Textbook of Usability

Weber's Law, formulated by the nineteenth-century German physiologist Ernst Heinrich Weber, states that the just-noticeable difference (JND) in a stimulus is a constant proportion of the baseline intensity:

$$\frac{\Delta I}{I} = k$$

where $\Delta I$ is the smallest detectable change, $I$ is the baseline intensity, and $k$ is a constant (Weber's fraction) specific to the sensory modality.

Applied to vision, Weber's Law describes why contrast is what matters, not absolute brightness. Text that is perfectly legible on a bright display in a dim office may become invisible outdoors in sunlight — not because the display got dimmer but because the ambient light raised the reference level.

Design implications:

• Contrast ratios (not absolute colours) determine legibility
• WCAG specifies minimum contrast ratios (4.5:1 for normal text, 3:1 for large text) derived from contrast sensitivity research
• Ageing reduces contrast sensitivity, so designs should exceed the minimum where possible
• Proportional changes are more perceptible than absolute ones: a 10% volume increase is noticeable regardless of starting volume

Weber's Law is one of the earliest quantitative laws in psychology and remains a foundational principle in visual and auditory perception.