Colored glass, stacked high in stone walls, first acted as theology in pictures and then, without any formal theory, as an optical device. Medieval workshops learned that adding metals to molten glass altered how panes absorbed and transmitted specific wavelengths, even if nobody used that term. Narratives of saints and miracles depended on controlling brightness, contrast, and color fields across the window, turning a devotional surface into a primitive interface for managing light.
Lead cames created a matrix that segmented incoming solar radiation into hundreds of discrete channels, each filtering and scattering light according to its composition and thickness. Variations in glass density, surface roughness, and paint layers changed refraction and diffuse reflection, shaping how halos glowed and backgrounds receded. As the sun moved, the interior behaved like a slow, analog experiment in photometry and human visual perception, testing how color saturation, luminance, and spatial arrangement could steer attention and emotion without a written theory of optics.
This tacit research, driven by liturgy rather than laboratory protocol, mapped the marginal effects of tiny recipe changes in glass batches onto the walls and floors of the nave. What began as storytelling in colored panels became a continuous, lived demonstration of selective transmission and spectral filtering, centuries before controlled prisms, spectroscopes, or formal laws of refraction gave those effects technical names.
