# Presentation: 424695

## SPECKLE PHOTOGRAPHY FOR REVEALING HIDDEN STRUCTURES

- APRIL, 24 2007
- STEVEN MERSCH
- smersch@pointsource-inc.com
- POINT SOURCE, INC.
- 1864 DAYTON PIKE
- GERMANTOWN, OH 45327

## 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**

## Slide 3

## 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.

## 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

## Basic Speckle Imaging System

## Dual Illumination Interferometric Method

## Example of speckle interferometry 

- Difference of the two speckle patterns

- Example of speckle interferometry

- Speckle pattern of undeformed surface

- Speckle pattern of deformed surface

## 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**

## Slide 10

## Slide 11

## 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.

## 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)

## 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.

## 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.