System parameter analysis of NIR-TRS spectra from homogeneous media with and without an absorbing boundary and from heterogeneous media with a single absorber.

Abstract

Traditional near infrared-time resolved spectroscopy (NIR-TRS) analysis includes fitting a spectrum with a known analytical solution to obtain the media absorption and scattering coefficients or computing mean time-of-flight for heterogeneity localization. The benefit of applying frequency response analysis to NIR-TRS data reduction is that magnitude ratio (MR) and phase shift (φ) information in a wide frequency range can be obtained from a single TRS spectrum. The sharper the input pulse is the wider the frequency range values one can obtain. This analysis has been applied to NIR-TRS spectra to obtain optical parameters from homogeneous systems (Kang et al., in press) and for localizing absorbers in heterogeneous systems (Kang et al., 1994). MR and φ can also be obtained from phase modulation spectroscopy (PMS). PMS, however, provides these values at a fixed modulation frequency unless the instrument is sophisticated enough to change the modulation frequency in a wide range. Another benefit is that multiple system parameters can be obtained from a single TRS spectrum, i.e., MR, φ, steady state gain (K), break frequency (fb)/time constant (τ), and system order (n). Steady state gain (K) is a valuable parameter that can be used to check system linearity. Other new parameters (fb/τ, n) are still to be studied in an optical sense.