Scanning oct off-wall particle sizing

A method for determining a characteristic of a flowing fluid in a sample space by Fourier domain optical coherence tomography (FD-OCT), the fluid comprising particles, wherein the method comprises:

• estimating a velocity of the fluid in the sample space;
• controlling an optical scanner to radiate a beam of light along an optical path to the fluid in the sample space and to sense a signal of interference of (i) measurement light-scattered back along the optical path mixed with (ii) reference reflected light, while moving the optical scanner, wherein the beam of light is moved with a scanner velocity being aligned with a velocity component of the velocity of the fluid perpendicular to the optical axis of the beam of light, wherein a ratio of a magnitude of the scanner velocity and a magnitude of the velocity of the fluid is in the range of 0.1-10;
• processing the signal into a corresponding complex-valued optical path length, z, resolved OCT signal, a(t, z), wherein the OCT signal a(t, z) represents the fluid in the sample space, and
• determining the characteristic of the fluid based on the OCT signal a(t, z), wherein the characteristic of the fluid comprises one or more of a size of the particles in the fluid, a shape of the particles in the fluid, a diffusion coefficient of the particles in the fluid, a particle size distribution (PSD) of particles in the fluid, a local velocity, νl, of the fluid, a velocity profile of the fluid in the sample space, and an mean velocity, <ν>, of the fluid in the sample space.