Simultaneous Multispectral Imaging in the Visible and Near-Infrared Region: Applications in Document Authentication and Determination of Chemical Inhomogeneity of Copolymers Chieu D. Tran,* Yan Cui, and Sergey Smirnov Department of Chemistry, Marquette University P.O. Box 1881, Milwaukee, Wisconsin 53201 A new multispectral imaging spectrometer capable of simultaneously recording spectral images in the visible and near-infrared has been developed. In this instru- ment, an acoustooptic tunable filter is used to diffract an unpolarized incident light into two diffracted beams with orthogonal polarization; one of them is detected by a silicon camera for the visible region while the other beam is detected in the near-infrared region (from 1 to 1 .7 μm) with a NIR camera. The imaging spectrometer is sensi- tive, inexpensive, and field deployable because it is based on the recently available InGaAs focal plane arrays camera, which is low cost and can be sensitively operated at room temperature. Preliminary applications of the imaging spectrometer include measurements of the visible and NIR absorption spectra of ink used to print U.S. currency. Such results may help to characterize samples as well as to control and to ensure the quality of the samples during the production processes. More impor- tant are the results obtained on ethylene/ vinyl acetate copolymers. The NIR spectral images obtained clearly indicate that these copolymers exhibit a high degree of chemical inhomogeneity. Because of the possibility of inhomogeneity, it is very important that the homogeneity of polymers or copolymers be thoroughly understood before the NIR methods, especially those based on NIR spectrometers equipped with a single-element detector, are used for measurements. A multispectral imaging spectrometer is an instrument that can simultaneously record spectral and spatial information of a sample; i.e., the recorded images contain signals that are gener- ated by molecules or units in a sample, plotted as a function of spectral and spatial distribution. 1 Chemical homogeneity of the sample can be elucidated from such images. This type of information is of particular importance since it is known that chemical as well as physical properties of materials are dependent on the chemical distribution within the samples. As a conse- quence, considerable efforts have been made in the past few years to develop multispectral imaging instruments. In these instru- ments, the spatial distribution of the sample is obtained by a camera, and the spectral information is gained by scanning a dispersive element to record spectra for each image. 1 Mechanical scanning dispersive devices (filter wheels, monochromators) are not desirable because they are slow and prone to vibrations. Spectral tunable filters based on electronic tuning such as an acoustooptic tunable filter, are desirable as they are fast and compact and have no moving parts. An acoustooptic tunable filter (AOTF) is a solid-state electroni- cally tunable spectral band-pass filter which is based on the diffraction of light by an acoustic wave in an anisotropic crystal. 2-10 Compared to other dispersive devices, the AOTF offers such advantages as being all solid state, having rapid scanning ability ( μs), high diffraction efficiency, and wide spectral tuning range, giving high resolution (2-6 Å), and offering imaging capability. 2-10 The filter is, therefore, particularly suited for multispectral imaging instruments. Multispectral imaging instruments based on AOTF have, in fact, been developed. In these instruments, the tunable filter was used to disperse light in either the visible or the near-infrared (NIR) region, and the corresponding spatial information was then obtained with either a silicon CCD camera or an indium anti- monide (InSb) camera. 11-18 As such, these instruments have not exploited the full potentials of the AOTF. 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