Laser and White-Light Speckle Techniques: A Tutorial Review

Abstract

This instructional review begins with descriptions of simple experiments that illustrate the phenomenon of coherent-light speckle and its properties that cause it to be useful for measurement of displacements and strains, namely the sensitivity of speckle brightness to object motion and the relationship between speckle size and imaging aperture. The concepts of laser speckle interferometry, speckle shearography, speckle photography, and white-light speckle photography are presented, and basic apparatus setups for implementation are illustrated. Exceptional data density and flexible displacement sensitivities cause these techniques to be favored for whole-field non-contacting measurements of displacement, strain, and vibrations, as well as for nondestructive inspection. The concept of using phase shifting to increase the precision of speckle interferometry far beyond that offered by analyzing fringe patterns is presented along with details of one method of execution. A simple MATLAB^® code for phase-shifting interferometry is included. Illustrative and suggestive applications are drawn from studies in geomechanics, biomechanics, fluid mechanics, NDI, strain analysis, residual stresses, and vibration characterization.

A.1 Appendix

Dr. S. Hong has kindly prepared for our use a short well-annotated MATLAB^® routine that performs a complete phase-stepping interferometry analysis. The program was written for digital speckle interferometry, but it can be made to serve for other types of interferometry. The code initializes the input and output devices, sets the phase stepping increment, acquires before-load and after-load intensity maps at each of four phase steps, calculates the before- and after-load phase difference maps using the four-step algorithm, subtracts the two phase difference maps to obtain the change-of-phase-difference array, wraps those results back to the range −π to +π, smooths the wrapped map to remove random noise, and finally unwraps it.

%============================================================== % A MATLAB script for phase-shifting digital speckle pattern interferometry. % Written by Dr. Soonsung Hong % The script assumes that the optical system consist of a IEEE-1394 (firewire) digital camera and an analog output device for phase shifting. % MATLAB Image Acquisition Toolbox and Data Acquisition Toolbox are used. %============================================================== % Initialize a digital camera (Image Acquisition Toolbox) vidobj = videoinput('dcam',1); % Initialize an analog output device (Data Acquisition Toolbox) ao = analogoutput('nidaq','Dev1'); ch = addchannel(ao,0); % Define the voltage step assuming the increment of 1.5 volt % introduces a 2*pi phase shift. vstep=1.5/4; % Acquire four phase-shifted speckle patterns in the undeformed state. for ii=1:4, putsample(ao,vstep*(ii-1)); %Send a voltage output to PZT controller pause(0.1); %Wait for 0.1 second i0(:,:,ii)=double(getsnapshot(vidobj)); %Capture and store an image end putsample(ao,0); pause %Wait for a loading to apply. Press any key to continue. % Acquire four phase-shifted speckle patterns in the deformed state. for ii=1:4, putsample(ao,vstep*(ii-1)); %Send a voltage output to PZT controller pause(0.1); %Wait for 0.1 second i1(:,:,ii)=double(getsnapshot(vidobj)); %Capture and store an image end putsample(ao,0); %Calculate phase maps using the four-step algorithm p0=atan2(i0(:,:,4)-i0(:,:,2),i0(:,:,1)-i0(:,:,3)); p1=atan2(i1(:,:,4)-i1(:,:,2),i1(:,:,1)-i1(:,:,3)); %Calculate the phase-change map and wrap back to –pi to +pi dp=atan2(sin(p1-p0),cos(p1-p0)); %Remove random noise in the phase-change map n=5; % Size of the smoothing window sindp=conv2(sin(dp),ones(n)/n^2,'same'); cosdp=conv2(cos(dp),ones(n)/n^2,'same'); sdp=atan2(sindp,cosdp); %the smoothed wrapped map %Unwrap the smooth phase-change map usdp=unwrap(unwrap(sdp,[],1),[],2);