The test image needs pixel accurate display. Use Wikipedia picture attribute frame and web browser zoom 100%. The average lightness of the 3%/97% lightness checkered area is integrated in the observers eye to 50% lightness. The non-checkered area lightness is 0.5^(1/gamma). If the system gamma (operating system, graphics card, monitor) is equal to the test image gamma, checkered and non-checkered areas have the same lightness.

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