Digital holographic interferometry in the long-wave infrared domain has been developed by combining a CO2 laser and a microbolometer array The long wavelength allows large deformation measurements, which are of interest in the case of large space reflectors undergoing thermal changes when in orbit We review holography at such wavelengths and present some specific aspects related to this spectral range on our measurements For the design of our digital holographic interferometer, we studied the possibility of illuminating specular objects by a reflective diffuser We discuss the development of the interferometer and the results obtained on a representative space reflector, first in the laboratory and then during vacuum cryogenic test

Digital holographic interferometry with CO2 lasers and diffuse illumination applied to large space reflectors metrology

ew Tech n iq u es in D ig ial H o lo g ap h y E d ied b y P acal P iart This book is devoted to special techniques in digital holography. The authors establish a synthetic state of the art of important advances in the field of digital holography. They are interested in detailing advances related to fundamentals of digital holography, in-line holography, digital color holographic metrology, digital holographic microscopy, infrared holography and special techniques in full-field vibrometry.

New Techniques in Digital Holography

We describe three different interferometric techniques (electronic speckle pattern interferometry, digital holographic interferometry, and digital shearography), using a long-wave infrared radiation produced by a CO(2) laser and recorded on a microbolometer array. Experimental results showing how these methods can be used for nondestructive testing are presented. Advantages and disadvantages of these approaches are discussed.

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Nondestructive testing by using long-wave infrared interferometric techniques with CO2 lasers and microbolometer arrays.

Electronic speckle pattern interferometry and digital holographic interferometry are investigated at long infrared wavelengths. Using such wavelengths allows one to extend the measurement range and decrease the sensitivity of the techniques to external perturbations. We discuss the behavior of reflection by the object surfaces due to the long wavelength. We have developed different experimental configurations associating a CO2 laser emitting at 10.6μm and microbolometer arrays. Phase-shifting in-plane and out-of-plane electronic speckle pattern interferometry and lensless digital holographic interferometry are demonstrated on rotation measurements of a solid object.

Electronic speckle pattern interferometry and digital holographic interferometry with microbolometer arrays at 10.6 μm

A search for recording media suitable for holography at wavelengths comparable to the 10.6 μm of a CO2 laser has led to the successful use of Saran Wrap plastic films and of thick acrylic plates. Holographic phase recording has been made with a 1-W single mode cw CO2 laser yielding incident intensities up to 3.5 W/cm2. Holographic reconstruction at a wavelength of 632.8 nm has given a diffraction efficiency up to 20%. At a wavelength of 10.6 μm, reconstruction by reflection on the silver-coated surface of an acrylic plate had a diffraction efficiency of 0.5%. It was observed that the recording is less linear on acrylic plates than on Saran Wrap films.

W. A. Simpson

Papers

Real-time visual reconstruction of infrared holograms.