Liquid-crystal based hyperspectral image projectors.

摘要

Traditional color cameras are able to detect limited spectral content: red, green, and blue. This is useful for simulating detection by the human eye, but is insufficient for some applications. This need for high resolution spectral imaging led to the formation of a new class of image acquisition, which is referred to as hyperspectral imaging or imaging spectroscopy. This technique combines the benefits of spectroscopy, which allows the ability to identify chemical compounds within a sample through measurements of reflectance as a function of wavelength, with imaging such that chemical compounds can be mapped spatially within the scene.The ability to realize the spectral resolution offered by hyperspectral imagers hinges on the ability to calibrate and qualify the detectors. This need has inspired the National Institute of Standards and Technology (NIST) to develop hyperspectral image projectors (HIP), that are used to generate realistic scenes for the development of standardized calibration and system performance verification protocols for hyperspectral imagers. The current HIP system at NIST uses micromirror arrays as intensity modulators. However, there are several drawbacks to this approach. First, micromirror arrays produce grayscale modulation through rapid dithering between two states, where one state transmits through the HIP system, and the other state diffracts light to a block. This approach is not compatible with all sensors that one may wish to test. Second, micromirror arrays have a small pixel pitch, resulting in significant diffraction in the MWIR and LWIR. Last, micromirror arrays tilt from one off-axis position to another off-axis position, resulting in an awkward off-axis optical layout. These issues motivated consideration of an alternative, liquid-crystal based approach.In this thesis a hyperspectral image projector (HIP) is introduced that is built with liquid crystal (LC) based spatial light modulators (SLM) as opposed to micromirror arrays. The use of an LC based SLM as a broadband intensity modulator presents several benefits to this application. With slight modifications to the SLM design, SLMs can be built for a wide range of spectral regimes, ranging from the ultraviolet (UV) to the long-wave infrared (LWIR). SLMs can have a large pixel pitch, significantly reducing diffraction in the mid-wavelength infrared (MWIR) and LWIR. Liquid crystal based devices offer direct analog intensity modulation, thus eliminating flicker from time-sequential drive schemes. SLMs allow for an on-axis configuration, enabling a simple and compact optical layout.The design of the HIP system is broken into two parts consisting of a spectral and spatial engine. In the spectral engine a diffraction grating is used to disperse a broadband source into spectral components, where an SLM modulates the relative intensity of the components to dynamically generate complex spectra. The recombined output is fed to the spatial engine which is used to construct two-dimensional scenes.The system is used to simulate a broad range of real world environments, and will be delivered to the National Institute of Standards and Technology as an enabling tool for the development of calibration standards and performance testing techniques for multispectral and hyperspectral imagers. The focus of this thesis is on a visible-band HIP system however, related work is presented with regard to SLM use in the MWIR and LWIR.