Making Images with Caustic Light
Probably the first thing you want to know is what 'caustic' is when dealing with light. It is the effect of light being reflected and/or refracted as it interacts with a surface. For example, ripples in a pool of water will cause bright and dark spots on the bottom of the pool. Now researchers at the Ecole Polytechnique Fédérale de Lausanne are exploiting this effect to produce detailed images with transparent materials, such as clear acrylic.
Another way to describe the effect would be to describe a magnifying lens. The curvature of the lens focuses the light that passes through it to a focal point, where it is brighter than the area around it. What the researchers have done, in simplistic terms, is made a very complex lens to focus the light in certain planes and away from others. Designing the surface of a material to produce an image like this actually involves just reversing an already well known process. For computer animated media, such as movies and video games, light passes through glass and reflects off materials quite often, and how those materials affect the light is often incorporated into the media somehow.
Potentially this method could be used in architecture and as a means to decorate glasses. Imagine a storefront window that projects an image just whenever the Sun shines through it, but does not have anything printed on it.