Design Realization lecture 25

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Retro-reflector gain

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.

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Application: personal displays

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.

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Application: Artificial backgrounds

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).

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Convex Lenses

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.

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Lenses

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.

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Lenses

When there are two focal points f1 , f2 (sometimes called conjugates), then they satisfy:

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Spherical Lenses

If the lens consists of spherical surfaces with radii r1 and r2, then the focal length satisfies 1/f = ( - 1) (1/r1 - 1/r2)

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Spherical aberration

Spherical lenses cannot achieve perfect focus, and always have some aberration:

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Spherical aberration

Compound lenses, comprising convex, concave or hybrid elements, are used to minimize aberration.