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 almost reaches the resolution achieved by interferometric systems, but avoids 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. At the same time, it does not suffer from vignetting, digitization effects or sample positioning; and it is auto-calibrated.
This method can be used for detecting surface shape, quality, defects and inhomogeneities of transparent objects, thin films, gas-liquid interfaces and reflective surfaces. Partnerships are welcomed, both for direct use or integration of the technology, and for dedicated application or development of the method into a product.
The Schlieren method itself is not new, and is popular due to:
- its relative ease of implementation,
- its low cost,
- its use of conventional optics and light sources,
- its variable sensitivity to fit the object under study,
- its robustness to vibrations (well adapted to industrial environment).
Combining Schlieren with phase-shifting methods leads to additional benefits:
- It has an inherent high in-plane resolution, which only depends on the spatial resolution of the camera detector.
- It provides a high out-of-plane resolution (0.002°), as phase-shifting provides a real measured gray values per camera pixel, without digitization or resolving effects limiting the resolution.
- The method does not suffer from small vignetting.
- The method creates large dynamic angular range, and thus the ability to characterize 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,
- visualize convection pockets at a gas-liquid surface,
- identify transparent material inhomogeneities,
- quantify flatness/polish of glass and mirrors.