Alt text for authors

Images are an important way of communicating data in a user-friendly format. But did you know that unless an image also has a textual description, it is not accessible to all readers? Alternative text, or alt text, is a textual image description.

It communicates the important visual information and function of the image to people who use assistive technology, such as read-aloud screen readers.

It also enhances content discoverability because alt text is indexed by search engines.

With the help of alt text, everyone can equitably access and understand visual content.

The length of alt text can be short or long depending on the complexity of the image. For example, there are simple images that can be described in under 200 characters.

These only need a short alt text, approximately 20-30 words.

Complex images such as anatomical illustrations or populous flow charts cannot be described meaningfully with short text.

For these images, the alt text is accompanied by a long description, which gives an extended, detailed equivalent to the image.

Now, many figures have captions which are also written texts associated with the image. However, there are key distinctions between alt texts and figure captions, and they can’t be used to replace each other.

Here’s an example of an image and its figure caption: