In optics, a caustic is the envelope of light rays reflected or refracted by a curved surface or object, or the projection of that envelope of rays on another surface. The caustic is a curve or surface to which each of the light rays is tangent, defining a boundary of an envelope of rays as a curve of concentrated light.
A piece of milled plexiglass acting as a projecting lens; via the Computer Graphics and Geometry Lab at the École Polytechnique Fédérale de Lausanne
New milling techniques applied to glass and plexiglass panels could be used to “create windows that are also cryptic projectors, summoning ghostly images from sunlight.”
[Pauly and Bompas] hope that the technique will be used in architectural design, to create windows that mould sunlight and throw images or patterns onto walls or floors,” which, if timed, milled, and manipulated just right, could produce a slowly animated sequence of images being projected by an otherwise empty window during different times of day.
To control the shape of a caustic pattern generated by a specular or refractive surface, we need to solve the inverse problem: how can we change the surface geometry, such that incident light is redirected to produce a desired caustic image?
Le Corbusier, the greatest architect of the last century, noted that 'architecture is the learned game, correct and magnificent, of forms assembled in light', demonstrating to what extent light has been prioritized in the Western tradition. Tanizaki, on the other hand, spoke of the important of shadows, of extended eaves. Rather than the light that shines directly into a room, he praised the soft light that penetrates a space after being reflected off the floor, and again from the ceiling.
...In Japanese architecture, the gentle light that passes through shoji screens serves a key purpose. It reaches right to the back of the room, so that the space feels bright, even without the aid of artificial light. The soft light filtering through the white film at Takanawa Gateway Station represents a form of light that was forgotten about by Japanese Modernism.