The principle of both instrumental techniques, conventional and FT-Raman spectroscopy are the same. However, the significant difference in FT-Raman versus conventional Raman deals with instrumentation. In FT-Raman spectroscopy the laser excitation source contains a wavelength that is in the near-IR range in order to eliminate fluorescence that may occur from the sample. Also, in conventional Raman spectroscopy a dispersive grating is used to analyze scattered light. In FT-Raman, the advance alternative to this dispersive grating incorporates the use of a Michelson interferometer for analyzing scattered light. Other instrumental components that are comprised within an FT-Raman instrument ...view middle of the document...
In working in the near infrared region, when a sample is heated it may also emit radiation that can interfere with the signal of interest by appearing in the same spectral region. This factor of thermal effects does not pose as a problem in visible Raman spectroscopy. In order to drastically alter the intensity signals in the visible region due to thermal effects, the sample would have to be heated to an extremely high temperature to produce black-body emission. However, in the near infrared region, even the slightest degree of heat can cause background emission.
3.3 Noise Distribution
Noise distribution is a factor that is only unique to Fourier transform spectroscopy as opposed to dispersive Raman measurements in the visible region. The reason being is because in FT-Raman there is a simultaneous detection of all frequencies of light; due to the Felgett advantage, which was previously discussed above. Considering the fact that there is a simultaneous detection of all frequencies of light, the signal and noise components become indistinguishable. Therefore, they will become mixed at the detector and there will be no way to determine at what particular optical frequency is the noise response coming from. For instance, if the optical filtering device becomes inadequate for whatever reason, then this may lead in an escape of Rayleigh scattering. As a result, the strong noise line that is associated with Rayleigh scattering will be included in the overall spectrum by the Fourier transform process. Therefore, the development of more efficient devices that are capable of completely blocking Rayleigh scattering is always a goal in FT-Raman spectroscopy .
Diagnosis of malignant melanoma and basal cell carcinoma by in vivo NIR-FT Raman spectroscopy is independent of skin pigmentation.
4.1 Overview of Objective
In this application Philipson et. al. were using FT-Raman spectroscopy as a method of diagnosing skin tumors i.e. malignant melanoma and basal cell carcinoma in vivo. Their main objective was to demonstrate how FT-Raman spectroscopy diagnoses are influenced by physical properties of skin such as pigmentation. Due to the fact that Raman spectroscopy measured molecular structure, they consider this method to be a beneficial tool for diagnosing skin tumors in vivo.
4.2 Overview of Experiment
In their experiment they were able to obtain Raman spectra, in vivo, from normal skin samples from 55 healthy patient volunteers. There was a wide variety of skin pigmentations (Fitzpatrick skin type I–VI) amongst these patient volunteers. The other skin samples came from patients who contained some sort of skin disease. The statistics from those skin samples are listed as follows: 25 basal cell carcinomas, 41 pigmented nevi and 15 malignant melanomas. The spectral background caused by fluorescence was as a result of an increase in skin pigmentations across the sample pool. However, they were able to perform background corrections...