How many layers you can have is limited by discriminable lightness gaps

What it is

The mockup lists eight elevations, 0 through 24. On a dark device you can see three layers: ground, card, overlay. The other greys sit below a just noticeable difference, which means they were not designed. How many layers a dark theme can support is capped by discriminable lightness gaps, not by token names.

Why it happens

Weber’s law still roughly holds for lightness: two greys look different only if the relative gap is large enough. A dark theme squeezes usable L* into the low end — from “almost black” to “still a dark surface” is a short interval; go higher and the surface starts to look like a light theme and the dark register is lost. Pack eight stops into that interval and adjacent gaps fall below what the session can tell apart — on top of calibration, ambient light and simultaneous contrast eating the difference.

Light themes can lean on shadow radius to split stops. Dark themes mostly have L*, so the same “eight elevations” collapse earlier. The direction can still be right (higher still lighter); the ruler is too short.

Studying it

Measure adjacent-surface discrimination with a staircase: two dark greys side by side, ΔL* adjusted until observers stably say “different”. Independent variables: absolute L* level, ambient light, viewing distance. Dependent variable: ΔL* at 75% correct. Then take the product’s elevation table and count how many pairs sit below that ΔL*.

Add a condition with other UI in between; side-by-side swatches overestimate how many stops you can have.

Where it stops holding

• Large areas discriminate more easily than small ones; a gap invisible on a narrow menu may show on a full-screen dialog. Set the count by the smallest surface, not by the dialog.
• Stroke and spacing can split layers when lightness is insufficient, but that is no longer lightness elevation — you cannot claim “we still have eight surfaces”.
• High-contrast mode cuts out mid greys, so even fewer stops remain.

Applying it

• Lock three or four dark surfaces that actually separate on device, then merge components onto those stops.
• Do not translate a light-theme eight-stop shadow table into eight L* values one-for-one.
• If a new product wants a fifth layer, ask whether it can replace rather than stack, instead of inserting another 2% white.
• How to check: make equal-size swatches of every elevation surface, shuffle them, and ask for a bottom-to-top sort. Merge stops that will not sort stably, until the sort holds.

Related

• Same group: F5.08.1 In dark mode, height is surface lightness, not shadow · F5.08.2 Higher layers are lighter surfaces
• Nearby: F5.09 Perceptual requirements of palettes · F5.13 Building and maintaining colour scales
• Search terms: just noticeable difference · lightness discrimination · elevation steps