Laser speckle reduction based on partial spatial coherence and microlens-array screens

摘要

We present, a novel speckle reduction scheme for application in laser-based projection systems. The scheme combines the use of a microlens array (MLA) as screen material with the concept of reduced spatial coherence. Incorporating the screen in the speckle reduction process reduces laser projector cost and complexity. On a typical screen, random scattering of coherent light would cause random interference, i.e. speckle. On an MLA screen however, the interference between the fields emitted by different microlenses is inhibited if the spatial coherence area of the incident light is made smaller than the microlens footprint. We tested both a MLA with randomly arranged lenses of varying size, averaging 120 μm in diameter, and a MLA with regularly spaced lenses with a fixed diameter of 100 μm. We benchmarked the performance of these MLA screens and a regular diffusive screen. Using a small-scale projection setup with a CCD camera as observer, we experimentally quantified the speckle contrast observed on these screens. Objective speckle contrast measurements on the irregular MLA yield results close to the subjective human speckle detection limit. Besides the experimental validation of the proposed speckle reduction scheme, we constructed a quantitative model to describe the speckle characteristics of the different screens. The model corresponds very well with experimental results and allows us to quantify the relative contributions of the different speckle reduction processes at play. Our approach can benefit any laser-based projection system, such as for example 3D cinema.