SIMULTANEOUS MULTISPECTRAL IMAGING IN THE VISIBLE AND NEAR-INFRARED REGION - APPLICATIONS IN DOCUMENT AUTHENTICATION AND DETERMINATION OF CHEMICAL INHOMOGENEITY OF COPOLYMERS

A new multispectral imaging spectrometer capable of simultaneously rec ording spectral images in the visible and near-infrared has been devel oped. In this instrument, an acoustooptic tunable filter is used to di ffract an unpolarized incident light into two diffracted beams with or thogonal polarization; one of them is detected by a silicon camera for the visible region while the other beam is detected in the near-infra red region (li om I to 1.7 mu m) with a NIR camera. The imaging spectr ometer is sensitive, inexpensive, and field deployable because it is b ased on the recently available InGaAs focal plane arrays camera, which is low cost and can be sensitively operated at room temperature. Prel iminary applications of the imaging spectrometer include measurements of the visible and NIR absorption spectra of ink used to print U.S. cu rrency. Such results may help to characterize samples as well as to co ntrol and to ensure the quality of the samples during the production p rocesses; More important are the results obtained on ethylene/vinyl ac etate copolymers. The NIR spectral images obtained clearly indicate th at these copolymers exhibit a high degree of chemical inhomogeneity. B ecause of the possibility of inhomogeneity, it is very important that the homogeneity of polymers or copolymers be thoroughly understood bef ore the NIR methods, especially those based on NIR spectrometers equip ped with a single-element detector, are used for measurements.