Principles and Applications of Frequency-Domain Measurements of Light Propagation

In the previous chapter the propagation and measurement of short light pulses in turbid media such as tissue was described. In this chapter we will discuss the propagation of light produced by sinusoidally modulated sources. The relationship between the two is illustrated in Fig. 10.1. If an infinitesimally short pulse is applied to a scattering medium, this pulse is broadened in time as it propagates due to the many possible photon paths between source and detector. The observed quantity in the time domain is h(t) the number of photons reaching the detector per unit time at a given time, t. If instead we have a sinusoidally modulated light source, the photon flux at the detector will also be sinusoidal in time but the oscillation will be delayed in time relative to the source and reduced in amplitude relative to the average flux. The observed quantities in this case are the phase angle between the detected and source (or some reference) signals and the amplitude of the oscillation relative to the DC level. This latter quantity, defined as A/B in Fig. 10.1, is referred to as the modulation.