Understanding the exposure-time effect on speckle contrast measurement for laser projection with rotating diffuser

Abstract

To evaluate the influence of observer’s exposure time over speckle noise of laser displays, we developed the high-speed measurement method which can measure speckle contrast over a wide range of exposure time without adjusting the brightness of laser displays in our previous work. We attempted to observe the exposure-time effect in speckle contrast for several commercially available laser displays using this method. However, their speckle contrasts were nearly constant at various exposure times in our experiments, although it is a more reasonable argument that the speckle contrast of laser display equipped with a temporally-averaging speckle-reduction device is dependent on the exposure time. Since we assumed that the temporal averaging factors of their laser displays were underestimated, we prototyped a speckle-reduction device using an intentionally slow-rotating diffused plate and measured the speckle contrast for a laser projection applying this device in this work. The speckle contrast was increased at the lower linear velocity of the diffused plate than 3.8 mm/s when the exposure time was fixed at the human response time. And, the speckle contrast was increased in the region of short exposure time when the linear velocity was fixed at 0.12, 0.47 and 1.9 mm/s. By analyzing the cross-correlation and the speckle average grain size, we confirmed that this phenomenon was caused by the exposure-time effect. In addition, we verified the relationship between the linear velocity, the exposure time and the temporal averaging factor by comparing the measurement results with theoretical fitting function.