Turn off ANY light around your monitor. If there is a wall behind it that reflects ambient light, turn of those lights too. Make it as dark as possible. Even obscure or turn of any bright white displayed objects on the screen. Your eyes tune their sensitivity to the brightest spot they can see. So it kills your ability to detect smal changes in intensity in the darker areas on the screen, if there are bright elements in your view. Hit the Del key (keypad .) to remove the user interface overlay.
To make it realistic, only have a red light nearby to allow you to see your keyboard. But that could also distort your perception of yellow/orange.red colors on flags.
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