Manufacturing and testing of large (1-4 m diameter) reflectors is an important technological problem for space agencies. Traditional optical testing methods typically use laser-based metrology where the phase difference between two beams – one reflected from a high quality reference mirror and the other reflected from the test mirror – is measured by recording their interference pattern. The phase data can be converted to an optical path difference, thereby giving quantitative information about the surface profile. Laser interferometry at visible wavelengths poses several challenges for large reflectors, particularly when they may have aspheric complex structures and the deformations to be measured can be as high as a few hundred micrometers. Such large path differences correspond to a very large number of fringes that are difficult to resolve. One possible solution is to use LWIR wavelengths, e.g. 10.6 μm CO₂ laser illumination. The recording medium for LWIR holograms is another technical issue to be resolved. Further, it is important to note that the testing must be performed by replicating possible conditions in space-based systems in a cryogenic environment.

Georges et al. demonstrate Digital Holographic (DH) interferometry for testing large reflectors by continuing the recent trend of using uncooled microbolometer arrays. Although microbolometers have been used mostly with passive thermal imaging for security applications, they are well suited for holography/imaging with active illumination as well. The wavelength of CO₂ lasers corresponds to ambient thermal radiation at approximately 20° C. The thermal background detected by the microbolometer is however incoherent in nature and its contribution from the interference term in the hologram may be separated using spatial filtering methods. It is important to note that at LWIR wavelengths, the reflectors are more specular as compared to visible wavelengths and as a result the authors also demonstrate the possibility of using diffuse illumination as in an ESPI (Electronic Speckle Pattern Interferometry) configuration. The authors show the use of in-line holographic configuration with phase shifting done on time scales shorter than changes in deformation during the testing time. The most important advantage of the DH approach as shown here is that the setup is much simpler than those of conventional null testing methods with reference objects, and the metrology can be performed with off-the-shelf components. The present work expands ever growing applications of DH to a new area of large reflector testing and will have high impact on how large optics for space applications will be tested in future.

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