How to Write Accessible Alt Text for Academic Images

Alt text for a stock photo on a marketing website is straightforward. Alt text for a complex biochemistry diagram, a statistical chart, or a historical map is not. Academic images present unique challenges because they often carry information that is essential to understanding the course material.

This guide covers the principles and specific techniques for writing effective alt text in an academic context.

The Core Principle

Alt text should convey the same information to a screen reader user that a sighted student gets from looking at the image. Not a description of what the image looks like — a description of what it communicates.

Ask yourself: if a student could not see this image, what would they need to know to understand the lecture or complete the assignment?

Charts and Graphs

For data visualizations, describe the conclusion the chart supports, then provide the key data points.

Instead of: "Bar chart"

Write: "Bar chart showing student enrollment by department from 2020 to 2025. Computer Science grew from 1,200 to 2,800 students, the largest increase. English declined from 900 to 650. All other departments remained stable within 10 percent."

For complex charts with many data points, provide a brief summary in the alt text and link to a data table with the full dataset.

Diagrams and Flowcharts

Describe the process or relationship the diagram illustrates, not the shapes and arrows.

Instead of: "Diagram with boxes and arrows"

Write: "Flowchart of the scientific method. Steps: Observation leads to Question, which leads to Hypothesis, then Experiment, then Analysis. Analysis leads to either Conclusion (if hypothesis supported) or back to Hypothesis (if not supported)."

Mathematical Equations

For simple equations, write them out in words or use standard notation:

Instead of: an image of the quadratic formula

Write: "The quadratic formula: x equals negative b plus or minus the square root of b squared minus 4ac, all divided by 2a"

For complex equations, use MathML markup when your platform supports it. MathML provides semantic structure that screen readers can interpret, which is far more effective than alt text for mathematical notation.

Scientific Figures

Lab photos, microscopy images, and scientific illustrations require context-specific descriptions:

Instead of: "Microscope image"

Write: "Light microscopy image of human red blood cells at 400x magnification. Cells appear as biconcave discs approximately 7 micrometers in diameter, stained pink with Wright stain. Three white blood cells are visible, identifiable by their larger size and purple-stained nuclei."

Include the clinically or educationally relevant observations, not just what is visible.

Maps and Spatial Data

Describe the geographic relationships and patterns the map shows:

Instead of: "Map of migration patterns"

Write: "Map of the United States showing internal migration patterns from 2020 to 2025. Arrows indicate net population movement from the Northeast and Midwest toward the Southeast and Southwest. Texas, Florida, and Arizona show the largest net gains, while New York, Illinois, and California show the largest net losses."

When Alt Text Is Not Enough

Some images carry so much information that alt text alone cannot adequately describe them. In these cases:

• Provide a brief summary as alt text (two to three sentences)
• Add a detailed long description in the surrounding text, a linked document, or a "long description" field
• For data visualizations, always provide the underlying data table as an alternative

Decorative vs Functional Images

Not every image needs alt text. Decorative images — visual flourishes, background patterns, or stock photos used for aesthetics — should be marked as decorative (empty alt attribute) so screen readers skip them entirely.

The test: if you removed this image, would any information be lost? If not, it is decorative.

Common Mistakes

• Writing "image of" or "picture of" — screen readers already announce that it is an image
• Describing colors without providing the information those colors convey
• Writing alt text that is too long (aim for under 150 words; use long descriptions for complex images)
• Copying the figure caption as alt text — captions and alt text serve different purposes
• Using OCR text from a scanned image as alt text without editing for clarity

Making It Sustainable

Writing good alt text takes practice but not much time once you develop the habit. The key is to write alt text when you create the content, not retroactively. When you are building a lecture and adding an image, spend 30 seconds writing the description while the context is fresh in your mind.