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video Video is an electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active d ...

video
, luma represents the brightness in an image (the "black-and-white" or achromatic portion of the image). Luma is typically paired with
chrominance Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture, separately from the accompanying Luma (video), luma signal (or Y' for short). Chrominance is usually represented as two ...
. Luma represents the
achromatic
achromatic
image, while the chroma components represent the
color Color (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American Engli ...

color
information. Converting R′G′B′ sources (such as the output of a
three-CCD camera A 3CCD imaging block consisting of a color separation prism of Philips type on which 3 CCDs are mounted A three-CCD (3CCD) camera is a camera A camera is an optical instrument used to capture an image An SAR radar imaging, radar i ...
) into luma and chroma allows for
chroma subsampling Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information Information is processed, organised and structured data. It provides context for data and enables decision making process. For examp ...
: because human vision has finer spatial sensitivity to luminance ("black and white") differences than chromatic differences, video systems can store and transmit chromatic information at lower resolution, optimizing perceived detail at a particular bandwidth.


Luma versus relative luminance

Luma is the weighted sum of gamma-compressed R′G′B′ components of a color video—the ''prime symbols'' ′ denote
gamma compression Gamma correction or gamma is a nonlinear operation used to encode and decode Relative luminance, luminance or CIE 1931 color space#Tristimulus values, tristimulus values in video or still image systems. Gamma correction is, in the simplest cases, ...
. The word was proposed to prevent confusion between luma as implemented in video engineering and
relative luminance Relative luminance follows the photometric definition of luminance, but with the values normalized to 1 or 100 for a reference white. Like the photometric definition, it is related to the luminous flux density in a particular direction, which is ...
as used in color science (i.e. as defined by
CIE CIE may refer to: Organizations * Cambridge International Examinations, an international examination board * Center for International Education at the University of Massachusetts-Amherst * Cleveland Institute of Electronics, a private technical an ...
). Relative luminance is formed as a weighted sum of ''linear'' RGB components, not gamma-compressed ones. Even so, luma is sometimes erroneously called luminance.
SMPTE The Society of Motion Picture and Television Engineers (SMPTE) (, rarely ), founded in 1916 as the Society of Motion Picture Engineers or SMPE, is a global professional association of engineers, technologists, and executives working in the m ...
EG 28 recommends the symbol Y′ to denote luma and the symbol Y to denote relative luminance.


Use of relative luminance

While luma is more often encountered, relative luminance is sometimes used in video engineering when referring to the brightness of a monitor. The formula used to calculate relative luminance uses coefficients based on the CIE color matching functions and the relevant standard chromaticities of red, green, and blue (e.g., the original
NTSC The National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12-19, with Some supplementary references cited in the Reports, and the Petition for adoption of transm ...
primaries, SMPTE C, or
Rec. 709 Rec. 709, also known as Rec.709, BT.709, and ITU 709, is a standard developed by for image encoding and signal characteristics of . The most recent version is BT.709-6 released in 2015. BT.709-6 defines the Picture characteristics as havin ...
). For the Rec. 709 (and sRGB) primaries, the linear combination, based on pure colorimetric considerations and the definition of relative luminance is: : Y = 0.2126 R + 0.7152 G + 0.0722 B The formula used to calculate luma in the Rec. 709 spec arbitrarily also uses these same coefficients, but with gamma-compressed components: : Y' = 0.2126 R' + 0.7152 G' + 0.0722 B', where the prime symbol ′ denotes
gamma compression Gamma correction or gamma is a nonlinear operation used to encode and decode Relative luminance, luminance or CIE 1931 color space#Tristimulus values, tristimulus values in video or still image systems. Gamma correction is, in the simplest cases, ...
.


Rec. 601 luma versus Rec. 709 luma coefficients

For digital formats following
CCIR 601 ITU-R The ITU Radiocommunication Sector (ITU-R) is one of the three sectors (divisions or units) of the International Telecommunication Union 260px, ITU Monument, Bern The International Telecommunication Union is a specialized agency of ...
(i.e. most digital standard definition formats), luma is calculated with this formula: : Y'_\text = 0.299 R' + 0.587 G' + 0.114 B' Formats following
ITU-R The ITU Radiocommunication Sector (ITU-R) is one of the three sectors (divisions or units) of the International Telecommunication Union 260px, ITU Monument, Bern The International Telecommunication Union is a specialized agency of the Unit ...
Recommendation BT. 709 use a different formula: : Y'_\text = 0.2126 R' + 0.7152 G' + 0.0722 B' Modern
HDTV High-definition television (HD or HDTV) describes a television system providing a substantially higher image resolution Image resolution is the detail an holds. The term applies to s, film images, and other types of images. Higher resolution m ...
systems use the 709 coefficients, while transitional 1035i HDTV formats may use the SMPTE 240M coefficients: : Y'_\text = 0.212 R' + 0.701 G' + 0.087 B' = Y'_\text These coefficients correspond to the SMPTE RP 145 primaries (also known as "SMPTE C") in use at the time the standard was created. The change in the luma coefficients is to provide the "theoretically correct" coefficients that reflect the corresponding standard chromaticities ('colors') of the primaries red, green, and blue. However, there is some controversy regarding this decision. The difference in luma coefficients requires that component signals must be converted between Rec. 601 and Rec. 709 to provide accurate colors. In consumer equipment, the matrix required to perform this conversion may be omitted (to reduce cost), resulting in inaccurate color. As well, the Rec. 709 luma coefficients may not necessarily provide better performance. Because of the difference between luma and relative luminance, luma does not exactly represent the luminance in an image. As a result, errors in chroma can affect luminance. Luma alone does not perfectly represent luminance; accurate luminance requires both accurate luma and chroma. Hence, errors in chroma "bleed" into the luminance of an image. Due to the widespread usage of
chroma subsampling Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information Information is processed, organised and structured data. It provides context for data and enables decision making process. For examp ...
, errors in chroma typically occur when it is lowered in resolution/bandwidth. This lowered bandwidth, coupled with high frequency chroma components, can cause visible errors in luminance. An example of a high frequency chroma component would be the line between the green and magenta bars of the
SMPTE color bars Rendition of SD SMPTE color bars SMPTE color bars are a trademarked television Television (TV), sometimes shortened to tele or telly, is a telecommunication medium used for transmitting moving images in monochrome (black and white), ...

SMPTE color bars
test pattern. Error in luminance can be seen as a dark band that occurs in this area.Constant Luminance
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See also

*
Chroma subsampling Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information Information is processed, organised and structured data. It provides context for data and enables decision making process. For examp ...
*
Chrominance Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture, separately from the accompanying Luma (video), luma signal (or Y' for short). Chrominance is usually represented as two ...
*
Gamma correction Gamma correction or gamma is a nonlinear In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, ...


References

{{Analogue TV transmitter topics Film and video technology