SPECKLE PHOTOGRAPHY FOR REVEALING HIDDEN STRUCTURES

1. SPECKLE PHOTOGRAPHY FOR REVEALING HIDDEN STRUCTURES APRIL, 24 2007 STEVEN MERSCH smersch@pointsource-inc.com POINT SOURCE, INC. 1864 DAYTON PIKE GERMANTOWN, OH 45327

2. Speckle photography / speckle interferometry holds the potential to provide a fast, simple, compact, portable and affordable means of checking a surface for features potentially hidden behind or beneath the surface. THE CONCEPT

4. Requirements for performing speckle photography • A laser (coherent) light source for subject illumination (see white light exception). • A subject with a diffuse (rough) surface. • A high resolution (electronic) camera. • Image processor that is capable of storing a reference, performing image subtraction, and applying image smoothing algorithms. • An energy means for introducing a disturbance into the subject.

5. Names of Speckle Techniques • Laser Speckle Interferometry • Laser Speckle Photography • White Light Speckle Photography • Sunlight Speckle Photography • Electronic Speckle Pattern Interferometry (ESPI) • Laser Speckle Contrast Imaging • Laser Speckle Imaging (LSI) • Laser Speckle Contrast Analysis (LASCA) • Speckle Metrology

6. Basic Speckle Imaging System

7. Dual Illumination Interferometric Method

8. Example of speckle interferometry Difference of the two speckle patterns Speckle pattern of undeformed surface Speckle pattern of deformed surface

9. Electronic Speckle Pattern Interferometry (ESPI) • Use TV system to record speckle instead of film • Gives real-time measurements • Minimum speckle size limited by camera resolution • Can perform computer analysis of speckle data

12. Using Laser Speckle to Image Subsurface Void A 5mm hole covered by a sheet of white paper. First image is an unprocessed photo. Second and third are subtracted and processed speckle images.

13. White Light Speckle Photography • Has been used for studying movement or distortions in large areas (large subjects). • Incoherent light source → Lower absolute sensitivity. • Can use the sun as a light source but must use a very high resolution camera for inspection of large areas and must collect images with the sun in nearly the same orientation (e.g. glacier movement study)

14. Concept for Finding Tunnels with Speckle Photography • Illuminate the surface to be checked with laser or controlled white light source. • Image the surface with a high resolution camera. • Store the image as the background image. • Cause a disturbance (i.e. from a small underground detonation) to propagate through the area of interest. • Collect images while the disturbance propagates through the area of interest. • Perform image subtraction of the background stored image. • Perform further smoothing of the “difference” image. • Examine the resulting difference image for anomalies. • The effect of a subsurface disturbance on the surface is a function of subsurface features.

15. References • Nothdurft and Yao, “Imaging Obscured Subsurface Inhomogeneity Using Laser Speckle”, Dec, 2006, Vol. 13, No 25, Optics Express. • Conley and Genin, “Application of Speckle Metrology at a Nuclear Waste Repository”, SPIE Vol. 1332, 1990, pg 798. • Briers, David, “Laser Speckle Contrast Imaging for Measuring Blood Flow”, Proceedings of the Symposium on Photonics Technology for 7th Framework Program, 2006, pg 328. • Stetson, “A Review of Speckle Photography and Interferometry”, Optical Engineering, Vol. 14, Sept-Oct 1975, pp 482-489. • North, Smith and Browne, “Development of Sunlight Speckle Photography”, Proc SPIE Vol. 2921, March, 1997, pp393-398. • Aguanno, Lakestani, Whelan, Connelley, “Speckle interferometry using a CMOS-DSP camera for static and dynamic deformation measurements”, ICEM12 – 12th International Conference on Experimental Mechanics, Sept, 2004. • Conley and Morgan, “Speckle Photography Applied to Measure Deformations of Very Large Structures”, SPIE Vol. 2446, pp161-168.