Unlocking Low-Frequency Laser Doppler Holography Spectra for Functional Retinal and Choroidal Microvascular Phenotyping
The low-frequency part of optical fluctuation spectra recorded by laser Doppler holography at a frame rate of 37~kHz is typically filtered out to reveal functional blood-flow waveforms in major retinal and choroidal vessels. We construct a time-varying spectral-excess (SE) distribution in capillary-associated territories by selecting complete cardiac cycles and applying successive spatial and beat medians in regions where vessels are unresolved and intermittent eye motion can dominate the signal. At each phase, spectral excess compares the integrated Doppler spectral area between two fixed low-frequency peaks near $\pm2$~kHz with a reference area defined by the high-frequency tail near $\pm18.5$~kHz. The resulting waveform, displayed against normalized cardiac phase, retains variation throughout the cardiac cycle and provides a basis for exploring its temporal organization. We summarize spectral-excess strength by the spectral excess index (SEI), defined as the temporal median of SE, and also describe an alternative evaluation after spectral pooling. A representative recording illustrates the construction. Physiological specificity and repeatability remain to be quantified.