An industrial optical engineering firm from Belgium has developed and patented the phase-shifting Schlieren method, filling a niche in the range of optical measurement techniques.
This submicron method comes close to the resolution achieved by interferometric systems but without their inherent limitations (cost, environment insulation, sensitivity, limited shape steepness). It has a variable sensitivity, both high in spatial (in-plane) and height (out-of-plane) resolution; it does not suffer from vignetting, digitalisation effects or sample positioning; and it is auto-calibrated.
It can be used for detecting surface shape/quality/defects/inhomogeneities of transparent objects, thin films, gas-liquid interfaces and reflective surfaces.
Partnerships are welcomed; both for direct use/integration of the technology and for dedicated application/development of the method into a product.
The Schlieren method itself is not new, and is popular due to:
- its relative ease of implementation
- low cost
- use of conventional optics and light sources
- variable sensitivity to fit the object under study.
- robustness to vibrations (well adapted to industrial environment)
Combining Schlieren with phase-shifting methods leads to additional benefits:
- An inherent high in-plane resolution, only depending on the spatial resolution of the camera detector.
- A high out-of-plane resolution (0.002°), as phase-shifting provides a real measured gray values per camera pixel, without digitalization or resolving effects limiting the resolution.
- The method does not suffer from small vignetting.
- A large dynamic angular range, and thus the ability to characterise objects with large slopes.
The method is mature and has already been successfully applied to
- perform in-line quality control of ophthalmic lenses;
- quantify shape and defects in glass spectacles;
- investigate multi-layered computer displays;
- visualise convection pockets at a gas-liquid surface;
- identify transparent material inhomogeneities;
- quantify flatness/polish of glass and mirrors