METHOD OF IMAGING TRANSPARENT OBJECTS WITH COHERENT LIGHT

1,217,385. Holograms. HOLOTRON CORP. May 10, 1968 [May 12, 1967], No.22295/68. Heading G2J. In a method of forming an image of a non-diffuse transparent object a beam of coherent light is passed through a phase modulator placed in close proximity to a coincident with the object and then through the object whereby the beam passing through the object has a constant amplitude and random phase. The method may be used to form a hologram, wherein a beam of coherent light 13 passes through a phase modulator 27 and the transparent object 15 and impinges upon a detector surface 17 where it interferes with a reference beam 19. The phase modulator may be a sheet of anti-glare glass polished flat on one side, a sheet of glass in which shallow etches have been provided or on which a coating has been formed. It is located as near as possible to the object so that the scattered light rays do not interfere with themselves before passing through the object. Alternatively the beam of whereat light may be passed through the phase modulator 27 and using an optical system an image of the modulator 27a is formed coincident with the object 15. To determine the focal plane of an imaging system, the beam of light is passed through a modulator and the object, a suitable optical system producing a grain free image and two out of focus graining images, the focal plane of the system being determined by the position of the grain free image. In a further embodiment a series of modulators each of known but different band width is used and those producing grain free images determined to calculate the frequency response of the system.