An Imaging Spectrometer using an Acousto-Optic Tunable Filter

Abstract

Spectroscopy is a powerful tool for analysis in a widerange of scientific and industrial fields. Spectroscopic measurements obtaininformation about the interaction between matter and radiated energy.This information may be utilized for detection and classification of objectsand substances. There is a wealth of different methods and technologiesdeveloped for different applications, from space- and air-borne remotesensing to medical sample analysis.In this report we present a spectral imaging system consisting of a digitalcamera with an acousto-optical tunable filter (AOTF) as the spectralband selective element. An evaluation of the properties of the imager ispresented, and we investigate the feasibility of real-time spectral imagingvideo based on this system. A comparison with other spectral imagingtechnologies is made. Custom application software for camera control andimage acquisition is presented. A special optical layout that minimizes theoptical aberrations produced by the diffraction process in the AOTF is alsopresented.The system produce a stack of monochrome images at different wavelength bandsforming an "image cube" containing a combinationof spatial an spectral information. The system uses the visible part ofthe electromagnetic spectrum to collect spectral information at every pointin an image plane. Our results show that the camera have good lightsensitivity and a high signal-to-noise ratio, but that the image throughputand spatial resolution is not optimal for a high image rate. The AOTFis a highly flexible device with random access to wavelength bands andselectable band-pass shape. But the many configurable parameters of thesystem also complicates usage. Many parameters need to be calibrated,and we present some methods on this. A major challenge in an spectralimaging system is to get enough light through to the sensor, and we findthat there is high loss of light caused by the diffraction process in the filter.It will be shown that the system is not suitable for real-time spectral videoimaging because of the low frame rate. Suggestions for improvements aregiven.