How did Francesco Borromini use light as part of his architectural designs?

Francesco Borromini treated light as a fundamental architectonic component, using it to manipulate space and create dramatic, illusionistic effects. By strategically concealing light sources, he could transform the perception of his interiors, making structural elements appear weightless or suspended.123

A primary example of this technique is found in his design for San Carlo alle Quattro Fontane. In this church, Borromini utilized calculated lighting to intensify the interior space, hiding the dome’s springing point and light sources from the view of those in the zone below. This deliberate concealment creates the illusion that the dome is floating above the church like a hallucinatory vision.23

Borromini’s awareness that light could serve as a major compositional element was rooted in his study of medieval architectural experiences, which he integrated into his radical Baroque designs. By denying the restrictive, enclosing qualities of traditional walls, he successfully used light to expand and contract space, challenging the architectural conventions of his time.21