A Comparison of Electronic Heterodyne Moire Deflectometry and Electroni,; Heterodyne Holographic Interferometry for Flow Measurements

Electronic heterodyne moire deflectometry and electronic heterodyne holographic interferometry are compared as methods for the accurate measurement of refractive index and density change distributions of phase object p . Experimental results are presented to show that the two methods have comparable accuracy for measuring the first derivative of the intE-ferometric fringe shift. The phase object for 'he -easurements is a large crystal of RD*P, w..ose refrac,:ive index distribution can be changed v accurately and repeatably for the comparison. N although the refractive index change causes only LL about one interferometric fringe shift over the entire crystal, t,e derivative shows considerable detail for the comparison. A3 electronic phase measurement methods, both methods are very accurate and are intrinsically compatible with computer controlled readout and data processing. Heterodyne moire is relatively inexpensive and has high variable sensitivity. Heterodyne holographic interferometry is better developed, and can be used with poor quality optical access to the experiment. IN THIS PAPER, electronic heterodyne moire deflectometry and electronic heterodyne holographic interferometry are compared for the accurate measurement of refractive index and density distributions of phase objzcts. The work, on which the comparison is based, was done at NASA Lewis Research Center ab part of a program to study the possibilities of automated, precise fringe measurement systems for use in wind tunnels. The two techniques, compared in this report, also h-ve other applice':ors, such as structural analysis and heat transfer measurements. The advantages of an electronic heterodyne method are accuracy, the ability to interpolate continuously be t ween fringe maxima, and computer compatibility. In using an electronic heterodyne method, the relative phase between two points in a time varying fringe pattern is r-Pasu red. This phase is then related to the difference in fringe number between the two points. Since phase measurements can be accomplished to better than a degree, fringe interpolation accuracies better than 1/360 fringe are possible. Electronic heterodyne interferometry, holography, and holographic interferometry have been undo: development for more than a decade (1-6)*. A fringe-interpolation accuracy of 1/1000 of a fringe has been demonstrated by Dindliker in measuring the bending of a structure using electronic heterodyne holographic interferometry (2). Farrel, Springer, and Vest have used the technique to measure concentrations and temperatures of gas mixtures (5). A commercial multichannel elEc.trouic heterodyne interferometer is ava: 1 FDle (6), and the image dissector has been demonstrated for the rapid scanning and heterodyne measurement of a fringe pattern (3). The problem with using electronic heterudyne holography are those of holography itself: noise and nonlinearities. The method also cequires interferometric stability for readout. Deflectometry, whose best known representative is Toppler schlieren has recently been rediscovered to be quantitative as well as qualitative (7-10). Moire deflectometry allows both the magnitude and direction of a light-ray deflection to be measured. The application of the heterodyne readout technique to moire fringes is quite new (11-13), but it offers the same advantages to moire as to interferometry. The relative states of development of electronic heterodyne moire and interferometry can be summarized. The measurement of fringes in electronic heterodyne holography and interferometry has been studied thoroughly, and *Numbers in parentheses designate references at end of paper.