c	Speed of light.
f	Modulation frequency of incident and detected light.
f*	Dimensionless modulation frequency, f*= fl*/c.
L	Optical path length.
〈L〉	Mean of the distribution of optical path lengths.
l*	Mean isotropic scattering length (= 1/ μs′).
M	Amplitude modulation of emitted light with respect to the incident.
Mabsence	Amplitude modulation of emitted light from a homogeneous medium.
Mpresence	Amplitude modulation of emitted light in the presence of an embedded absorber.
Mr	Modulation ratio, = Mpresence/Mabsence.
n*, n′, ntot	Number of scattering events or scattering steps.
$P_s\left(\mathcal{X},\zeta ,n\right)$	Probability that a photon is emitted at a source sample position $\left(\mathcal{X},\zeta \right)$ in n scattering steps.
$P_D\left(\mathcal{X},\zeta ,n\right)$	Probability that a photon is emitted at a detector sample position $\left(\mathcal{X},\zeta \right)$ in n scattering steps.
$P\left(\mathcal{X},\zeta ,n\right)$	Probability that a photon is emitted at a source and detected at the detector sample position $\left(\mathcal{X},\zeta \right)$ in n scattering steps.
t	Photon time of flight.
x, y, z	Directions of the Cartesian system.
ZCF	Zero-crossing frequency at which Δθ = 0.
θ	Phase shift of emitted modulated light with respect to the incident light.
θabsence	Phase shift of emitted light from a homogeneous medium.
θpresence	Phase shift of emitted light in the presence of an embedded absorber.
Δθ	Phase-shift difference, θpresence – θabsence.
ρ	Source-detector separation.
ρ*	Dimensionless source-detector separation, = ρ/l*.
μs′	Isotropic scattering coefficient (length−1).
μa	Absorption coefficient (length−1).
$\mathcal{X},\zeta$	Nondimensional x and z directions $\mathcal{X}=x/l*$ and ζ = z/l*.