Research On Digital Micromirror Devices For Use In Infrared Scene Projector

The infrared(IR)scene projection is a core part of IR hardware-in-the-loop(HWIL)simulation system and provides IR target and background imaging in the simulation system.There are several kinds of typical IR scene projectors developed in succession,including infrared CRT,laser diode arrays,resistive arrays,IR liquid crystal light valves and digital micro-mirror devices(DMDs).Compared with other types of scene projections,DMD-based scene projection is developed rapidly because of its low cost,ultra-stability and high-quality imaging.Referring to the current status of DMD-based scene projection,several technical issues have to be addressed for its application in IR scene hardware-in-the-loop simulation systems.In order to resolve these issues,further study and comprehensive research are done in this thesis.The DMD is a reflected spatial light modulator invented by Texas Instruments Incorporated in 1987 and is widely used in DLP projection display,HD theater,spectral imaging,Photolithography and so on.Currently the DMD on sales by TI works at a wavelength range from 320 to 2500 nm and it can’t work well at wavelengths longer than approximately 2.7μm.Therefore,for the use in traditional mid-and long-wave IR bands(3-5μm and 8-12μm)the DMD package window has to be replaced by an IR transmitting window.After a careful study on infrared materials,we use Zn Se window to replace the standard window.Firstly the Zn Se window is treated by thin film coating,and the transmittance of the coated ZnSe window is more than 95% and more than 80% in 3-5μm band and in 8-12μm band,respectively.Then we develop a process of micro-nano manufacture technology for removing the standard window and installing the ZnSe window while maintaining 100% operability of the mirrors.It is shown with series of tests that the replaced DMD can work well in the mid and long wave IR bands.Diffraction effects play a significant role for DMD-based scene projectors used in IR waveband.What’s worse,the diffraction effects are more seriously as the increase of incident wavelengths.In 8-12μm bands,the image contrast of the projector systems can become noticeably worse due to the diffraction of the mirrors.For this issue,a metallic diffraction grating,in accordance with the structure and the operation of the DMD,is presented.Both of scalar diffraction and vector diffraction theory are adopted to characterize the diffraction of the DMD in IR waveband.Simulation with scalar theory comes to conclusion that in 3-5μm bands,the projector can be with the best contrast as an incident azimuth angle of 28°.We analyze the diffraction effects of the DMD in 8-12μm bands with vector diffraction theory.It is shown that the diffraction effects of the DMD-based scene projectors can be weakened greatly with TM polarization light at an incident azimuth angle of 48°.A series of experiments are conducted to further verify the simulation analysis.Firstly an experimental system is set up to verify the vector diffraction grating model in 8-12μm bands.The experiment is well consistent with the simulation results.Then quantitative measures of the main parameters that influence the contrast of the DMD-based scene projectors in 8-12μm bands are given.Finally,the method to improve the contrast of the DMD-based scene projectors is verified.It is concluded that in 8-12μm bands,a good imaging contrast of the DMD-based scene projectors can be obtained with TM polarization light at an incident azimuth angle of 48°.We develop a long waveband infrared DMD-based scene projector.Firstly,the illumination optics and projection optics are designed with the consideration of the simulation and experimental results previously.The illumination optics condenses and relays the energy from the illumination source onto the DMD at the proper angle of illumination.It is designed to have complete and uniform illumination on the DMD,and with higher energy efficiency.The projector optics,a telecentric structure,is designed to collimate the image of the DMD at an infinite conjugate.Then imaging performance of the system is tested.The result shows that the DMD-based scene projector performs well with an imaging contrast about 0.85 in the longwave IR band.It can provide high quality of IR image for Hardware-In-the-Loop simulation in longwave IR band.