Didactic laser speckle experiments with a lensless camera

Abstract

Laser speckles are granular patterns created through the interference of coherent light scattered by a rough surface. Besides comprising an appealing optical phenomenon, the speckle patterns enable applications in physical and biochemical measurements. This paper proposes didactic far-field (objective) speckle experiments using a visible laser source and a lensless webcam for implementation in optics labs or at home. Firstly, a ceramic mug works as the scattering surface to investigate the effect of the target-observation plane distance on the average size of light granules. Secondly, the speckle pattern deviations produced by the liquid inside the recipient are analysed to measure mass and temperature changes. Finally, the mug is replaced with a leaf to evaluate its biological activity based on the scattered light. Image processing tools carry out the quantification of speckle parameters like average size and correlation coefficient, achieving a good agreement between theory and practical results. The proposed far-field speckle experiments explore topics related to optics, instrumentation, and image computing straightforwardly, developing the desired skills for undergraduate and graduate students of physics and engineering courses.