Normalizing tissue measurements by standard measurements

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A "MEASUREMENT" is defined as a tissue measurement normalized by a measurement of the standard. For example, one of the red or green curves on the previous slide would be normalized by the black curve. This normalization causes the factors S, A, and D to cancel.

The factor S is the wavelength-dependent power of the light source [W].

The factor D is the wavelength-dependent sensitivity of the detector [counts/W].

The factors Ttissue and Tstd describe the transport of light from the source through the tissue and standard, respectively, to reach the collection port, in units of [cm-2]. The source power, S [W], times the transport, T [cm-2], yields the fluence rate, F [W cm-2], at the collection fiber.

The factor A indicates the area of the collection port for the collection fiber [cm2]. The power that impinges on the collection port is the fluence rate, F [W cm-2], times the area of collection, A [cm2].

The factors ftissue and fstd describe the fraction of light reaching the collection port that actually enters the collection fiber and is successfully guided up the optical fiber to the detector. The values are similar for the tissue and standard measurements and are in the neighborhood of 0.06-0.10. But the tissue is measured topically while the standard is measured by immersion in liquid, and so there is the possibility of a slight difference in the values. Contact between a fiber and a tissue is notorious as a source of variability in optical fiber measurements. But the value const = ftissue/fstd should be wavelength independent. The factor const will scale the magnitude of M but not influence the wavelength dependence of M.

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