High resolution mapping with an improved Schlieren method
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 resolution achieved by interferometric systems but without their inherent limitations (cost, environment insulation, sensitivity, limited shape steepness).
The Phase-shifting Schlieren method presented here combines the classical optical Schlieren method with fringe phase-shifting methods from interferometry; thus effectively combining advantages from both methods. Schlieren 'a light wavefront measuring technique' is still a popular technique for depicting deviations of light beams induced by density, shape or deformation of an optical medium; and delivers its phase data in the form of fringe images. Phase-shifting on the other hand is a powerful technique to evaluate phase (~light deviation) data encoded in fringe images.
The combined technology that has been developed ? though using in essence simple optics ? fills the niche that existed for a long time between classical nanometric interferometry methods and moiré or Schlieren methods whose resolution is in the order of tens of microns. It also bridges the gap between interferometric and mechanical measurements. What?s more, it is relatively fast (compared to interferometric scanning and tactile probing).
The technology provider has an international team of experienced optical engineers and the company is strongly growing in markets in- and outside space. The phase-shifting Schlieren method was developed in close collaboration with the academic world.
Potential applications include fluid dynamics, jet/flow characterisation, heat transfer, surface shape and deformation, surface roughness, optical defect characterisation, microscopy and tomography, refractive index variation mapping, alignment of optical instruments, etc. The method is mature as it has already been successfully applied to in-line quality control of ophthalmic lenses (intra-ocular lenses, contact lenses and spectacles) to quantify shape and defects in glass spectacles; to investigate multi-layered computer displays; to visualise gas-liquid interfaces; to identify transparent material inhomogeneities; and to quantify flatness/polish of glass, mirrors etc.
Innovations & Advantages
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 does not suffer from instabilities due to light interference.
- A high sensitivity to engraving defects.
Phase-shifting Schlieren needs calibration once per measurement setup, and then remains auto-calibrated.
The Schlieren method resolution can be adapted to the problem, and ranges from 60nm to 30µm. In other words, it can measure optical lens powers ranging from 0.05 up to 100 dioptre.
An operational limit of the phase-shifting Schlieren method lies in the fact that multiple fringe images need to be recorded. However, an alternative method is to combine single-shot Schlieren with fast Fourier analysis to achieve high measuring speed.
Current and Potential Domains of Application
Potential applications include:
- fluid dynamics
- jet/flow characterisation
- heat transfer
- surface shape and deformation
- surface roughness
- optical defect characterisation
- microscopy and tomography
- refractive index variation mapping
- wavefront mapper
- optical system alignment
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