The visual system actively boosts contrast at luminance step edges, producing Mach bands

What it is

In a pattern where luminance changes in discrete steps — say, several equal-width gray bands each slightly brighter than the last — a narrow strip right next to each luminance jump looks brighter than the band itself on the lighter side, and darker than the band itself on the darker side, even though a photometer shows the actual physical luminance is perfectly uniform within each band. This "subjectively boosted or suppressed edge" illusion is called Mach bands, named after the physicist Ernst Mach, who first described it systematically. It is not a display defect or an artifact introduced by rendering or printing — it is a genuine, repeatable subjective enhancement the visual system applies to luminance step edges.

Why it happens

The visual system does not simply read out the luminance at each retinal location and report it as-is; it applies an active, edge-detecting layer of processing to luminance patterns — at locations where luminance changes, this layer amplifies the change signal itself, while in flat, unchanging regions the output settles and is no longer specially emphasized. Mach bands are the trace this active processing leaves at a step edge: the process itself overshoots near the boundary, pushing the brighter side even brighter and the darker side even darker, so the edge looks steeper and higher-contrast than the two bands actually are on either side. This is not a physical effect from the optical system (lens blur, backlight diffusion in a display, ink bleed in print) — optical effects only soften and blur an edge, whereas Mach bands do the opposite: they make an edge look sharper and more contrasty than it physically is.

Studying it

• Staircase-grating and ramp-stimulus paradigms: presenting a staircase pattern of equal-width bands with steadily increasing luminance, or a continuous linear luminance ramp flanked by uniform patches, and having participants report or adjust a matching display to reproduce the perceived luminance profile, then comparing it against the instrument-measured physical profile — the gap between the two characterizes the strength and width of the Mach bands.
• Typical independent variables: the magnitude of the luminance step, band width (which corresponds to spatial frequency on the retina), and the steepness of the ramp.
• Typical dependent variables: the magnitude of deviation between perceived and physical luminance profiles, the detection threshold for the illusory band, and the reported width of the bright/dark fringes.
• Use in interface research: this paradigm is often borrowed to test whether a given rendering or encoding scheme produces more subjective contrast than intended at a step or near-step edge.

Where it stops holding

• Mach bands require the spatial scale of the luminance change to fall within the range the visual system's edge-detecting processing is sensitive to: a transition that is too gradual (spanning a large visual angle) or too steep and dense (beyond acuity's resolving power) weakens or eliminates the effect — not every luminance transition produces a noticeable Mach band.
• Effect strength depends on viewing distance and display size, since both jointly determine the spatial frequency the luminance change corresponds to on the retina; the same pattern viewed from a different distance can show a very different degree of banding.
• This entry concerns the luminance (achromatic) channel specifically, not hue or saturation; whether a similar subjective enhancement occurs at edges defined purely by hue change is a separate question that this conclusion cannot be assumed to cover.

Related

• Same group: A1.31.2 Edge enhancement comes from lateral inhibition, not a change in physical luminance · A1.31.3 Mach banding in smooth gradients gets misread as color banding or posterization defects · A1.31.4 Adding smoother transitions or dither noise weakens edge-enhancement artifacts
• Nearby: A1.04 Contrast Sensitivity and Spatial Frequency Channels · A1.02 Foveal vs. Peripheral Vision Division of Labor
• Search terms: Mach bands · edge enhancement · brightness overshoot