Holography with a Double Focus Lens : a Simple Lecture Demonstration

Abstract

THE simplest forms of holography require that, when light waves carrying information about an object are incident on the recording material, a uniform light wave which is to some degree coherent with the first should fall simultaneously over the same area. The various practical forms of this principle differ principally in the way in which these two mutually coherent waves are derived^1–3. It is often convenient for the reference wave to be spherical, so that a point source can be used as a reference in reconstruction. At the recording stage coherent waves of different curvature must be provided, and for transilluminated objects such as transparencies the difference of curvature is provided by the illuminating optical system. Even when the object is illuminated through a diffuser, it may be advantageous to arrange for a plane wave to illuminate each point of the transparency simultaneously, while the reference wave converges to a point in the same plane. We have found that a double focus lens can perform this function very efficiently, and the scheme lends itself to an effective demonstration of holography.