Description
A luminance channel is a linear weighted combination of an image's linear red, green, and blue channels [1][2]:
A luma channel is a linear weighted combination of an image's gamma-encoded red, green, and blue channels:
Luminance Standards
The following standards define luminance weights.
| Luminance Standard | Red Weight | Green Weight | Blue Weight | Reference(s) | 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 | [6] Item 3.2 | This is replaced by ITU-R BT.709-6. |
| ITU-R BT.709-6 | 0.2126 | 0.7152 | 0.0722 | [7] Item 3.2 | These are the same weights as IEC-61966-2-1. |
| ITU-R BT.2020-2 | 0.2627 | 0.678 | 0.0593 | [8] Table 4 | This is sometimes referred to as UHDTV. |
| Display P3 (D65) |
0.22897 |
0.69174 |
0.07929 | [9], [10] Eq G-7 |
Choosing Luminance Weights
There are many standards and papers that 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 [11] 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 [12] 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 [13] lists the following normative references:
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 [14] 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[15]. 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 [16] 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 [17] lists ITU-R BT.709-5 [6] 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 [18] lists ITU-R BT.709-5 [6] 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 Value | 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 | |
| Display P3 (D65) |
0.22897 |
0.69174 |
0.07929 | 6 |
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 Value | 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 [19]. When rounded to 3 decimal places, these weights are the same as ITU-R BT.601-7. |
| Display P3 (D65) |
0.22897 |
0.69174 |
0.07929 | 10 | |
| 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
- “Luma (video).” Wikipedia. 2025. https://en.wikipedia.org/wiki/Luma_(video). ↩
- “Relative luminance.” Wikipedia. 2025. https://en.wikipedia.org/wiki/Relative_luminance. ↩
- Studio Encoding Parameters of Digital Television for Standard 4:3 and Wide-Screen 16:9 Aspect Ratios, ITU-R BT.601-7. [Online]. Available: https://www.itu.int/rec/R-REC-BT.601. ↩
- Parameter Values for the HDTV Standards for Production and International Programme Exchange, ITU-R BT.709-1. [Online]. Available: https://www.itu.int/rec/r-rec-bt.709. ↩
- Parameter Values for the HDTV Standards for Production and International Programme Exchange, ITU-R BT.709-6. [Online]. Available: https://www.itu.int/rec/r-rec-bt.709. ↩
- Parameter Values for the HDTV Standards for Production and International Programme Exchange, ITU-R BT.709-5. [Online]. Available: https://www.itu.int/rec/r-rec-bt.709. ↩a ↩b ↩c
- Multimedia Systems and Equipment: Colour Measurement and Management, Part 2-1: Colour Management, Default RGB Colour Space, sRGB, IEC 61966-2-1. [Online]. Available: https://webstore.iec.ch/en/publication/6169. ↩
- Parameter Values for Ultra-High Definition Television Systems for Production and International Programme Exchange, ITU-R BT.2020-1, 2020. [Online]. Available: https://www.itu.int/rec/R-REC-BT.2020. ↩
- Colorimetry of P3 Color Spaces, SMPTE ST 2113. ↩
- Digital Source Processing: Color Processing for D-Cinema, SMPTE EG 432-1:2010, 2010. ↩
- IEEE Standard for Automotive System Image Quality, IEEE Std 2020-2024, 2024. [Online]. Available: https://standards.ieee.org/ieee/2020/6765/. ↩
- Photography: Digital Still Cameras, Determination of Exposure Index, ISO Speed Ratings, Standard Output Sensitivity, and Recommended Exposure Index, ISO 12232:2019, 2019. [Online]. Available: https://www.iso.org/standard/73758.html. ↩
- Photography: Electronic Still-Picture Imaging, Noise Measurements, ISO 15739:2023, 2023. ↩
- Photography: Digital Cameras, Image Flare Measurement, ISO 18844:2017, 2017. [Online]. Available: https://www.iso.org/standard/63552.html. ↩
- Photography: Digital Cameras, Part 2: Texture Analysis Using Stochastic Pattern, ISO/TS 19567-2. ↩
- Kelly and Keelan, “ISO 12232 revision: Determination of chrominance noise weights for noise-based ISO calculation,” in Proc. SPIE Electronic Imaging, 2005, doi: 10.1117/12.582447. ↩
- Photography: Electronic Still-Picture Imaging, Noise Measurements, ISO 15739:2013, 2013. [Online]. Available: http://www.iso.org/iso/catalogue_detail.htm?csnumber=59420. ↩
- Photography: Electronic Still-Picture Imaging, Noise Measurements, ISO 15739:2017, 2017. ↩
- “rgb2gray.” MATLAB Documentation, MathWorks. 2025. https://www.mathworks.com/help/matlab/ref/rgb2gray.html. ↩