Theoretical calculation and experimental investigation of speckle reduction by multiple wavelength lasers in laser projector with different angular diversities

Abstract

It is well known that speckle reduction can be achieved by using multiple wavelength lasers and angular diversity in laser projectors. However, few studies have been reported about the dependency between the speckle reduction by angular diversity and speckle reduction using multiple wavelength lasers. Here, the speckle reduction using 1–5 lasers with different wavelengths was measured on matte and silver screens. The image size was chosen from two sizes to change the angular diversity. The results were compared with the theoretical calculation. It was shown that speckle reduction using multiple wavelength lasers depends on the image size and screens. On a silver screen, the amount of speckle reduction by wavelength diversity depends on the image size; wavelength interval of two lasers needed to reduce speckle was smaller when the image was larger. On the other hand, on matte screen, speckle reduction by wavelength diversity does not depend on the image size. This is because effective surface roughness of matte screen is larger than that of the silver screen due to multiple or volume scattering. In addition to wavelength diversity, the difference of speckle reduction between matte and silver screens is caused by polarization diversity. The speckle contrast decreases by polarization diversity by a factor of 2 on matte screen, whereas it does not decrease on silver screen because silver screen preserves polarization. Speckle contrast was calculated by using illumination light property, screen property, and speckle measurement camera settings. The experimental results were in a good agreement with the theoretical calculation.