The retro-reflection response of a screen is normally rated in terms of gain.
Gain = ratio of peak reflected light energy to the energy reflected by a Lambertian surface.
Gains may be 1000 or more.
Light source only needs 1/1000 of the light energy to illuminate the screen, as long as the viewer is close enough to the source.
Each user has a personal projector (e.g. a PDA with a single lens in front of it), and projects on the same retro-reflective screen.
Projector and camera along same optical axis, project scene onto actors and retro-reflective background.
Cameras sees background only on screen, not on the actors (3M received technical academy award for this in 1985).
A refractive disk with one or two convex spherical surfaces converges parallel light rays almost to a point.
The distance to this point is the focal length of the lens.
If light comes from a point source that is further away than the focal length, it will focus to another point on the other side.
When there are two focal points f1 , f2 (sometimes called conjugates), then they satisfy:
If the lens consists of spherical surfaces with radii r1 and r2, then the focal length satisfies 1/f = ( - 1) (1/r1 - 1/r2)
Spherical lenses cannot achieve perfect focus, and always have some aberration:
Compound lenses, comprising convex, concave or hybrid elements, are used to minimize aberration.