Friday 12th May 2023 THE SCIENCE OF INTERFEROMETRY Optical Coatings Thickness Measurement A technique called spectral reflectance interferometry is typically used for non-contact measurement of thin films such as coatings on beverage cans. HOW REFLECTANCE INTERFEROMETRY WORKS A beam of light is transmitted through a fibre optic and focused onto the can surface. The reflected light is detected by the same optic fibre and transmitted back to a spectrometer. Most of the light waves reflect from the surface of the coating, but some pass through the coating to reflect off the metal of the can body. Light passing through the coating is refracted along a different path from the light that reflects off the top surface. Precise differences detected between these reflected light waves provide information which is used to calculate the coating thickness. DIAGRAM 1: Interference patterns caused by phase shifts (A) in the reflected light beam are used to calculate the path difference (B) of the refracted reflections, which is mathematically correlated to the coating thickness (C). PROBLEMS WITH 'WHITE LIGHT' INTERFEROMETRY Optical coatings thickness gauges which use standard interferometry solutions that are typically based on visible or 'white' light have problems measuring coatings over colored inks or tinted base coats. This is because some wavelengths are absorbed by the colored pigments, creating unpredictable gaps in the data and leading to unreliable results. DIAGRAM 2: With standard 'white light' interferometry, some wavelengths are absorbed by colored pigments; only the wavelengths of the perceived colors are reflected. This produces incomplete data which makes coatings thickness calculation unreliable. A CUSTOM DESIGNED SOLUTION USING INFRA-RED Sencon recognized the inherent deficiencies of off-the-shelf interferometry technologies so, together with a leading technical university in the UK, developed a solution specifically for canmaking applications. During this 3-year research and development partnership, a custom-designed optical metrology and signal processing technique for measuring coatings thickness on beverage cans was designed and implemented. This proprietary Film-Scan™ technology uses a form of reflectance interferometry based on infra-red light, which has proven reliable for measuring all kinds of external coatings on decorated cans, including black and matte coatings. The technology can also successfully assess the minimum thickness of tactile coatings as well. DIAGRAM 3: Infra-red light waves are beyond the visible spectrum, which avoids the problem of selective wavelengths being absorbed by colored pigments. Together with Sencon's specially developed signal processing, this technique produces highly reliable coatings thickness measurement on all kinds of beverage cans. Film-Scan™ Optical Coatings Measurement Sencon’s Film-Scan™ optical coatings gauge carries out a full 360° optical scan of both internal and external coatings. World class GR&R is now possible on more locations on the can, including the neck, plug, and dome areas, as well as the notoriously difficult reverse wall and rim coating. Instead of needing physical lacquer samples, the gauge works from a list of qualified coatings using data obtained and updated by the customer from their preferred coatings suppliers. If you would like more detailed information on the Science of Interferometry please request a copy of our White Paper below. For more information on Sencon's range of Optical Measurement systems please contact your regional Sencon office or agent.