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?
Historically, Japan's shrines have been built in order to worship the gods who live in the sacred mountains or seas; They don't reside in the shrine itself, but in the space beyond it. This belief that the spirits and deities exist beyond the confines of the shrine, and that the shrine itself acts not as a centre, but as a kind of gateway, is very different to the grand, imposing churches and cathedrals of Christianity.
The majority of shrines are not found in the mountains or in the middle of the fields, therefore, but at the borders of mountain villages – which is to say, at what is seen as the edge of the mountains. The tori gate, marking the entrance to a shrine, indicates that there are gods on the other side of it.