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A very rugged, compact (3x3x10 inches), gas purged "PINHOLE CAMERA" has been developed for viewing electron beam materials processing (e.g. melting or vaporizing metal). The video image is computer processed, providing dimensional and temperature measurements of objects within the field of view, using an IBM PC. The "pinhole camera" concept is similar to a TRW optics system for viewing into a coal combustor through a 2 mm hole. Gas is purged through the hole to repel particulates from optical surfaces. In our system light from the molten metal passes through the 2 mm hole "PINHOLE", reflects off an aluminum coated glass substrate and passes through a window into a vacuum tight container holding the camera and optics at atmospheric pressure. The mirror filters out X-rays which pass through the AL layer and are absorbed in the glass mirror substrate. Since metallic coatings are usually reflective, the image quality is not severely degraded by small amounts of vapor that overcome the gas purge to reach the mirror. Coating thicknesses of up to 2 microns can be tolerated. The mirror is the only element needing occasional servicing. We used a telescope eyepiece as a convenient optical design, but with the traditional optical path reversed. The eyepiece images a scene through a small entrance aperture onto an image plane where a CCD camera is placed. Since the iris of the eyepiece is fixed and the scene intensity varies it was necessary to employ a variable neutral density filter for brightness control. Devices used for this purpose include PLZT light valve from Motorola, mechanically rotated linear polarizer sheets, and nematic liquid crystal light valves. These were placed after the mirror and entrance aperture but before the lens to operate as a voltage variable neutral density filter. The molten metal surface temp being viewed varies from 4000 to 1200 degrees Kelvin. The resultant intensity change (at 488 nm with 10 nm bandwidth) is seven orders of magnitude. This surface intensity variation is contrast reduced if the observation wavelength is a narrow band as far red as high intensity blooming will allow an observable picture. A three eyepiece camera allows an image plane where photo gray glass functions as a neutral density filter only over the high intensity portion of the image, thus reducing blooming. This system is enclosed in a water-cooled housing which can dissipate 15 watts/cm2, keeping the camera below 40 degrees Celsius. Single frames of video output are acquired for feature enhancement and location by a Data Translation DT2803 image processing board housed in an IBM PC.
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