Eye read that in eye tests red and green colors are used. For example:

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But in Snellen/Eye chart, these are not background colors. These are lines only. What is the purpose of it?

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Its for a 'Red-Green duochrome test'.

A primary task of the eye care professional is determining the refraction, or optical correction, of a patient. The duochrome red-green test is a standard tool for verification of the final refraction. Traditionally, it is recommended for use both prior to and subsequent to determining the cylindrical or astigmatic component of the refraction.

Red-Green Duochrome testing

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