DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

The Rec. 601 luma coefficients were
computed using the technique that
I explained in Luminance coefficients,
on page 250, using the NTSC
primaries and white point of
Table 22.2, on page 238.
from those of Rec. 709. For tristimulus values greater
than a few percent, use these equations:
045 .
R'240 = 1. 1115R −0.
1115
045 .
G'240 = 1. 1115G −0.
1115
045 .
B'240 = 1. 1115B −0.
1115
Eq 24.4
The sRGB specification for desktop computing uses
numerical values slightly different again (page 267). For
tristimulus values greater than a few percent:
R'sRGB = 1. 055R24 . −0.
055
1
G'sRGB = 1. 055R24 . −0.
055
1
B'sRGB = 1. 055R24 . −0.
055
Rec. 601 luma
The following luma equation is standardized in
Rec. 601 for SDTV, and also applies to JPEG/JFIF (in
computing) and Exif (in digital still photography):
As mentioned a moment ago, the E and prime symbols
originally used for video signals have been elided over
the course of time, and this has led to ambiguity of the
Y symbol between color science and television.
The coefficients in the luma equation are based upon
the sensitivity of human vision to each of the RGB
primaries standardized for the coding. The low value of
the blue coefficient is a consequence of saturated blue
colors having low lightness. The luma coefficients are
also a function of the white point, or more properly, the
chromaticity of reference white.
In principle, luma coefficients should be derived from
the primary and white chromaticities. The Rec. 601
luma coefficients of Equation 24.6 were established in
1953 by the NTSC from the primaries and white point
then in use. Primaries have changed over the years
since the adoption of NTSC. The primaries in use for
480i today are approximately those specified in SMPTE
RP 145; the primaries in use for 576i are approximately
those specified in EBU Tech. 3213. (These
CHAPTER 24 LUMA AND COLOR DIFFERENCES 291
1
Eq 24.5
601
Y'= 0.299 R'+ 0. 587 G'+ 0. 114 B'
Eq 24.6