What is Alt Text?

A picture is worth a thousand words—but what if someone can’t see that picture? Images are inaccessible to individuals who are blind unless a description is provided. The best way to provide a description for most images in the digital world is through the use of alternative (or “alt”) text.

Alt text is a “behind-the-scenes” description of an image. It’s not typically visible in a document or webpage; it is read out to people using screen reading software. When alt text has been provided for an image, a screen reader user will typically hear “image” or “graphic”, followed by the alt text (e.g., “graphic, CU Boulder logo”). Screen reading technology is unable to interpret or describe images, so blind users will not be able to access the content of an image if alt text has not been provided by the content creator.

How Do I Create Alt Text?

The process to add alt text to an image will differ depending on the software in which you are creating content. However, there are some general guidelines that you can follow regardless of which platform you’re using.

Alt text should be concise and only as descriptive as necessary to convey the information contained in the image. A general rule of thumb for length is 1-2 sentences. When there is text in an image, you should make sure the image text is provided in the alt text.

If a longer description is needed, that description should be provided in the main body of the document or in supplementary documentation. Long descriptions are often needed for complex diagrams such as graphs and flowcharts. If you provide a long description elsewhere, you should still provide alt text (e.g., “diagram of Krebs Cycle; longer description to follow”).

Some technical diagrams can be challenging to describe. If you are unsure of how to describe a complex chart or image, the DIAGRAM Center provides helpful recommendations on how to describe a variety of types of images, from geographic maps to Venn diagrams.

The only time an image should not have alt text is if it is purely decorative. A decorative image is one that is only used for visual appeal or is redundant given the content of the text. (For example, a horizontal line used to separate two sections of text typically does not need alt text.) If this is the case, then null alt text should be used. Some platforms have a checkbox you can select to indicate if an image is decorative. (If you’re editing the HTML directly, you would just use alt=""). When a screen reader reaches an image that has null alt text, it does not announce the image at all.

If possible, it is best to have the content creator write the alt text, since they have the most complete understanding of the purpose of that image in context and the necessary subject matter expertise.

Writing alt text can be more of an art than a science. Ask yourself why the image is important in the context of the document, and let that guide your decisions on which details to include in the description. It is best to put the most important idea at the beginning of the alt text. Because screen readers announce that the content is an image, you don't need to start alt text with "image of...". Lastly, avoid providing subjective interpretation; just describe what is visually presented in a neutral tone.

Try it out, and let us know if you encounter any questions along the way!

January Challenges

  • Pick a document or web page you manage that contains images. Add alt text to the images, or review the existing alt text for accuracy and quality.
  • Pick one complex image or diagram that you use in your work and write a longer description for it. Feel free to send it to us for review and feedback!

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