Alt text is not a second caption. The caption names and credits the figure for everyone; the text alternative conveys its function when the image cannot be seen. Repeating only “Figure 3” or a filename leaves the essential content unavailable.
Start with meaning rather than pixels
Ask what a reader would miss if the image failed to load. In a historical portrait analysis, posture, dress or photographic setting may matter. In a logo comparison, the relevant information is the difference in form. In a bar chart, the axis, categories, values and trend discussed in the prose are central. Context decides which visual properties belong in the description.
Phrases such as “image of” are often unnecessary because assistive technology already announces the element as an image. Begin with the content. Stay concise where one sentence communicates the purpose. When relationships, multiple values or spatial structure need explanation, separate the short identification from a fuller account.
Complex charts need two descriptive levels
The W3C recommends a short identification and a more complete textual equivalent for complex images. The short alt text can state the chart type, subject and headline finding. A long description records axes, units, significant values, relationships and exceptions. When a chart contains many numbers, a real table is usually more accurate and navigable than an extended paragraph.
“Values rise” is not enough. Give the period, scale and relevant break where those details support your interpretation. Do not introduce a causal conclusion that the visual cannot establish. If the surrounding prose already explains all essential values, the alt text may point readers to that section, but the destination must be clear and easy to find.
Three figures require three different solutions
A process diagram
“Flow chart with arrows” is too vague. A more useful alternative is: “Four-stage review from source capture through citation matching to final approval; a missing source sends the process back to research.” A structured long description can then explain each decision in sequence.
A bar chart
The short description states the useful finding: “Bar chart: documented citation errors decrease across three drafts, with the largest reduction between drafts one and two.” An adjacent table provides the actual values. Readers can therefore test the summary rather than accepting an inaccessible interpretation.
A historical photograph
If the thesis examines spatial arrangement, describe that arrangement. Do not guess identity, ethnicity, mood or cause. The caption records photographer, subject, date, collection and source. Historically sensitive labels can be reported accurately and discussed critically in the prose.
Test the document without looking at the figure
Hide the image or listen to the final file with text-to-speech. Can you still understand the proposition? Can you find the detailed data and source? Is the long description structured with navigable headings and lists? A table needs genuine row and column headers; an image of a table remains inaccessible.
Check contrast, colour coding and reading order as well. Do not distinguish results only with red and green. Add patterns, symbols or direct labels. Export to the actual submission format and inspect it again, because PDF tags and alternative text can disappear during conversion even when the source document was correctly prepared.
Accessibility does not replace attribution
Alt text explains function; the caption and reference trail establish provenance. Use the figure-list source audit to connect those records. The media citation hub covers specialised visual cases, while our process overview explains the separate role of a plagiarism check.