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How does natural daylight change the appearance of stained glass during the day?

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The north rose window at Chartres Cathedral in France, featuring intricate stained glass with vibrant geometric and biblical designs in a large circular and five tall vertical panels. — stained glass The stained-glass rose window in the north transept of Chartres Cathedral, France.

The appearance of stained glass is not static; it is a dynamic medium that relies on the constant interaction between light and the viewer's vision. Because stained glass is essentially an art of painting with light, its aesthetic qualities shift throughout the day based on the intensity, direction, atmospheric diffusion, and color of natural daylight.1

As the sun moves across the sky, different windows come to life while others fade into twilight. For example, in a cathedral, east-facing windows may glow brilliantly at sunrise, only to lose that intensity as the sun shifts toward the south by midday. Direct sunlight can make specific colors, such as deep blues, appear incredibly intense, while the arrival of clouds diffuses the light, causing windows to lose their sharp brilliance and altering the overall tonality of the interior space.1

As sunset approaches, the changing color temperature of the sun significantly impacts the glass; warmer colors like rubies may become fiery and assertive, often appearing to displace cooler tones as the primary colors of the composition. Ultimately, the "luminous life" of the glass is a continuous, organic process that changes moment to moment.1