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The basic components of a holographic system are the laser, lenses and camera. The laser light must first be expanded and then collimated (ie parallel, the beam is not expanding or focusing)**. When the light passes through the test section, some of it hits objects and deflects, but some passes straight through. The combination of the direct light (this is called the Reference beam) and the deflected light hitting the CCD chip creates an 'interference pattern'. Any other objects, such as dust on the lens, will also deflect the light and appear in the image. The spacing of the interference pattern is the information used to reconstruct the hologram. The result is a photograph that can be refocused at different depths, rather than being in focus at only one position. The laser light needs to cover the whole image area, which is why it is expanded at the start. This is also why a camera lens is not needed; you don't want to focus the light from infinity to a point, instead you want it to cover the entire CCD chip. This method of holography is called "in-line" holography, logically because all of the components are in a line! The image you get is actually the shadow of whatever lies in your field of view, so you cannot see the color of an object using this method. The laser light provides all of the illumination, and other lights will detract from the quality of the hologram, so all of the images on this site were taken in a mostly-dark room. Digital reconstruction is accomplished using Fourier transforms. For more information on this contact Jian Sheng. He wrote the program that was used to reconstruct all of the holograms on this website. It reconstructs one image at a time by calculating the appropriate filter for a given depth, wavelength of light, and resolution (microns/pixel). This filter is convolved over the entire image, generating one point per filter position, to create the reconstructed image. You can download 'DigiHoloWin' by clicking this link. Eventually we will have a help file to explain the data field entries (in the meantime you can contact us for more information). Note that this program is the property of Jian Sheng- all rights reserved. Please contact us if you plan to use it. The advantage of holography lies mainly in small-scale applications. When you take a photograph of a room, many things are in focus (like your roomate, his dog, and the wall behind them). The depth of objects in focus is called the 'depth of field'. So in a large-scale photo the depth of field might be several meters. The depth of field is related to the smallest resolution of the image, and when the resolution becomes much smaller, the depth of field becomes even more dramatically small. So on the microscopic level the depth of field might be on the order of microns! This is where holography is at a huge advantage, because the image can be refocused at any depth ('any' depth is not truly accurate; the depth of field that can be focused is around 100 times greater).
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