The three measurements recorded by the oximeter are the amplitude of the light intensity (AC), the average light intensity (DC), and the phase (φ). With increasing distance from a point source of light, AC and DC decrease, whereas the phase increases. The rate of change in AC, DC, and phase with respect to distance is an optical property of the medium.
9 Knowing these values at different source-detector separations allows for the slopes of AC, DC, and phase to be calculated (
SAC,
SDC, and
Sφ). Absorption (μ
a) and reduced scattering (μ
s′), coefficients can be calculated from the slope of
Sφ and either
SAC or
SDC. As
SAC is less affected by room lighting,
SAC and
Sφ are used.
SAC corresponds to the slope of ln(
r2AC) plotted against
r, and
Sφ denotes the slope of phase against
r, where
r is the source/detector separation. The following equations can be used to calculate the absorption and reduced scattering coefficients
10:
In these equations, ν is the speed of light in the tissue and ω is the angular modulation frequency. The concentration of Hb and HbO can be calculated, by using the absorption coefficient and the principles of the Beer-Lambert law:
where ε is the extinction coefficient, λ represents the wavelength of light used, and
B denotes the background absorption.
9 Two other values that the oximeter computes are the total hemoglobin concentration (THC) and the percentage of oxygen saturation in tissue (StO
2), where THC = Hb + HbO, and StO
2 = HbO/THC × 100%.