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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