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Re: [csswg-drafts] [css-color-4] sRGB doesn't really use 80 cd/m^2 white luminance (#3435)

From: Joshua Krämer via GitHub <sysbot+gh@w3.org>
Date: Thu, 01 Jul 2021 17:45:12 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-872434757-1625161510-sysbot+gh@w3.org>
The US Department of Energy (DOE) recommends an S-shaped curve to adjust TV luminance depending on ambient illumination:
![DOE](https://user-images.githubusercontent.com/10373011/124156118-8323a400-da97-11eb-8218-af00d332ef60.png)
(https://www1.eere.energy.gov/buildings/appliance_standards/pdfs/tv_tpnopr_room_illuminance_abc_031912.pdf)

The luminance levels for dark room conditions are based on a recommendation by the Imaging Science Foundation (ISF), while the luminance levels for brighter conditions are based on a japanese study from 2010 on preferred luminance by Kishimoto et al. (https://www.jstage.jst.go.jp/article/itej/64/6/64_6_881/_pdf/-char/en). That study evaluated preferred luminances depending on ambient illuminance, angle of view and average picture luminance. Tested ambient illuminances were 30 lx, 100 lx and 300 lx. Subjects were divided by age into 24 young subjects (mean age 22) and 24 old subjects (mean age 71). Separate regression analyses for the two age groups lead to the following formulas:

Young: log PL = 2.40 + 0.27 log E - 0.22 log SA - 0.32 log AL
Old: log PL = 2.87 + 0.13 log E - 0.34 log SA - 0.21 log AL
PL: preferred display luminance (cd/m²), E: ambient illuminance (lx), SA: horizontal angle of view (°), AL: average picture luminance (% of max)

According to the model, at an illumination of 100 lx, a viewing angle of 20° and an average picture luminance of 25 %, the preferred luminance for young people is 161 cd/m², and for old people it is 248 cd/m². The DOE curve is probably based on a mean of values for young and old people.

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