Adjacent colors at exactly equal light intensity lose their visible boundary — the edge vanishes

The opposite of vibrating boundaries: when two adjacent colors have exactly the same light intensity (lightness/value), the boundary between them disappears perceptually even if their hues differ. Albers calls this ‘equal light intensity’ (deliberately avoiding the misused ‘equal value’) and considers it the rarest and most exciting color phenomenon. It is almost never achieved in practice — it demands extreme precision: most people claiming ‘equal value’ have never actually seen it. It occurs naturally on cumulus cloud edges against blue sky, where shaded cloud grey equals the sky blue in luminance. In digital contexts this is precisely calculable: set the L* channel equal in CIE Lab space, vary only hue and chroma.

Examples

Cumulus cloud underside (grey) merging with blue sky at equal luminance: you cannot see where cloud ends and sky begins. In GLSL: set two adjacent colors to equal CIELAB L* with different hue angles — the boundary vanishes.

Assessment

Using a digital color tool, find two colors of different hues with identical L* in CIE Lab. Place them adjacent and verify the boundary vanishes. Then explain why setting equal RGB average brightness would not reliably produce the same result.