Cleaning in some form or other plays an important part in the manufacturing of complete spectacles, lenses, frames and contact lenses. This need to clean the products we produce has become even more important with the introduction of organic lens materials and also hard and anti-reflection coatings. The prerequisite of cleanliness is no more important than within the coating laboratory. All lens surfaces must be absolutely clean, otherwise the finished coating will not be mechanically stable. Although part of this cleaning process is completed during the coating cycle, by electrostatic discharge, the surface must still be pre-cleaned, to remove all but the most stubborn particles. Manual cleaning can achieve a possibly acceptable result, but there is always the risk of mechanical damage to the surface and the process will be labour intensive and time consuming. The introduction of ultrasonic cleaners has led to much shorter cleaning times and, it has to be said, a cleaner surface. This improvement in cleaning has been one of the factors behind today's superior coatings, both hard and anti-reflection. The word 'ultrasonic' covers the subject of waves with the same nature as sound, but with frequencies above that of audio frequencies, i.e., above the normal range of the human ear. These waves have a frequency above 20kHz, the human ear being able to hear frequencies from about 20Hz to the 20kHz level. Ultrasonic wave energy can be transmitted through liquids and it is this property that is used in ultrasonic cleaning techniques. To make use of these ultrasonic waves, the article to be cleaned is immersed in a liquid within a suitable container or tank - normally made of stainless steel. Ultrasonic waves are generated by a transducer or transducers attached to the tank; these are usually piezoelectric crystal oscillators placed between metal resonators. The waves propagate through the cleaning liquid causing areas of high and low pressure. In turn these alternating pressure waves cause countless microscopic bubbles to form. These increase in size and then finally collapse, or implode. This collapse, called cavitation, causes turbulence within the liquid - thus introducing a 'flow' of liquid across the surface of the immersed object which is thus 'scrubbed' very gently. This constant action of pressure change or cavitation causes the bubble creation to form on all surfaces of even an intricately shaped object (even inside hollow objects which can be penetrated by the liquid) making the process eminently suitable for cleaning jewellery and delicate mechanical objects such as watch mechanisms. The process can also be used to clean spectacle frames, even when the lenses are still in place. It is sometimes almost impossible to clean a frame manually, particularly around the pad arms of a metal frame and also in the joint charniers.
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