Displacement measurements of a parabolic reflector by holographic interferometry in the long-wavelength infrared

Deformation metrology of complex shape space structure and reflectors is a recurrent problem addressed by ESA.
Consequently, suitable measurement techniques have to be developed and validated to support relevant on-ground
qualification and verification testing.
Holographic techniques in the visible wavelengths are common for measuring the surface displacement of object.
However the short wavelength induces high stability criteria which are so severe that they often prevent the
measurement of large space structures under good conditions.
We propose to use a wavelength longer than the usual visible ones to render such holographic displacement
measurement techniques less sensitive to external perturbations. For this we consider CO2 laser emitting at 10.6 μm
which correspond to the long-wavelength infrared (LWIR) spectrum.
The most important element for performing holography is to use an adequate recording medium. However no
convincing holographic recording media exists in LWIR and we decided to investigate the possibility of electronic
hologram recording, such as electronic speckle pattern interferometry (ESPI). This possibility is feasible nowadays
thanks to various thermal imaging technologies such has uncooled microbolometers sensors commonly found in
thermographic cameras.
In this study, we analyse how it is possible to measure surface displacements of specular objects, which do not
produce speckle, using speckle interferometry by the mean of a scattering diffuser. We then apply the ESPI
technique at LWIR for measuring displacements of a large parabolic reflector.