Dependence of speckle contrast in laser lighting on laser and phosphor characteristics

A challenge using lasers for general lighting is the ability of lasers to form speckles. Speckles arise due to the coherence of the laser light. In laser lighting, a blue laser is typically converted to white light through luminescence in a phosphor material. The correct mixing of scattered blue laser light and the yellow light, generated by luminescence, creates the impression of white light. However, elastic scattering of the coherent laser light in phosphor materials causes speckles to appear in the blue part of the generated white light. The presence of speckles affects the uniformity of the light and therefore efforts to reduce the contrast of the speckles are needed. We present a thorough investigation of the influence of different laser and phosphor properties on the degree of speckle contrast in the white light. In this study, we consider objective speckles, observed on a surface, illuminated by the white light. We find that the speckle contrast depends significantly on the coherence properties of the laser as well as on the properties of the phosphor material. These findings are beneficial in order to design low-speckle laser lighting systems for general lighting applications.