John H. Mcleod

The Axicon: A New Type of Optical Element

A search for a universal-focus lens has led to a new class of optical elements. These are called axicons. There are many different kinds of axicons but probably the most important one is a glass cone. It may be either transmitting or reflecting. Axicons form a continuous straight line of images from small sources.One application is in a telescope. The usual spherical objective is replaced by a cone. This axicon telescope is in focus for targets from a foot or so to infinity without the necessity of moving any parts. It can be used to view simultaneously two or more small sources placed along the line of sight.If a source of light is suitably added to the telescope it becomes an autocollimator. Like ordinary autocollimators it can be used to determine the perpendicularity of a mirror. In addition, it can simultaneously act as a telescope for a point target which may be an illuminated pinhole in the mirror.The axicon autocollimator is also a projector which projects a straight line of images into space.

Axicons and Their Uses

The most common axicon is a flat cone. A small source of light on the axis of the cone is imaged into a line along a portion of the axis. In lenses the spot diagram has been useful in evaluating image quality. In axicons a corresponding line diagram where lines take the place of dots is useful. In general, axicon instruments correspond to the usual optical instruments. For example, an axicon may be used as an objective to form a telescope. The resulting axicon telescope may be used in aligning machinery such as paper mills. Similarly, an axicon autocollimator may be used to precisely set mirrors perpendicular to a line. One form of axicon microscope has been tried out for the special purpose of locating the position of shiny surfaces without touching them. A most useful form of optical aligner is the reflection cone axicon. It is used as a straight edge. One example is a reflecting cone of 6 in. diam and maximum range of 40 ft with precision of 5 or 6 wavelengths over the entire range. Another example is a 5 in. diam cone with a range of 10 ft and precisions of about 1 wavelength. In this case the use of a suitable radius for the reflecting surface had the effect of making the image brightness substantially uniform over the 10 ft range. Photo cell pickup has been shown to be successful with very high precisions of setting. This opens the way for automatic machine guiding to very high precisions.

Thin Sheet Plastic Fresnel Lenses of High Aperture

Fresnel type lenses of high precision and excellent surface quality have been made in thin sheet plastic materials. The prismatic elements are very fine—about 0.003″ to 0.005″—and are not visible to the average unaided eye. A high degree of correction for spherical aberration has been achieved. Molded from high precision molds, lenses have been made in diameters of two to fifteen inches and focal lengths of 212 to 2212 inches. Relative apertures in excess of f/1.0 have been made. These lenses have found many applications as light collecting elements where weight and space are limited. Such applications include uses for large condensers, large field lenses in finders, camera viewing screens, and translucent screens for projection.