Reduction in scattering from a turbid medium by photo-bleaching
A method is proposed whereby photo-bleaching is used not only to change the absorption and fluorescence of a sample but is also employed to change its scattering characteristics. When the compounds which are bleached are contained in regions wherein the real part of the index of refraction is greater than or equal to the average index of the medium, the bleaching will result in reduction in the scattering at wavelengths longer than the wavelength of the bleaching source. This reduction can be useful in measuring the concentration of analytes located at significant depths within turbid media.
1. A method for reducing elastic scattering from a sample comprising an analyte residing at a significant depth in the sample and a non-uniformly distributed photo-bleachable substance, the method comprising:
photo-bleaching the non-uniformly distributed photo-bleachable substance in the sample to induce a reduction in elastic scattering in the sample;
generating analyte spectra from inelastic scattering from the sample, wherein some or all of the analyte resides at a depth in the sample greater than an effective elastic scattering length of the sample, wherein the effective elastic scattering length is 1/u s ', wherein u s ' is an isotropic scattering coefficient of the sample, and wherein the inelastic scattering is relatively weak in relation to the elastic scattering; and
collecting the analyte spectra, wherein a sufficient amount of the analyte resides at the depth in the sample greater than an effective elastic scattering length of the sample such that the collected analyte spectra has a higher signal to noise ratio than without photo-bleaching.
2. The method of claim 1 , wherein the inelastic scattering comprises fluorescence.
3. The method of claim 1 , wherein the inelastic scattering comprises Raman scattering.
4. The method of claim 1 , wherein the step of photo-bleaching is performed by a source of optical radiation that performs the step of generating analyte spectra.
5. The method of claim 1 , wherein the step of photo-bleaching is performed by a source of optical radiation that operates at a different wavelength than a source that performs the step of generating analyte spectra.
6. The method of claim 5 , wherein the source that performs the step of generating analyte spectra operates at a longer wavelength than the source performing photo-bleaching.
7. The method of claim 5 , wherein the source of optical radiation that performs photo-bleaching operates at a first wavelength, and the source of optical radiation that performs the step of generating analyte spectra operates at a second wavelength, and difference between the first and second wavelengths as expressed in wavenumbers is greater than 1000 cm −1 .
8. The method of claim 1 , wherein the step of photo-bleaching occurs before the step of generating analyte spectra.
9. The method of claim 1 , wherein the sample comprises human skin.