Digital Accessibility Essentials: Images
What Is Alt Text?
Alternative text (or alt text) is descriptive text that communicates the meaning or purpose of an image for individuals who may not be able to view the image. Alt text is typically not visible in a document but can be read by screen reader users and displayed if an image fails to load on a webpage.
Images must be described using alt text so that everyone can access the information they convey.
For the image below, an example of appropriate alt text might be “Aerial view of the CU Anschutz campus.”
Decorative Images
The only time an image should not have alt text is if the image is purely decorative.
Decorative images do not add meaningful information and are only used for visual appeal or repeat information already provided in surrounding text. Some tools, like Microsoft Word, allow you to mark an image as decorative instead of providing alt text.
To determine if an image should be marked as decorative, imagine if it was removed from the document or webpage. Ask yourself: Without this image, does the meaning or context of the content change?
No: The image is decorative.
Yes: The image needs alt text.
If the tool you are using does not allow you to mark images as decorative, then you still need to provide alt text.
Images of Text
Avoid using images of text because (unlike digital text) they cannot be enhanced or magnified properly, creating a poor user experience for everyone, including many assistive technology users.
If an image of text cannot be avoided (such as a logo), add alt text to the image.
Why Alt Text Matters
Everyone needs to be able to access the meaningful information provided by images. When alt text is missing, screen readers typically announce “image” or “graphic” followed by the file name. People who rely on screen readers cannot access the content of an image without alt text.
The following is what a screen reader user would hear for this image with and without alt text:
Woman participating in a virtual meeting at a desk.
Image picture100f947e8302864d9a5bfff0a001ce385
How To Write Alt Text
Alt text should be a concise description of one or two sentences that conveys the important visual information in an image.
Alt Text Guidelines
- Be concise and describe only what is important. Focus on the image’s purpose, not every visual detail.
- Aim for no more than approximately 150 characters.
- Do not start alt text with “Image of...”. Screen readers already announce that they are reading alt text for an image.
- Some applications (such as Microsoft PowerPoint and Outlook) may automatically generate alt text for images. Review this alt text and make edits for clarity and accuracy.
Alt Text for Complex Images
Complex images and graphics may require longer descriptions than the 1 or 2 sentences used for alt text.
- If the purpose of a complex image (such as a chart with data) is to convey an overall trend or summarize data, provide the trend or summary information using 1 or 2 sentences of alt text.
- If the complex image (such as a chart with data) conveys specific data points that need to be known, studied or used in an activity, provide a full text description of the data alongside the image. Include alt text for this kind of complex image, for example: “COVID-19 infection rates chart, longer description to follow.”
Images As Course Materials
Ensuring that images used in course materials are accessible can be challenging. This is especially true in advanced fields of study, such as medicine, where complex visuals (like detailed diagrams, charts, and imaging scans) play a critical role in teaching and learning.
Below are examples of real course materials and alt text used at CU Anschutz. The level of detail in the alt text or image description varies based on the image’s purpose in the course.
Alt Text: Papillary renal cell carcinoma histopathology showing fibrovascular cores.
Image Description: Treatment targets to inhibit proteins activated by HIF. HIF activates VEGF and other proteins involved in tumor growth. Normally, VHL would degrade HIF in normoxia but if VHL tumor suppressor function is lost, then HIF and its targets are activated. Inhibiting those targets helps inhibit tumor growth.
Practical Application
Use the following resources to learn how to add alt text in common tools. You can also explore the topic of identity and inclusion in alt text: