Abstract
BackgroundHomogenizing properties of beam shaping diffusers illuminated with coherent laser light was studied with special regard on their use in vision applications.ExperimentalTwo diffusers projecting circular pattern at different scattering angles (Thorlabs Engineered Diffuser™) were illuminated with different spatial intensity distribution (elliptical Gaussian, nearly flat-top and granular multimode fiber output, respectively) collimated beams.ResultsThe intensity distribution measured at high angular resolution with a CMOS sensor revealed the granular (speckle) pattern of the projected light having contrast of nearly 1. The smallest speckle size (highest speckle density) could be obtained at the largest aperture size using nearly flat-top and multimode fiber illumination.ConclusionAs an important measure of the homogeneity for vision applications the apparent local contrast was calculated as the function of the spatial averaging. Although, the homogeneity requirements depend on the actual application, the usability of such diffusers in combination with coherent light sources is in general strongly limited in megapixel resolution imaging applications, unless time-varying speckle averaging methods can be applied.
Highlights
The laser beam shaping and homogenization is of high importance in various industrial, medical and scientific areas, including material processing, interferometric and vision applications, etc
The pattern produced by the diffuser was recorded at different working distances (WD, 25 and 35 cm, respectively) by the sensor of an 8-bit industrial monochrome camera (Pixelink PL-B741F, Cypress IBIS 5B Global Shutter Complementary Metal-Oxide Semiconductor (CMOS) sensor, resolution 1280 × 1024, 6.7 μm × 6.7 μm pixel size) allowing high resolution imaging of the spatial intensity distribution
While in average there is a nearly inversely proportional decrease when increasing the diameter of the illuminating spot on the diffuser surface, the corresponding speckle sizes were practically similar for the flat-top and Multimode optical fiber (MM) illuminations and slightly higher for Gaussian beam distribution
Summary
The intensity distribution measured at high angular resolution with a CMOS sensor revealed the granular (speckle) pattern of the projected light having contrast of nearly 1. The smallest speckle size (highest speckle density) could be obtained at the largest aperture size using nearly flat-top and multimode fiber illumination
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