Dependency of speckle reduction by wavelength diversity on angular diversity in laser projection system

Abstract

Speckle is one of the most serious problems in laser projection system. Most speckle reduction methods in a laser projector can be categorized into wavelength diversity, angular diversity, and polarization diversity. In order to reduce speckle to the acceptable level, the combination of some or all of these methods are used. When the speckle reduction by wavelength diversity and angular diversity are used at the same time, the total speckle reduction can be different from the product of each speckle reduction effect when each of them is used separately. Here, the theoretical model of speckle reduction by wavelength diversity and angular diversity when they are used at the same time is derived. The calculation results by the theoretical model are compared with the experimental results. It is shown that theoretical calculation agrees well with the experimental results.

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

“…However, the normalized speckle contrast did not decrease by a factor of √2 even when the wavelength interval was large enough in the experiment. Thus, in our experiment, the value of C non-speckle was chosen to be 6% for the silver screen so that the calculated contrasts agree with the experimental results when the wavelength interval of the two lasers is large enough [18].…”

“…The light emitted from the projector illuminated a silver screen with effective surface roughness σ h of 3.2 μm (Finesilver 240, Kikuchi Science Laboratory Inc.), or a matte screen with effective surface roughness of 30 μm (Matt Plus, Harkness Screens Ldt.) at the normal angle [18]. Since the light emitted from each laser has nearly the same illumination angles, angular diversity did not change when the number of laser was changed.…”

“…Since the effect of speckle reduction by wavelength diversity depends on the observation angle [17], we fixed the observation angle throughout the experiments. The focal length of the objective lens of the CCD camera was 50 mm, and a circular aperture with a diameter of 1.0 mm was mounted in front of the camera to simulate the human eye [18,20]. The pixel size of the CCD sensor is 7.4 μm, and the speckle size on the CCD sensor was measured around 35 μm in diameter for both the matte and silver screens [21].…”

“…When those speckle reduction methods are used at the same time, they are treated independently in many cases [8,16]. However, in [17,18], we showed that speckle reduction by using lasers with two different wavelengths is dependent on angular diversity, and the theoretical model to express the speckle reduction by wavelength and angular diversities was derived.…”