Photoacoustic excitation sensing enhanced by cross-correlated unfocused speckle images

A part inspection system comprising:

• a photoacoustic excitation unit comprising at least one pulsed laser source configured to illuminate at least one predetermined position on a part and generate thereby ultrasonic excitation of the part;
• a coherent illuminator configured to illuminate at least one predetermined location on the part with coherent illumination, the at least one predetermined position illuminated by the at least one pulsed laser source is at a predetermined lateral shift with respect to the at least one predetermined location illuminated by the coherent illuminator;
• a detector;
• an optical unit in association with the detector, wherein the optical unit is configured to gather an illumination scattered from the at least one predetermined location to the detector, and wherein the detector is configured to generate, based on the scattered illumination, at least one sequence, wherein each of the at least one sequence comprising at least two temporally-sequential de-focused speckle pattern images of one of the at least one predetermined illuminated location;
• and
• an image analysis unit configured to:
• receive the at least one sequence;
• and
• determine at least one set of speckle pattern image translations, each of the at least one set is determined based on one of the at least one sequence, wherein each speckle pattern image translation in each of the at least one set is determined based on two temporally-sequential speckle patterns images in the respective sequence, and
• wherein the at least one predetermined position illuminated by the at least one pulsed laser source is at a predetermined lateral shift (δx, δz) with respect to the at least one predetermined location illuminated by the coherent illuminator, and
• wherein the part inspection system is configured to determine and control the lateral shift, by selecting the lateral shift which provides a maximal signal of translation derived from correlation between speckle patterns.