, 235]. Thus, the digital 8 bit reference White Level is 235 and reference Black Level is 16. The headroom above 235 and the footroom below 16 accommodate signal overshoots due to filtering and specular highlights.
Color Range
Standards define the RGB to YUV and vice versa matrix coefficients in relative units, e.g. [0,1] or [0%,100%]. Digital signals are usually defined not in percents, but in 8, 10 or 12 bit levels. So white pixel that has RGB values of (255, 255, 255) in 8 bit (2^8) is also (1023, 1023, 1023) in 10 bit (2^10), (4095, 4095, 4095) in 12 bit (2^12) and so on.
Colour systems that will output either full data range images (0-255 or 0-1023), or TV legal levels (16-235 or 64-940), and displays that expect the input signal to be either full data range images, or TV legal levels, and will display accordingly.
Full range of Luma, Chroma (Cb/Cr) and RGB is [0, 1023] for 10-bit and [0, 4092] for 12-bit. SDI level (narrow range) of Luma & RGB is [64 –940] for 10-bit and [256, 3760] for 12-bit. Also narrow range of Chroma (Cb/Cr) is [64, 960] for 10-bit and [256, 3840] for 12-bit.
IRE
A pixel can have integer values between 0-100% or 0-100 IRE. Institute of Radio Engineers (IRE) devised a unit of measurement for color i.e. IRE. The sync pulse is normally 40 IRE below the zero value. So, peak to peak signal should be equal to 140 IRE. IRE is a relative measurement (percent) because a video signal may be any amplitude. This unit is used in the ITU-R BT.470 which defines PAL, NTSC and SECAM.
In analog video, the electrical reference for white is defined as 100 IRE. In 50 Hz countries it is 700 mV, but in America it is 10000/14 = 714.285714 mV. The electrical reference for black depends on what video format you are using, and even what country you are in. PAL and the Japanese version of NTSC always use 0 IRE to define black. Composite NTSC video in North America uses an electrical value of 7.5 IRE.
Below diagram shows the narrow range level mapping. For YUV signals the narrow Range is [16, 235] for Y and [16, 240] for UV.
