What additional information does the phase measurement in a frequency-domain imaging technique provide compared with the continuous wave technique that measures only the amplitude of the diffuse light?
A phase shift of a frequency modulated light source is almost equivalent to a change in mean flight time of the photons and hence provides information about the mean free path length of the photons through the tissue. This information is distinct to that provided by a change in amplitude, which is the only variable measured in continuous wave (CW), and helps in distinguishing the degree to which attenuation is a result of either scattering or absorption events. In certain cases, such as diffuse optical tomography, it is possible to separate scattering and absorption using CW measurements by solving a regularised inverse problem, however frequency domain measurements will typically improve this separation by reducing the non-uniqueness.
A phase shift of a frequency modulated light source is almost equivalent to a change in mean flight time of the photons and hence provides information about the mean free path length of the photons through the tissue. This information is distinct to that provided by a change in amplitude, which is the only variable measured in continuous wave (CW), and helps in distinguishing the degree to which attenuation is a result of either scattering or absorption events. In certain cases, such as diffuse optical tomography, it is possible to separate scattering and absorption using CW measurements by solving a regularised inverse problem, however frequency domain measurements will typically improve this separation by reducing the non-uniqueness.
Adding to the above answers, phase measurement provides idea about superposition properties of the wave. If the device or light being measured is at an intermediate position in the device or process, phase measurement is useful to understand what effect would the device or light or image will have on the output or the transmitted wave.
The phase frequency response information is very important for the integrity of complex waveforms. In case of linear phase versus frequency the there will be phase distortion of the signal meaning the the complex waveform remain coherent. In case of nonlinear phase dependence on the frequency the there will be phase distortion and dispersion of the frequency components of a complex waveform. The slope of the phase versus frequency response is the group delay time. To decrease the waveform dispersion the group delay time variations must be kept limited within specific range.
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FFT takes us from time to frequency.
Time tells us the signal at t.
Frequency tells us the signal at w.
at Frequency w we have amplitude and phase.
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