Measurement of the light reflected from a 90% reflectance standard was achieved by placing the source/collector probe a distance of about 10 cm from a white card (which had been calibrated by comparison with a Spectralon standard to be about 90% reflective (Rstd = 0.90) over the visible/near-infrared wavelength range).
Measurement of the skin was accomplished by placing the probe on the skin.
The reflectance was calculated:
which allows cancellation of the wavelength-dependent source spectrum (S) and detector responsivitiy (D), and accounts for the geometry of collection (G, 0 < G < 1),
The transport through the tissue was called T, and constituted the "reflectance" of the tissue. The reflected light from the white card standard was called Tstd and equaled 0.90. The factor Gstd described the collection efficiency from the white card standard and G described the optical fiber collection efficiency as the probe contacted the tissue. The factor GG, the ratio of these two collection efficiencies, was relatively constant with respect to wavelength (but not perfectly, see
---> PR Bargo, SL Jacques, R Sleven, T Goodell, Optical properties effects upon the collection efficiency of multifiber probe configurations, IEEE J. Selected Topics in Quantum Electronics 9:314-321, 2003
---> Paulo R. Bargo, Scott A. Prahl, and Steven L. Jacques, Collection efficiency of a single optical fiber in turbid media, Appl. Optics 42:3187-3197, 2003).
The tissue reflectance, expressed as T = MTstd/GG, was used for analysis. (In some images, Tstd is denoted Rstd.)
A typical experimental result is shown below. This is not great data !!! The system was thrown together in just a short time for a student demonstration. The coupling of light into the source fiber was poor, so there was little light and the signal-to-noise was low. NEVERTHELESS, this spectrum can be analyzed effectively.
