On the optical efficiency of collecting scattered radiation or telescopically imaging the measurement area in airborne acoustic pressure evaluations

Photon correlation is an optical method for calculating acoustic pressures through particle velocity measurements, involving the intersection of two focused Gaussian beams forming an interference fringe pattern. Particles within this measurement region exhibit sinusoidal motion as a result of a propagating acoustic pressure wave over the fringes, consequently scattering photons. This scattered radiation is then collected and processed to determine the particle velocity and then the acoustic pressure. In order to analyse the efficiency of the optical collection system, decoupled optical delivery configurations were implemented. In this case, instead of relying on particles moving over static fringes, moving fringes can be made to pass over an apparently static particle. To simulate this effect, a single laser beam can be modulated with a 3-D printed pattern simulating sinusoidal particle motion across fringes. Having established this alternative delivery system, suitable optical collection systems can be implemented, so that their efficiency can be analysed and assessed. This paper reports on the details of two such collection systems; analysing spherically propagated radiation and telescopically capturing the measurement area respectively.