Speckle reduction for a laser light sectioning sensor

Abstract

Abstract. Automated optical inspection is an important test procedure in electronic circuits assembly. Frequently 3d information is required and laser light sectioning sensors are often applied. However, some effects complicate the reliable automatic detection of the shape of such assemblies and their components. The packages of electronic components often are made of black plastics or ceramics so that the intensity available for the optical detection is quite low, especially in comparison to the surface of the PCBs where the components are mounted on. In addition due to rough surfaces of the components and the coherence length of the laser light speckle structures arise. In the work presented here a piezo actuator is used to oscillate the illuminating laser lines along the direction of the lines. The aim is to reduce the visibility of the speckle structures by averaging while maintaining the geometrical shape of the lines. In addition, image processing methods like segmentation and skeletonization are used to allow the detection of the shape of components and assemblies also if materials with distinct differences in the reflectivity are involved. Investigations include the influence of the parameters amplitude and frequency of the piezo actuator.

Imaging sensor data modelling and evaluation based on optical composite characteristics

“…Due to this behaviour and following Thompson et al [55] and Tutsch et al [56] speckle can be reduced substantially when the average speckle sized s is smaller than the dimension of a pixel s px which meansd s < s px . For the pixel size of the installed sensor, this implies a desired aperture number of f a < 5.876.…”

“…The average speckle grain dimensiond s can be approximated from Eq. 3 [54] which was applied by Thompson et al [55] and used from Tutsch et al [56].…”

“…4 is required. Inspired from Tutsch et al [56] the maximal blur tolerance u should be one pixel. In order to provide suitable information even for more geometrically complex components and larger fibre placement defects, this should be at least between DOF = [1.5,2] mm.…”