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Technique G17:Ensuring that a contrast ratio of at least 7:1 exists between text (and images of text) and background behind the text

Applicability

Any technology that produces visual output.

This technique relates to 1.4.6: Contrast (Enhanced) (Sufficient).

Description

The objective of this technique is to make sure that users can read text that is presented over a background. This technique goes beyond the 4.5:1 contrast technique to provide a higher level of contrast to make it easier for people with low vision to read.

If the background is a solid color (or all black or all white) then the contrast ratio of the text can be maintained by making sure that each of the text letters have a 7:1 contrast ratio with the background.

If the background or the letters vary in relative luminance (or are patterned), then the background around the letters can be chosen or shaded so that the letters maintain a 7:1 contrast ratio with the background behind them even if they do not have that contrast ratio with the entire background.

The contrast ratio can sometimes be maintained by changing the relative luminance of the letters as the relative luminance of the background changes across the page.

Another method is to provide a halo around the text that provides the necessary contrast ratio if the background image or color would not normally be sufficiently different in relative luminance.

Examples

  • A black background is chosen so that light colored letters that match the company's logo can be used.
  • Text is placed over a picture of the college campus. Since a wide variety of colors and darknesses appear in the picture the area behind the text is fogged white so that the picture is very faint and the maximum darkness is still light enough to maintain a 7:1 contrast ratio with the black text written over the picture.

Other sources

No endorsement implied.

Tests

Procedure

  1. Measure the relative luminance of each letter (unless they are all uniform) using the formula:

    • L = 0.2126 * R + 0.7152 * G + 0.0722 * B where R, G and B are defined as:

      • if R sRGB <= 0.04045 then R = R sRGB /12.92 else R = ((R sRGB +0.055)/1.055) ^ 2.4
      • if G sRGB <= 0.04045 then G = G sRGB /12.92 else G = ((G sRGB +0.055)/1.055) ^ 2.4
      • if B sRGB <= 0.04045 then B = B sRGB /12.92 else B = ((B sRGB +0.055)/1.055) ^ 2.4

      and R sRGB, G sRGB, and B sRGB are defined as:

      • R sRGB = R 8bit /255
      • G sRGB = G 8bit /255
      • B sRGB = B 8bit /255

      The "^" character is the exponentiation operator.

    For aliased letters, use the relative luminance value found two pixels in from the edge of the letter.

  2. Measure the relative luminance of the background pixels immediately next to the letter using same formula.

  3. Calculate the contrast ratio using the following formula.

    • (L1 + 0.05) / (L2 + 0.05), where

  4. Check that the contrast ratio is equal to or greater than 7:1

Expected Results

  • #4 is true

Before May 2021 the value of 0.04045 in the definition was different (0.03928). It was taken from an older version of the specification and has been updated. It has no practical effect on the calculations in the context of these guidelines.

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