Polarization-Division Interferometry: Approach for Biomedical Infrared Imaging

Abstract

Conventional Fourier transform infrared absorption spectroscopy is not suitable for most biological and biomedical studies because of strong infrared absorption by water. For wavelengths where samples absorb more than one unit of absorbance, most measurements with conventional Fourier transform infrared spectroscopy suffer from photometric inaccuracies. However, water is not sensitive to the polarization of the infrared light, but living systems can exhibit differential interaction to the polarized nature of light. In a method where the measured signal directly represents the difference in polarized absorptions, the water component becomes spectrally transparent (although the number of photons reaching the detector is still attenuated by water) and facilitates the measurements appropriate for biological systems. Verification of this principle is reported here for the use of a new midinfrared polarizationdivision interferometer developed in our laboratory. With the amide I vibrational band of a nylonll film which overlaps with the water absorption band at 1647 cm-', the associated dichroism is measured in the presence of varying amounts of interfering water absorption. It is found that dichroism can be measured even in the presence of strong interfering absorption from water. This observation suggests that the present method provides a unique approach, as well as the necessary impetus, for initiating biomedical infrared imaging studies.