CANNON’s range of Bending Beam Rheometers, which are used in asphalt testing for pavements around the USA, can also be applied to the measurement of materials used for amazingly fine engineering – the polishing of precision optics.
Typical optical surfaces have a maximum deviation from the allowable of ¼ of a wave of red light (160nm or 6 microinches). The very best, used in interferometry, might be 1/100th wave (6nm or 0.25microinches)! These are all achieved by rubbing with ultra fine abrasives on pitch laps. The polishing process is a mechanochemical one; some action is by direct removal of material, some is by chemical means.
Fine optics are always polished on pitch which is usually derived from organic sources. The finest grades are supposedly made from pine pitch and other ingredients or from petroleum sources. The unique properties of pitch allows it to flow minutely and conform to the optical surface whilst being resilient enough to allow the development of the macro surface. The CANNON TE-BBR can be applied to measuring optical pitch with no modifications.
CANNON article supplied by-Steve Taylor, Rheologist
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