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What
is Digital Holography? |
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Digital holography is a term that was coined to describe holography using recordings made on the new, large-format, digital cameras that became available at the end of the 20th century. Researchers realized that such cameras were capable of recording enough data that they could function as small holograms. Images could be recorded of the interference between light scattered from an object and a reference beam, and these could be reconstructed mathematically by digital computation using the Fresnel transforms or Fourier transforms. Such recordings offered the ability to generate images at different focal planes from a single recording in the same way as can be done with photographic holograms. The process, however, had severe limitations. The resolution of such cameras, although excellent for ordinary images, was far below that of photographic emulsions used in holography. Images were restricted to a narrow angle to the reference beam, and their size could not be very large. As time wore on, people began to use lenses to form images near the digital recording sensor. Furthermore, they began to record multiple exposures with phase steps in the reference beam to eliminate the need for an off-axis reference beam. Thus, these systems began to resemble those used since the 1980s for electronic holography in holographic interferometry.
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Digital
Holography with HoloFringe |
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The K100/HOL holography optical head illuminates an object with laser light and forms a high-quality image of the object on a digital TV camera. It combines this image with a mutually coherent reference beam located on axis with the exit aperture of the TV objective lens. When the program is run in its Time Lapse mode, it stores images that, taken as pairs, constitute holographic recordings. One is the cosine term of the complex holographic image, and the other is the sine term. The HoloFringe program can recall these images either to combine with other recordings of the same object after being subjected to stress, or to combine with a live holographic image of the object. The stored recordings can, however, be subjected to Fresnel or Fourier transformation to shift the focal plane of the images. This capability of the K100/HOL optical head and its operating programs has been unrecognized for over 20 years. It can greatly extend the capability of the measurement capabilities of these systems, especially when used in the field of holographic microscopy.
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