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

ID: G17

Technology: general

Type: Technique

## When to Use

Any technology that produces visual output.

## 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.

## 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

     - L1 is the relative luminance of the lighter of the foreground or background colors, and
     - L2 is the relative luminance of the darker of the foreground or background colors.
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.

## Related Techniques

- G148
- G174

## Resources

- TPGi Colour Contrast Analyser (CCA) – Application
- Contrast Ratio Analyser - online service
- Color Contrast Samples
- Atypical colour response
- Colors On the Web Color Contrast Analyzer
- Tool to convert images based on color loss so that contrast is restored as luminance contrast when there was only color contrast (that was lost due to color deficiency)
- List of color contrast tools