Image descriptions help those with visual impairments understand the meaning of an image. This description is often labeled alt text or alternative text.

When a screen reader reaches an image, it will read aloud the description that has been added. The user gets a sense of what that image is and its purpose in context.

Writing Helpful Alt Text

When authoring alt text, consider the context in which the image is being used. Your goal is to convey in words (two words, at a minimum) the information another user might gain visually. How would you describe the image to someone on the phone?

Professor Parno talks to students at a wooded dig site in Concord, Massachusetts

This image could be described in many ways, including:

As the author of the document or web page, it is your job to determine which alt text is the most useful in each case. In addition, more detailed descriptions can entice the visitor to read the article (if applicable). What do you know about the image and why is it on the the page?

Alt Text vs. Captions

Group of undergraduates dressed in caps and gowns standing and smiling in Gosman.All smiles on a momentous graduation day.

Authoring alt text is similar to writing a caption for an image, but the two are different. An image caption is text that displays on the screen. Alternate text is read aloud to someone using assistive technology, but is hidden from a sighted user.

If your image will also have a caption, consider including more descriptive detail of what you see in the picture in the alt text compared to the text in the caption. A screen reader will read both the alt text and the caption, and redundancy can be frustrating to the user.

In the example here, the alt text for the image is "Group of undergraduates dressed in caps and gowns standing and smiling in Gosman." You can see how that is different from the caption below the image, "All smiles on a momentous graduation day."