Digital holographic interferometry with CO₂lasers and diffuse illumination applied to large space reflector metrology [Invited]

Abstract

Digital holographic interferometry in the long-wave infrared domain has been developed by combining a CO(2) 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 in the Long-Wave Infrared for the Testing of Large Aspheric Space Reflectors

“…There are basically two schemes for dealing with specular reflectors and that were discussed in [5], but only one was implemented then. Without loss of generally we will present both in the c speciment (1.1 m diamet different configurations.…”

“…This spot fering with the reference parabola is more open or is possible to compute th using them in an imagin alternative configuration rabola with an extended the laser. This way it is po detector, even in case of details in [5] and we call the case of a truly scatte system. In Fig.2 we show lar tilt applied to the para ity, similar to fringes ob grams suffer from second elements in the object pa some parts of the mirror.…”

“…The advantage of using long wavelength is that large deformations can be measured at once, in addition to being more immune against environmental perturbations. In this paper we review different optical configurations of DH interferometer that led to test a parabolic mirror under thermal-vacuum test [5], as well as an off-axis ellipse tested in laboratory conditions, which is a new result. …”

Combined holography and thermography in a single sensor through image-plane holography at thermal infrared wavelengths

“…Photochemical hologram recording at this wavelength was studied already in the 1960s years. In a previous paper [7] we performed a literature survey of the different materials. We showed that there was no convincing material for recording holograms to be used in HI, mainly because of low resolution and some other practical considerations.…”

“…Since then that group showed numerous applications and advantages of LWIR DH. One advantage of LWIR DH is the possibility of reconstructing numerically objects larger than in visible DH (for a given distance between the object and the sensor) because the ratio of the wavelength and pixel size is generally 5 to 10 times larger in LWIR [7,10]. Recently it was demonstrated that lensless LWIR DH can capture laser light reflected by humans illuminated through smokes and flames and reconstruct numerically images of sufficient quality to recognize the presence of people located beyond [11].…”