The Use of 1-bit Numerical Correlation in the Measurement of an Object's Translation by Speckle Correlation

Modern optics offers several methods that are capable of non-contact and non-destructive measurements of an object's deformation (e. g. holographic interferometry, photoelasticity). It is already well known that interaction of a coherent beam and a rough object's surface results in an interference field called speckle. Speckle can be exploited for our demands - such as an object's translation measurement, which is presented in this paper. Coherent light generates a speckle field either by reflection or transmission through the measured object. We can use a camera or simply a sheet of paper to see specific sections of this field. The observed speckle pattern carries information about the object's surface roughness, but does not carry any information about the forces acting on the object and changing its properties. Such information is carried by the correlation of at least two snapshots of speckle pattern. By applying a correlation algorithm we are able to compare recorded signals before and after object deformation. There is a well-described theory that binds the signal shift and the object deformation. Signals are usually recorded in 0-255 values, so the method is called 8-bit speckle pattern correlation. We have tried to modify this method with downsampling to just one bit. Our motivation is to lower the bit rate to simplify the numerical calculations while preserving the accuracy of measurement. This method is applied to the measurement of an object's small translation. We analyze influences that bring the use of the binary correlation method to this measurement, especially its accuracy and repeatability.