LUMETRICS: ADVANTAGES OF LOW - COHERENCE INTERFEROMETRY OVER NUCLEAR - BASED GAUGES
Many film manufacturers depend on nuclear gauges (beta, gamma or x-ray) for quality control during manufacturing, e.g. monitoring layer thickness. Low coherence interferometry is a proven measurement technology capable of replacing aging and costly nuclear-based gauging systems. It offers number of advantages with respect to safety, precision, and total cost of ownership. Index Terms — nuclear, gauge, multiple layers, interferometry, metrology, optics, non-contact thickness measurements.
In addition to the obvious safety issue, one of the biggest pitfalls of nuclear gauging is its inability to measure multiple layers at once directly. If a film manufacturer wants to know the thickness of a coating on a roll of film using nuclear gauge, it typically has to use an indirect measurement method called subtractive measurement. Subtractive measurement requires two separate nuclear gauges: the first one placed in production line in a location before the coating is applied, and a second gauge placed in a location after the coating is applied. The difference in thickness between those spots is subtracted to calculate the thickness of the coating. It is important that these two nuclear gauges be positioned and timed in such a way that they measure the same location on the film. Therefore, such configuration not only doubles the cost of the measurement system, but requires complex alignment and maintenance. Lumetrics has developed and commercialized a lowcoherence interferometer called OptiGauge II, which uses invisible 1310nm light to measure the thicknesses of multiple layers at the same time in a non-contact manner. In the example of the film manufacturer, a single probe is positioned at a location after the coating is applied. The point-like measurements are conducted at speeds ranging from 50 to 200 times per second.
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