One white balance across a scene lit by two lamps
Drawn at its defaults, in what a scene does to a spectrum. It takes no options at all, so every essay calling it gets this exact drawing.
Called by 3 essays
the blast radius of changing it
The highlight is the lamp
Light reflected at the surface of a glossy material never entered it, so it never met a pigment. Fresnel's reflectance varies by 5.5% across the visible band and the paint underneath varies by a factor of twenty — which makes a highlight the one region of a photograph that reports what the light was rather than what the object is.
What a scene doesA scene has no white point
White balance is one transform applied to a whole image, and a scene lit by two lamps has no single answer for that transform to be. The failure is structural rather than a matter of a better estimator — and every colour-managed workflow in existence takes exactly one white point as an input, with no field in which to say there were two.
What light isTwo lamps do not average
Light adds, band by band, and every number a lamp is sold by is a projection of the sum rather than a sum of the projections. Two radiators sitting exactly on the Planckian locus mix into a light that is measurably pink; two poor lamps mix into a better one than either.