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?
I walked around with a map, penciling in X’s wherever there was a barn or shed. For the next three days, I covered four kilometers in all four directions. Living toward the outskirts of town, there are still a good many farmers in the vicinity. So it came to a considerable number of barns—sixteen altogether.
I carefully checked the condition of each of these, and from the sixteen I eliminated all those where there were houses in the immediate proximity or greenhouses alongside. I also eliminated those in which there were farm implements or chemicals or signs that they were still in active use. I didn’t imagine he’d want to burn tools or fertilizer. That left five barns.