I 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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2 Answers 2


These two lines can be used to detect color blindness in which red cannot be distinguished from green (deuteranopia). The person tested, often a child in that case, must be able to name the colors of the lines. (Reference: Ellen M. Chiocca, Advanced Pediatric Assessment, p 314):

Otherwise, it is also certainly a convenience to allow the test taker to say which line they are comfortable with, e.g., I can read the letters just above the red line but not the ones under it.

  • I thought it could also mean like eye sight is weak if you can't read below red line that is why colored red. And if you are above it, you are good, hence green.
    – Vikas
    Jul 2, 2022 at 5:16

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

  • 2
    I think the OP is not asking about duochrome tests, but specifically about the green and red lines in the Snellen chart, as shown in the picture
    – 今夜九
    Jul 1, 2022 at 17:45
  • @今夜九 Yes I mean same.
    – Vikas
    Jul 1, 2022 at 17:53

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