How do you write alt text for a digital lesson image?
Alt text for a digital lesson image is not a label describing every pixel. It makes the information that the image contributes to a particular learning task available in words. The same image can call for a different description in a different activity.
Identify the image’s role in the lesson
1. What information does the image carry? Start by stating what the learner is expected to do with it: identify a leaf part, interpret a trend in a graph, or merely see a decorative layout. W3C’s images tutorial recommends choosing the text alternative according to the image’s purpose in context. “Picture of a plant” is insufficient for a labelled biology drawing. Conversely, giving every inference needed to solve the question may answer the task for the student. Read the learning objective alongside the assessment prompt. The core of the alternative is the information a learner would lack if the picture were absent. This is a decision about instruction, not simply a property of the image file. 2. How do informative and decorative images differ? An arrow that shows the order of steps has a function; a colored border pattern may only decorate the page. An informative image needs a concise, specific explanation. For a purely decorative image, an empty alternative (`alt=""`) prevents a screen reader from announcing an irrelevant filename. Yet the same icon becomes functional when it is a button: its accessible text should convey an action such as “continue.” Repeating nearby prose word for word can also add listening burden without adding information. When the distinction is unclear, remove the image temporarily and ask whether the meaning of the lesson changes. Context, rather than visual complexity alone, determines the answer. 3. What belongs in a graph or diagram description? For a line chart, identify the axes and units, time span, compared series, and the pattern the learner needs to understand. “Graph” or “blue line” does not give access to the idea. A simple diagram can fit in short alt text. Complex data usually need a nearby table or longer description as well. The short alternative introduces the visual while the extended explanation makes the underlying information available; neither automatically replaces the other. If that explanation reveals an answer that students are meant to derive, redesign the question so all students face an equivalent reasoning task. Preserve access and the intended cognitive demand together.
Writing, implementation and learner testing
4. How can a sentence be concise yet sufficient? Opening with “image showing” is generally redundant because the screen reader already announces the image element. Use the lesson’s terminology accurately and avoid inventing unseen intentions or certain outcomes. Describe observable information, such as three students arranged from left to right at a laboratory bench; put any scientific interpretation in separately justified prose. Include numbers and direction when they matter. There is no universal word limit that determines a good alternative. The aim is to convey the image’s contribution to this task. If one sentence cannot do that, expand the explanation around the image rather than stuffing a complex dataset into an unwieldy attribute. 5. Where is alt text entered and maintained? On a web page, a meaningful image’s `alt` attribute travels with the image element. Slide and document editors have their own alternative-text fields. Do not leave a filename as the automatic description. Important words inside an image should also appear in accessible text. If an image acts as a link, describe the link destination or action rather than its decoration. A teacher or author should review automatically generated captions: a model might misidentify a laboratory instrument. Check that exported formats preserve the alternative after a slide or document is converted, and update it whenever the lesson image or task changes. A correct entry in an editor is not proof that the final distributed file remains accessible. 6. How is quality tested, and what remains outside its scope? Hide the image and listen to the page in reading order with a screen reader. Does the learner receive the information needed to do the task, and are they forced through needless repetition? Voluntary testing with blind learners can provide stronger feedback, while one person’s preference should not be treated as a universal rule. Alt text alone cannot repair a badly structured table, weak color contrast, or an activity operated only by dragging. Review the whole material flow and revise the description as the course content changes. The image and its text together should serve the actual learning objective rather than merely satisfy a form field.