'Alt text' is a contraction of 'alternative text.'
Alternative text for any non-text content, usually images, is provided so that the non-text content can be changed into other forms people need, such as large print, braille, speech, symbols, or simpler language.
Most of us are prompted for alt text whenever we add a picture to some form of a written report or a PowerPoint slide. Few technical professionals use alt text consistently.
Standard practice is that the alt text should not be longer than a sentence or two. Alt text should provide the key message and idea of the non-text content. Do not refer to the image of the surrounding texture, such as "a picture of." Stick with the key message.
Remember, web search engines use the alt text when they crawl your articles, papers, and website in search of keywords. The absence of alt text hampers being found on the internet, and Google uses the presence and quality of alt text in its ranking criteria.
Some applications also use image file names as a form of alt text. This points to the importance of another unappreciated aspect, image file names, but underscores the need to get the alt text correct in the first place.
Alt text is part of Structure in the FINESSE fishbone diagram. It also touches on Illustrations, Noise reduction, and Empathy in the trademarked mental model. Remember to do your alt text and do it well!
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