Browse Prior Art Database

Noncontact Holographic Thermal Imaging

IP.com Disclosure Number: IPCOM000039059D
Original Publication Date: 1987-Apr-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 4 page(s) / 48K

Publishing Venue

IBM

Related People

Ermert, H: AUTHOR [+2]

Abstract

Thermal wave imaging has been invented and developed over the past several years with the goal of imaging defects or structures beneath the surface of opaque materials. Generically, this technique involves a beam of energy (laser, electron, ion,...)focussed to a point and modulated (or pulsed) at some frequency. The thermal (or heat) wave emanating from the heated region of the sample is scattered by inhomogeneities within the sample. The detected thermal wave thus contains information regarding these inhomogeneities. This information can be used to "image" the inhomogeneities as the heat source is scanned, point by point, over the sample.

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Noncontact Holographic Thermal Imaging

Thermal wave imaging has been invented and developed over the past several years with the goal of imaging defects or structures beneath the surface of opaque materials. Generically, this technique involves a beam of energy (laser, electron, ion,...)focussed to a point and modulated (or pulsed) at some frequency. The thermal (or heat) wave emanating from the heated region of the sample is scattered by inhomogeneities within the sample. The detected thermal wave thus contains information regarding these inhomogeneities. This information can be used to "image" the inhomogeneities as the heat source is scanned, point by point, over the sample. Many schemes have been described to detect the thermal wave, including gas cell acoustic, piezoelectric, pyroelectric, infrared, photo-deflection, photo-refraction, and temperature-sensitive luminescent chemicals.

All of these detection schemes have the property that the "signal" is detected as the heat source is scanned point by point. Therefore, to acquire a high resolution image of a large area sample requires a long time. In order to obtain high speed thermal images of large area samples, holographic recording techniques and large area (as opposed to point of focus) heat sources are proposed. An embodiment of this invention is shown that incorporates a conventional holographic imaging system and several additional components, including the heat source HS1, a chopper C2, a lens L4, a phone shifter PS1, and a controller. These additional components provide thermal imaging information, for the detection of subsurface effects. The basic holographic system works as follows: The CW laser beam from laser LA1 is split by the partial mirror PM1 . One beam, the object beam OB is reflected from mirror M1, and defocussed by lens L2, to impinge upon the sample S1, with subsurface inhomogeneity IH1
. Sample S1 scatters the laser light to the holographic recording medium HRM1 . Simultaneously, the reference laser beam RB, is reflected by mirror M1, defocussed by lens L1, and then impinges upon HRM1 . The optical interference of OB and RB at HRM1 is recorded by HRM1 . This hologram can then be focussed by lens L3 onto the videocamera VC1, and then recorded by the video recorder VR1 for subsequent analysis. If the sample S1 is opaque to laser light, the hologram will contain information only about the morphology of the surface of the sample and the internal inhomogeneity IH1 will not be detected. Heat source HS1is modulated or pulsed by chopper C2 and defocussed by lens L4 to uniformly heat the surface of sample S1 .

This causes the sample surface to be heated by the heat source either periodically at frequency f0= 0/2f or pulsed, or modulated with some other convenient function of time. Simultaneously, the hologram laser source LA1is chopped by chopper C1 . The time function of the modulation of the hologram laser LA1 is similar to that of the heat source HS1 . However...