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?
The newborn baby and the [blind man suddenly gifted with sight] do not have to learn to see. Sight is given to them. But they do have to learn to perceive. Perception is learnt and learnt slowly. Skill is required for perception as for speech. We are largely unaware of the skill we exercise. None of the things we have to learn to perceive are self-evident, or, apparently, instinctively evident. No doubt, however, we have an instinctive aptitude for this learning, and once we have learnt we cannot easily see as though we had not.
As Ruskin says, one has to strive, if one is to see with the 'Innocent Eye'.