A DMD (Digital Micro-Mirror Device) Based Multi-Object Quasi-Imaging Spectrometer Trine Kirkhus, Britta Fismen, Øystein Skotheim, and Jon Tschudi SINTEF ICT Forskningsveien 1, N-0372 Oslo, Norway trk@sintef.no http://www.sintef.no/omd Abstract In order to increase flexibility in spectral measurements performed by a point detector spectrometer, we are using digital micro-mirror devices (DMD) to selectively both illuminate and pick out regions of interest from a scene. The goal is to make a robust, quasi-imaging spectrometer and reduce the effects of scattered light from the background and nearby objects. 1. Introduction For designing an optimal setup for a spectral measurement of a multi-object scene, a series of choices has to be made. Thorough analysis of the measurement situation unveils needs for spatial resolution, spectral resolution and wavelength band of interest and so forth. The solution might be a scanning point measurement, or some kind of imaging spectrometer, often including a dispersive element, a camera, and a scanning motion either using a mirror device (for example a DMD) or moving the sample. The latter is time consuming. A spectral reflectance system is suggested (in Figure 1) which makes use of a point measuring spectrometer, a camera and two DMDs. This ensures that both the illuminated areas and the imaged regions are fully programmable. The system is spectrally quasi-imaging in the sense that it can be adjusted to measure the reflectance from arbitrary regions of interest. In addition it will be possible to do multi-object spectroscopy by carefully designing a mask that allows several spectrally different objects in the scene to be rapidly measured in sequence. We will focus on spectral measurements in the visual and near-infrared part of the spectrum, as there are several applications in this wavelength region where such a system is of interest. 1.1. Related Work The use of DMDs for spectral measurements have been described earlier, but for other measurement strategies than the one presented here. Several groups have used micro-mirrors to select specific wavelengths from a rainbow light source    Sarvotham et al. [4] are using the DMD to build up an image using one single pixel photo detector. This enables them to image wavelengths from the scene that are not possible to image using present CCD or CMOS cameras. DMDs have also been used in spectral imaging, where the regions to be measured are selected using a line imaging spectrometer. Kearney et al. have characterized the DMD’s optical properties for this use in [5] and in [6] they describe how this can be implemented and used to make a spatial and spectral hypercube and how to adjust the integration time to avoid sensor saturation. Nayar et al. [7] used a DMD in front of an imaging device for doing programmable imaging in order to switch the light off for pixels that have been saturated, resulting in an image with increased dynamic range. DMD controller Image analyser Figure 1: Schematic overview of the concept idea. The scene is illuminated and imaged onto a camera (via two DMDs). Based on image analysis, the region of interest is located, here the red apple. A mask is generated for the illumination DMD to ensure that only the apple is illuminated, avoiding the background and the green leaf. The detection DMD picks up only the light reflected from the apple and this light is projected onto the entrance aperture of the spectrometer.