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Rec709 and sRGB

Author Steve

#1 | Posted: 6 Oct 2016 10:36 | Edited by: Steve 
With the latest release of LightSpace we have changed both the Rec709 and sRGB target profiles for Display Calibration.

This has been done for two different reasons.

Rec709 has been changed to a pre-set Gamma target of 2.4 (previously 2.2).
This change has been made as the industry has slowly been moving to a gamma of 2.4 for Rec709, based on attempting to match the underlying 2.4 gamma for BT1886.
We do not necessarily agree with this, as on any display with a black level that is not close to zero BT1886 generates a system gamma that is closer to 2.2, rather than 2.4. But, the likes of the EBU have decided to ignore this fact, and have now specified a 2.4 gamma in EBU Tech 3320.
So, we have decided to follow this standard - as any user can adjust the Rec709 pre-set within LightSpace to any gamma target they want, and re-save as a new pre-set.

sRGB is a bit more interesting, as even we had this incorrect within LightSpace!
There is a misconception that the sRGB colour space standard has a compound gamma, with a linear portion close to black.
This is incorrect, as this is the sRGB Encoding standard, not the target colour space standard for display calibration.
For display calibration the target colour space should use a simple power law gamma, 2.2.

This is the Encoding standard for sRGB: x <= 0.0031308 ? x * 12.92 : (1.055 * pow(x, 1/2.4)) - 0.055
(As defined in the IEC 61966-2-1:1999 Standard for sRGB.)
And this is the Encoding standard for Rec709: x < 0.018 ? (x * 4.5) : 1.099 * pow( x, (0.45) ) - 0.099
(As defined in the ITU-R Recommendation BT.709)

(See: http://www.lightillusion.com/lut_manual.html#maths)

Both have a linear portion close to black, with different overall system gamma.

For display calibration it is incorrect to use these standards, and pure power law gamma should be used, as is now the standard colour space pre-sets within LightSpace.

Steve Shaw
Mob Boss at Light Illusion

Author tsanga
#2 | Posted: 20 Feb 2021 01:11 | Edited by: tsanga 

I was about to post a question on this, but a quick search uncovered exactly the information I was looking for regarding Rec709 vs sRGB. Thank you for making it so clear.

When I see the Rec709 encoding equation, it becomes clear that calibrating sRGB to the encoding transfer function would be treating these two inconsistently. Now I'm somewhat alarmed that other calibration software continue to reflect a compound curve target for sRGB presets.

Author Steve

#3 | Posted: 20 Feb 2021 09:24 | Edited by: Steve 
Indeed, that is true.
A number of display calibration systems have this wrong, as do many PC monitor manufacturers.

There is another very simple point to remember about sRGB.
When sRGB was developed, all displays were CRTs, with fixed gamma that could NOT be adjusted.
And the gamma was a power law gamma, and at that time thought to be 2.2 (but later found to be closer to 2.4).

Therefore, any sRGB image expects to be displayed on a display (historically a CRT) calibrated with a 2.2 power law gamma.
It really is that simple.

Steve Shaw
Mob Boss at Light Illusion

Display Calibration Light Illusion Forums / Display Calibration /
 Rec709 and sRGB

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