Item to check linearity of screen brightness variation

[yrobic]

Graphic display of linearity response for brightness variation.
Get measured brightness value step by step over the entire range.

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[kamrik]

This seems useful. But please be aware that the WALT screen sensor is not calibrated to be linear in any meaningful units, it can also get saturated when used on a very bright screen.

Please submit your CLA so that we can merge the pull request.