Scattered light testing of digital micromirror devices (DMDs)

Digital Micromirror Devices (DMDs), a type of Micro-Opto-ElectroMechanical System (MOEMS) device, are commonly used in Digital Light Processing (DLP) televisions and projectors. These devices consist of an array of hundreds of thousands to millions of micron-scale mirrors, each of which can be programmed to tilt in one of two directions. DMDs have proven useful in astronomy instrumentation where they have been used as a programmable slit, allowing light from a star or galaxy to be separated from the remainder of the field by tilting those mirrors aligned to the target toward the spectroscopic arm of the spectrograph, while the remaining mirrors are tilted to direct light to an imaging camera. When mirrors are tilted away from the spectroscopic arm, some light may still scatter back towards it, increasing the background noise. Characterizing this noise source is crucial to determining the sensitivity of the spectrograph. In this paper, we present contrast ratio measurement results for a Texas Instruments DLP7000. Two methods were used to determine the contrast ratio: 1) the ratio of the light intensity with all mirrors turned “on” to the intensity with all mirrors turned “off”; and 2), the ratio of the total number of mirrors illuminated compared to the number of mirrors required to reproduce the back-scattered light intensity. Additionally, we measured the ratio of the total light incident on the DMD surface compared to the total light back scattered to determine how much of the unwanted light entering the system becomes light scattered into the spectrograph. A variety of LEDs were used in the testing, ranging from 385 nm to 1050 nm. Both silicon and InGaAs photodiodes were used to measure the reflected light. In this work we present the details of the setup used to conduct the scattered light measurements, compare the two measurement methods, discuss the results of our testing, and provide analysis of the measured contrast.