Fig. 2. A typical example of average spectral spectrum of a laser Doppler flowmetry (LDF) signal.  (A ) During unstimulated blood perfusion.  (B ) During iontophoresis with acetylcholine (ACh).  (C ) During iontophoresis with sodium nitroprusside (SNP). Measurements in arbitrary units. The  vertical lines indicate the outer limits of each frequency interval (0.0095–0.02, 0.02–0.06, 0.06–0.2, 0.2–0.6, and 0.6–1.6 Hz).