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?
Translation is always a treason, and as a Ming author observes, can at its best be only the reverse side of a brocade—all the threads are there, but not the subtlety of color or design. But, after all, what great doctrine is there which is easy to expound? The ancient sages never put their teachings in systematic form. They spoke in paradoxes, for they were afraid of uttering half-truths. They began by talking like fools and ended by making their hearers wise. Lau Tzu himself, with his quaint humor, says, "If people of inferior intelligence hear of the Tao, they laugh immensely. It would not be the Tao unless they laughed at it."