Apparatus and method for reducing spectral complexity in optical sampling

#The present invention generally relates to the field of diagnostic spectroscopy, and more specifically, to an optical sampling subsystem that reduces the spectral complexity of light exiting a heterogeneous sample and collected by a plurality of fiber optics. The present invention increases the signal-to-noise ratio while compensating for spectral consistency and quantitative performance by providing means for creating a generally uniform radiance at the input of a wavelength dispersive or modulating device.

#Spectral data arising from spectroscopic analysis provides practitioners with a wealth of detailed information about the identity, structure, concentration or constituents of samples. Spectral data derives from the detected and recorded energy change of a molecule through the emission or absorption of a photon.

#In particular, practitioners focus upon a molecule's vibration. Atoms within a molecular species vibrate back and forth about an average distance. Absorption of light by an atom at an appropriate energy causes the atoms to become excited, elevating the atom to a higher vibration level. The excitation of the atoms to an excited state occurs only at certain discrete energy levels, which are characteristic for that particular molecule. Infrared absorption spectroscopy is particularly useful for performing this type of analysis. In absorption spectroscopy, the net absorption of incident radiation at various wavelengths is measured.

#FIG. 1 is a schematic depiction of a non-invasive spectrometer system;

#FIG. 2 is a perspective view of elements of a tissue sampling subsystem;

#FIG. 3 is a plan view of the sampling surface of the tissue sampling subsystem of FIG. 2 showing an arrangement of illumination and receiver optical fiber ends;

#An ideal spectrophotometer system would use a perfectly efficient and stable point source in optimal alignment with optical components that contribute no aberrations or variances and a detector that is perfectly efficient, noise-free, and captures all photons emitted by the source. If no sample were present, a spectrum obtained from such an instrument system would be exactly representative of the source's emission. With the introduction of an absorbing sample into the optical path, the absolute transmission spectrum of the sample could be easily determined, because there would be no photometric errors or variances in the spectrum induced by the spectrophotometer system. Consequently, the spectrophotometer would not contribute any error or complications in qualitative or quantitative analysis.

#Because ideal elements for an instrument do not exist, practical instrument designs are forced to introduce variances or photometric errors. One subsystem of a spectrophotometer system which will introduce variances or photometric errors is the optical sampling subsystem. The significance of these variances are believed not to have been understood in most applications. It has been found, as disclosed herein, that sampling subsystem variances can lead to unacceptable errors in analysis for spectroscopic analysis of a heterogeneous sample such as tissue. The errors have been found to be more pronounced when the subsystem is designed to maximize light throughput so that light exiting the tissue after interaction and collected for analysis is sufficient to achieve an adequate signal-to-noise ratio. The present invention provides for an optical sampling subsystem that allows for the reduction or elimination of the effects of photometric errors or variances induced by the subsystem design, which is preferably used to analyze heterogeneous samples. Of particular importance is the subsystem design to eliminate or reduce photometric errors from the high light throughput in a heterogeneous sample and from wavenumber axis shift (frequency domain equivalent of wavelength axis shift).

#A preferred overall spectroscopic system is first presented, followed by specifics of a preferred optical sampling subsystem incorporated therein. Finally, experimental data is presented that details some of the variances introduced by a sampling subsystem and how the present invention reduces or eliminates the effects of these variances. Referring now to FIG. 1, a non-invasive spectroscopic system used in preferred embodiments is depicted in schematic view. The overall system includes six subsystems. The subsystems include an illumination subsystem 100 , a tissue sampling subsystem 200, a calibration maintenance subsystem 300 , an FTIR spectrometer subsystem 400 , a data acquisition subsystem 500 and an embedded processing subsystem 600. The subsystems have been designed and carefully integrated in order to ensure that the net analyte signal-to-noise ratio is preserved to the maximum amount. The net analyte signal is the portion of the near-infrared spectrum that is specific for determining the concentration of the analyte of interest because it varies orthogonally to other sources of spectral variance. The net analyte signal-to-noise ratio is directly related to the accuracy and precision of the non-invasive measurement of any desired analyte by quantitative near-infrared spectroscopy with the present invention.