On assuring the accurate parallel alignment of a laser sheet for planar and stereoscopic particle image velocimetry

The calibration of two-component–two-dimensional (2C-2D) particle image velocimetry (PIV) and three-component–two-dimensional (3C-2D) stereoscopic PIV (SPIV) systems generally requires a calibration target to be placed within the laser sheet that is parallel to it, covering the field of view. In practice, this is very difficult to achieve. Any misalignment between the calibration target and the laser sheet results in a position or velocity error. In 3C-2D SPIV, a number of methods use the disparity between the two cameras to estimate this misalignment. These techniques adjust the calibration or the 3C displacement field to reduce the error due to misalignment, but in doing so, they can introduce an unintended bias error in the measured 3C displacements. This paper introduces a novel method to ensure accurate parallel alignment of the laser sheet with the calibration target so that the error due to misalignment is minimized, if not completely avoided. The proposed method has been validated using two approaches: (i) by imaging the laser sheet and measuring the sheet rotation angle and (ii) using 3C-2D SPIV measurements of microparticles inside a block, which are illuminated using an aligned parallel laser sheet, while the block is translated by known displacements. This experiment demonstrates that the uncertainty in the 3C displacement is less than m (0.09 pixels), illustrating the usefulness of the new proposed alignment method for PIV calibration.