The thicker a cloud is, the larger is the diffuse halo around the entry point of a laser shining on the cloud, so we can estimate the thickness of a cloud layer by the size of the halo of backscattered light. In the diffusion domain, G is sensitive to cloud thickness, allowing this important quantity to be retrieved. These clouds of polystyrene microspheres suspended in water are analogous to real clouds of water droplets suspended in air. The microsphere size distribution is roughly lognormal, from 0.5 microns to 25 microns, similar to real clouds.
Detectors are flux and photon-counting PMTs, with a glass probe for precise positioning.
The stepper motor is accurate to 10 microns, and step size is selectable from the VI software. Far from the incident beam, the rate of exponential increase as the direction of the incident beam is approached scales as expected from diffusion theory, linearly with the cloud thickness, and inversely as the square root of the reduced optical thickness, and is independent of particle size. Near the beam the signal begins to increase faster than exponential, due to single and low-order scattering near the backward direction, and here the distribution depends on particle size.