Physical Description - The variable-focus retro-reflector is shown in the attached photo. Basically, it is a flexible plane mirror with mounting and pushing hardware. The mirror is a 12-inch diameter round segment of thin, high quality glass, metallized on the front side. The mounting and pushing hardware is an assembly of the following components:

(1) A backing plate with an O-ring that contacts the outer edge of the round glass mirror.

(2) A push ring on the front side of the mirror. .

(3) A push bar used to exert force on the push ring.

            (4) An aluminum base plate.

(5) Aluminum vertical mount with springs and adjustment screws.

How the Variable-Focus Retro-reflector works. - With no force on the push bar, the plane mirror reflects radiation without focusing. If force is then exerted by using the push bar to press on the push ring, the glass bends, becoming a concave focusing mirror. The harder the push, the shorter the focal length. If the pressure on the glass is uniform around the diameter of the push ring, the deformation of the mirror is uniform, giving it a nearly spherical figure. As the pressure is changed, the focal length of the mirror can be adjusted to anywhere between about 4 meters and infinity. The image has blurring and distortion, but the focussing is highly successful in bringing infrared radiation to a sensing element for a spectroscopic measurement.

How the Variable-Focus Retro-reflector Differs from Prior Art. - Long-radius mirrors are normally prepared through the art of the telescope maker in which a thick piece of glass is ground on a mandrel. Obtaining a desired focal length requires repetitive grinding and testing. When the mirror is finished, there is only one focal length, which cannot be changed. The finished product is a relatively heavy item. The variable-focus retro-reflector is superior in that it is moderate in cost, easy to manufacture, light in weight, and it can be adjusted to any desired focal length. Others have attempted to make such retro-reflectors, but no one else has used glass with a front-side push. The choice of glass is the key to the success of this invention, because glass is a semi-crystalline material that deforms uniformly and springs back to its original condition when the stress is relieved.

Main Application. The main intended use of the variable-focus retro-reflector is in the study of trace gases in the air. Radiation is projected from a spectrometer to the retro-reflector, which returns it to a detector. The retro-reflector may be used in multiple-passing arrangements in which there are many traversals back and forth. The focal length can be adjusted so that the optical system fits into a designated space, indoors or outdoors.

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