29th Jan 2021
Read our January 2021 newsletter.
Automated Inspection of Surface Quality
The surface quality of optics is specified and measured in many ways. Many applications require a particular surface shape, or form, whether this be for a lens or an optical flat. Additionally, surface finish / texture / roughness can be important, especially for high power laser applications. Optical technicians use a variety of instruments, commonly based around interferometry, to precisely inspect these parameters to nanometre precision.
Alongside this measurement of form and finish, there are various standards categorising the general quality of optical polish, identifying the level of local imperfections – or scratch-dig. The industry specification for this imposes limits on either the feature size (ISO 10110-7) or the magnitude of apparent scatter (MIL13838B) from an imperfection and, given the associated ‘accumulation rules’ is a much more elusive parameter to tie-down.
Recognising the frustration of this across the optics industry, Crystran commissioned RedLux Ltd – a highly respected UK metrology supplier – to develop a new instrument to assess scratch-dig automatically and completely objectively across the entire range of optical materials we polish and supply; the OptiLux SD.
Using the OptiLux SD is easy. The user inputs the optic’s dimensions (up to 300 mm) and a pre-scan is taken to electronically level and focus on the surface being measured – the rest is automatic. Using dark-field microscopy, and a high-resolution CCD, a step-and repeat process quickly scans the complete optic to a spatial resolution of better than 5 microns. An overall pass / fail against either the chosen MIL or ISO string is then presented alongside a surface map and summary against all lower-tier accumulation rules – all of which can be neatly generated into a report for customer viewing.
Compared with the traditional / manual method of assessing scratch-dig using a trained eye and comparator plates (an impossible task for most), the OptiLux SD has proven to be an invaluable tool, ensuring all our high-value parts are made precisely to specification with no subjectivity or compromise.
Crystran would be pleased to discuss the use of this instrument with you and measure any samples sent our way.