Broadband vs Single Wavelength Monitoring
The historical method of calling an end of a layer in optical coating was to monitor a wavelength and when a condition was met calling the end of the layer. In the earliest days this method was done using the human eye of a trained observer. In some cases this is still done today but with the most limited of success. The next advent in coating control came with the advent of the ability to measure automatically the intensity of light in transmission or reflection mode by wavelength.
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