Binary Holograms Research Articles

Signal/Noise In Analog And Binary Holograms

The common hologram is an analog hologram containing photographic grey tones. A binary hologram contains only black or white areas and is made either by computer or by photographic hardclipping of analog holograms. Theoretically and experimentally, the signal/noise superiority of binary over analog holograms is demonstrated in this paper. The diffraction efficiency of binary holograms is typically ten times higher than that of analog holograms, while the noise due to the photographic grains is lower at the same time by a factor one half in the examples discussed. The discussion is presented in the context of holographic data storage with references to other applications where the signal/noise improvement is important.

Hologram conversion by computer

A technique of converting Fresnel-tranform holograms into Fourier-transform holograms by computer is discussed. A sound-wave hologram is processed with 64×64 sampling points and images are reconstructed optically from this processed binary hologram in the Fourier-transform plane, that is, a focal plane of a lens.

New Coding Technique for Computer Generated Holograms

A coding technique is developed for recording computer generated holograms on a computer controlled CRT in which each resolution cell contains two beam spots of equal size and equal intensity. This provides a binary hologram in which only the position of the two dots is varied from cell to cell. The amplitude associated with each resolution cell is controlled by selectively diffracting unwanted light into a higher diffraction order. The recording of the holograms is fast and simple.

Minimization of Reconstruction Errors with Computer Generated Binary Holograms

Generation of holograms by computer allows the possibility of better controlling the hologram formation process and of displaying a synthesized image in the case where the object does not exist physically. However, limitations of equipment used to plot the hologram can cause degradation of the reconstructed image. We examine these degradations for the binary Fourier transform hologram and present a method by which the plotting procedure may be designed so as to yield a most faithful reconstructed image. Experimental results which support the analysis are included.

Computer-generated Binary Holograms

Holograms synthesized by computer are used for constructing optical wavefronts from numerically specified objects. Elimination of the need for a physical object has made new applications possible, for example, three-dimensional computer output displays, synthetic prototypes for interferometric testing, and filters for various optical data processing operations. Our computer holograms differ from a normal hologram in that the transmittance is binary, yet they are able to construct general wavefronts and images efficiently and have several practical advantages over holograms with a continuous range of transmittance. Recent improvements that simplify the production of binary holograms and improve their performance are described and experimental work showing reconstruction of two- and three-dimensional images is presented.

Microwave hologram by photoengraving

The photoetching technique was applied to the construction of a microwave hologram. The 34.26-GHz microwave field was mapped by Polaroid film and copied on a printed circuit board. The printed circuit board was illuminated by the same beam as used in the formation of the original hologram. The real image was focused onto liquid crystal film for visualization. A comparison was made between the reconstructed image of the binary microwave hologram and that of the photoetched plate. The photoetched board which can produce the halftone transmission of the microwave beam was shown to be a satisfactory means of making microwave holograms.

Three-Dimensional Fourier-Transform Method for Synthesizing Binary Holograms*

A new method for the synthesis of holograms, based on a Fourier transformation, has been theoretically derived and experimentally verified. The method extends the familiar two-dimensional optical Fourier-transform analysis to include objects located in a volume centered about the focal point of the transform lens, and is specifically developed to provide an easily synthesized binary hologram for the reconstruction of three-dimensional objects. The synthetic holograms produced display, in addition to three-dimensionality of the reconstructed image, all other properties associated with photographic holograms. This technique for synthesizing holograms has the potential advantage of computational simplicity and increased image resolution over the Fresnel-transform technique, and permits the synthesis of a variety of objects composed of discrete points, continuous lines, and continuous surfaces.

Three-dimensional images exhibiting a gray scale from binary computer generated fresnel holograms

Extremely simple mathematical concepts have been used to synthesize holograms by digital computer. The binary holograms produced possess all of the characteristics of conventionally recorded holograms including the reproduction of an image gray scale.

Computer Generated Spatial Filters for Coherent Optical Data Processing

The setup used in many optical data processing schemes is a coherent optical image forming system. The most important lement in this setup is the complex spatial filter. It can perform a large variety of linear operations upon the object or input. In general, it is difficult to produce complex filters, since both amplitude transmission and phase delay may vary across the filter plane in a complicated manner. Our own filters which are very similar to binary holograms, consist of many little transparent rectangles on opaque background. They can easily be drawn on a large scale by a computer-guided plotter, and then photographically reduced in size. We show that our filters, despite containing only amplitude values zero and one, can perform any data processing operation which could be performed by any complex filter. After explaining the principle, we present three groups of applications. First, we describe new versions of some classical methods: schlieren observation and phase contrast. Next, we report on spatial which perform differential operations upon the object in order to enhance gradients or corners. Finally, we use our binary filters for signal detection.

A sampled computer-generated binary hologram

A simple way of recording computer-generated holograms is reported. It involves recording the sign of the amplitude (hence the word binary) on computer printer paper. Also the resolution attained during reconstruction of a hologram is discussed and compared with the theoretical limit.

Binary Fraunhofer Holograms, Generated by Computer

When a hologram is desired from an object which does not exist physically but is known in mathematical terms, one can compute the hologram. An automatic plotter will make a drawing at a large scale which is then reduced photographically. Since the drawing can contain only black and white areas, we have developed a theory for binary holograms. They are equivalent in terms of image reconstruction with ordinary holograms. This has been proven theoretically and verified experimentally.

Complex Spatial Filtering with Binary Masks

Usually a hologram is produced by means of an interference experiment. Here, however, we let a computer- guided plotter draw the hologram. The plot, which has to be minified and recorded on film, contains no grey, only binary transmittance values. Our binary holograms yield reconstructed images of a quality equal to that of images obtained from usual holograms of comparable dimensions. When a Fourier hologram is inserted into the Fraunhofer plane of a coherent image forming system, it acts as a special type of a spatial filter, a so-called optical matched filter. Our binary matched filter is suitable for optical character recognition, the same as the usual optical matched filter introduced by Vander Lugt.

Synthesis of binary holograms

Synthesis of binary holograms