Description

A luminance channel is a linear weighted combination of an image’s linear red, green, and blue channels [1, 2]:

\( Y=w_r \cdot R + w_g \cdot G + w_b \cdot B \)

A luma channel is a linear weighted combination of an image’s gamma-encoded red, green, and blue channels:

\( Y’=w_r \cdot R’ + w_g \cdot G’ + w_b \cdot B’ \)

Luminance Standards

The following standards define luminance weights.

Luminance Standard	Red Weight	Green Weight	Blue Weight	Reference	Note(s)
IEC 61966-2-1	0.2126	0.7152	0.0722	[3]	These are the same weights as ITU-R BT.709-6.
ITU-R BT.601-7	0.299	0.587	0.114	[4] 2.5.1	This is sometimes referred to as NTSC.
ITU-R BT.709-1	0.2125	0.7154	0.0721	[5] Item 4.2	This is replaced by ITU-R BT.709-6.
ITU-R BT.709-5	0.2126	0.7152	0.0722	[16] Item 3.2	This is replaced by ITU-R BT.709-6.
ITU-R BT.709-6	0.2126	0.7152	0.0722	[6] Item 3.2	These are the same weights as IEC-61966-2-1.
ITU-R BT.2020-2	0.2627	0.678	0.0593	[7] Table 4	This is sometimes referred to as UHDTV.

Choosing Luminance Weights

There are many standards and papers which specify different weights.

When referencing a standard normatively in a standard, if a version is present, then that version shall be used. If no version is specified, then the latest version shall be used.

Active Standards/Papers

Standard/Paper	Luminance Weights	Notes
IEEE Std. 2020-2024 IEEE Standard for Automotive System Image Quality	Weights for colorspace, if known, otherwise ITU-R BT.601-7	The standard [17] informatively lists ITU-R BT.601-7 and ITU-R BT.709-6 as references. In §1.4.3, the standard recommends applying weights for the color space, if available, otherwise to use ITU-R BT.601.
ISO 12232:2019 Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index	ITU-R BT.709	The standard [8] lists ITU-R BT.709 as a normative reference. Within the standard’s text, Formula (8) specifies the ITU-R BT.709-6 weights.
ISO 15739:2023 Photography — Electronic still-picture imaging — Noise measurements	ITU-R BT.709	The standard [9] lists the following normative references: ITU-R BT.709 IEC 61966-2–1:1999 IEC 61966-2–1:1999/AMD1:2003 Formula (2) specifies “ITU-R BT.709” (no version) and uses the ITU-R BT.709-6 weights.
ISO 18844:2017 Photography — Digital cameras — Image flare measurement	Conflicting Normative Clauses	The standard [10] lists IEC 61966-2-1 as a normative reference. The standard lists ITU-R BT.601 as an informative reference. Formula (1) specifies the ITU-R BT.601-7 weights for computing the output luma channel. The next sentence says the calculation from the camera output signals to a luminance signal (Y) is provided in colour encoding specification. If sRGB, IEC 61966-2-1 shall be used.
ISO/TS 19567-2 Photography — Digital cameras Part 2: Texture analysis using stochastic pattern	IEC 61966-2-1	There are no normative references for this technical specification [11]. The text mentions IEC 61966-2-1 (sRGB) and specifies the luminance calculation in Formula 1.
Kelly and Keelan (2005) ISO 12232 revision: determination of chrominance noise weights for noise-based ISO calculation	ITU-R BT.709-1	Reference [12] Equation (2) uses the ITU-R BT.709-1 weights. No standard is listed.

Deprecated Standards

The following standards are deprecated. Unless compatibility with a legacy standard is needed, using the active version of the standard is recommended.

Standard/Paper	Luminance Weights	Notes
ISO 15739:2013 Photography — Electronic still-picture imaging — Noise measurements	Conflicting Instructions	The standard [13] lists ITU-R BT.709-5 [16] as a normative reference, which has the same weights as ITU-R BT.709-6. Formula (1) specifies “ITU-R BT.709” (no version) and uses the ITU-R BT.709-1 weights.
ISO 15739:2017 Photography — Electronic still-picture imaging — Noise measurements	Conflicting Instructions	Reference [14] lists ITU-R BT.709-5 [16] as a normative reference, which has the same weights as ITU-R BT.709-6. Formula (1) specifies “ITU-R BT.709” (no version) and uses the ITU-R BT.709-1 weights.

Standard/Paper

Luminance Weights

Notes

ISO 15739:2013 Photography — Electronic still-picture imaging — Noise measurements

Conflicting Instructions

The standard [13] lists ITU-R BT.709-5 [16] as a normative reference, which has the same weights as ITU-R BT.709-6.

Formula (1) specifies “ITU-R BT.709” (no version) and uses the ITU-R BT.709-1 weights.

ISO 15739:2017 Photography — Electronic still-picture imaging — Noise measurements

Conflicting Instructions

Reference [14] lists ITU-R BT.709-5 [16] as a normative reference, which has the same weights as ITU-R BT.709-6.

Formula (1) specifies “ITU-R BT.709” (no version) and uses the ITU-R BT.709-1 weights.

Settings

Classic Imatest

The luminance weights can be set from:

Calculation Settings window in the new interface
Options III in classic Imatest

The weights are controlled across all modules via the [imatest] lum_coeffs INI Key. The value in this key is a four-element numeric entry. The first 3 elements are the red, green, and blue weights. The fourth element is an index.

The following options are available:

Name	Red Weight	Green Weight	Blue Weight	Ini Index	Note(s)
NTSC Rounded	0.30	0.59	0.11	1	This is ITU-R BT.601-7 rounded to 2 decimal places.
ITU-R BT.709-1	0.2125	0.7154	0.0721	2
ITU-R BT.601-7	0.299	0.587	0.114	3
ITU-R BT.709-6 or IEC 61966-2-1	0.2126	0.7152	0.0722	4
ITU-R BT.2020-2	0.2627	0.678	0.0593	5

New Settings

The new settings apply to the following modules:

Stray Light, as part of the channel selection settings

The luminance weights are saved in the channel_lum_type INI key. The value is a single number index.

The following options are available:

Name	Red Weight	Green Weight	Blue Weight	Ini Index	Note(s)
IEC 61966-2-1	0.2126	0.7152	0.0722	1
ITU-R BT.601-7	0.299	0.587	0.114	2
ITU-R BT.709-6	0.2126	0.7152	0.0722	3
ITU-R BT.2020-2	0.2627	0.678	0.0593	4
MATLAB rgb2gray	0.298936021293775	0.587043074451121	0.114020904255103	6	This uses MATLAB’s `rgb2gray` function [15]. When rounded to 3 decimal places, these weights are the same as ITU-R BT.601-7.
Custom	User-Specified	User-Specified	User-Specified	7	This allows the specification of a custom set of RGB weights.

Custom weights are saved in the channel_lum_custom_rgb_weights INI Key. The value is a three-element array in square braces corresponding to red, green, and blue weights.

References

[1] https://en.wikipedia.org/wiki/Luma_(video) [Accessed April 21, 2025]

[2] https://en.wikipedia.org/wiki/Relative_luminance [Accessed April 21, 2025]

[3]ITU-R BT.609-7. Studio encoding parameters of digital television for standard 4:3 and wide-screen 16:9 aspect ratios. Available from https://www.itu.int/rec/R-REC-BT.601.

[4] ITU-R BT.709-1. Parameter values for the HDTV standards for production and international programme exchange. Available from https://www.itu.int/rec/r-rec-bt.709.

[5] ITU-R BT.709-6. Parameter values for the HDTV standards for production and international programme exchange. Available from https://www.itu.int/rec/r-rec-bt.709.

[6] IEC 61966-2-1. Multimedia systems and equipment – Colour measurement and management – Part 2-1: Colour management – Default RGB colour space – sRGB. Available from https://webstore.iec.ch/en/publication/6169.

[7] ITU-R BT.2020-1.Parameter values for ultra-high definition television systems for production and international programme exchange. Available from https://www.itu.int/rec/R-REC-BT.2020.

[8] ISO 12232:2019 Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index. Available from https://www.iso.org/standard/73758.html.

[9] ISO 15739:2023 Photography — Electronic still-picture imaging — Noise measurements.

[10] ISO 18844:2017. Photography — Digital cameras — Image flare measurement. Available from https://www.iso.org/standard/63552.html.

[11] ISO/TS 19567-2 Photography — Digital cameras Part 2: Texture analysis using stochastic pattern.

[12] Kelly and Keelan “ISO 12232 revision: determination of chrominance noise weights for noise-based ISO calculation”, Proc. SPIE 5668, Image Quality and System Performance II, (17 January 2005); https://doi.org/10.1117/12.582447.

[13] ISO 15739:2013 Photography — Electronic still-picture imaging — Noise measurements.

[14] ISO 15739:2017 Photography — Electronic still-picture imaging — Noise measurements.

[15] MATLAB rgb2gray https://www.mathworks.com/help/matlab/ref/rgb2gray.html [Accessed April 21, 2025]

[16] ITU-R BT.709-5. Parameter values for the HDTV standards for production and international programme exchange. Available from https://www.itu.int/rec/r-rec-bt.709.

[17] IEEE Std. 2020-2024. IEEE Standard for Automotive System Image Quality. Available from https://standards.ieee.org/ieee/2020/6765/.

Documentation – Alpha v25.1

Luminance