The motion picture industry is presently transitioning from traditional film-based projectors to digital or electronic cinema. This trend is accelerating due to the popularity of three-dimensional (3-D) movies. Even as digital cinema projection has matured and succeeded, largely based on the use of Digital Light Projection (DLP) technology, both the light sources and the DMD modulators have been evolving. In the case of lasers, high power compact visible lasers are becoming increasingly mature and cost competitive, enabling the development of laser digital cinema projectors. One such system is described in the paper “A Laser-Based Digital Cinema Projector”, by B. Silverstein et al. (SID Symposium Digest, Vol. 42, pp. 326-329, 2011).
At the core of the DLP technology, which was developed by Texas Instruments, is the digital micro-mirror device (DMD), which is a spatial light modulator that includes an array of micro-mirrors. DMD spatial light modulators have been successfully employed in digital projection systems, including digital cinema devices that meet the DC2K digital cinema resolution standard. Efficiency measurements have been performed on such systems that use the DC2K chip and have found the efficiency optimizing principles to be valid; however, for systems utilizing the new DC4K chip the principle was not found to work as well.
Early in the development of the DMD device technology, the individual micro-mirrors 55 or pixels were relatively large, at ˜30 μm square. Subsequently, device resolution has improved, with progressively smaller pixels, evolving from ˜17 μm square in the late 1990's, to 13.8×13.8 μm for the 2K digital cinema projectors (2005), and more recently, reaching 7.5 μm×7.5 μm pixels with the DC4K devices (2011). The projector described by Silverstein et al used 2K resolution DMD devices for image light modulation, and various aspects of DMD device behavior were taken into account. As another example, in the paper “Laser Digital Cinema Projector,” by G. Zheng et al., published in the Journal of Display Technology, Vol. 4 (2008), a laser projector is described that also uses the DC2K versions of the DMD devices, but with conventional DLP projection optics, including the TIR and Philips prism assembly. Notably, both Silverstein et al and Zheng et al describe projectors using the DC2K versions of the DMD devices, meaning that they were compliant with the DCI digital cinema projection specification and provided “2K” horizontal resolution.
However, as noted above, the recently released DC4K devices have much smaller pixels to support the higher horizontal resolution standard. As the size of the micro-mirrors decreases, diffraction effects become increasingly important, and the micro-mirror array can be thought of as a programmable blazed grating. Diffraction becomes an even larger concern when laser light interacts with these small pixel and sub-pixel features. Essentially, diffraction effects cause an efficiency loss versus a plane mirrored surface as some light is directed into other diffracted orders, which the blaze then partially restores.
Some aspects of the diffractive behavior of the DMD devices have been considered in the literature. In particular, Texas Instruments provided some guidance concerning the interaction of laser light, in their publication “Using Lasers with DLP® DMD technology”, TI Tech Note TI DN 2509927, September 2008. However, additional practical guidance on the diffraction behavior that aids projector design is needed. A subsequent paper “DMD diffraction measurements to support design of projectors for test and evaluation of multispectral and hyperspectral imaging sensors”, by J. P. Rice et al., published in the SPIE Proc., Vol. 7210 (2009) provides infrared (IR) light diffraction efficiency measurements for IR optimized 2K resolution DMD devices. However, this paper does not discuss the optical propagation behavior of the diffracted light, and how that might affect projector design, whether in the IR or other spectral ranges.
In summary, as micro-mirror array spatial modulators such as the DMD devices have migrated to yet smaller pixel dimensions, and laser projection systems have become increasingly feasible, the specifics of the interaction of the laser light with the micro-mirrors, which are dynamic diffractive structures, becomes more important. Therefore, opportunity exists to better understand the subtleties of micro-mirror array diffraction behavior and optimize projector design accordingly.