A new camera model combining an analytical model and a discrete correction to overcome refractive index variation challenges

Gwenaël Acher (1) , Lionel Thomas (1) , Benoît Tremblais (2) , Laurent David (1)

1 HydEE - Hydrodynamique et écoulements environnementaux [Institut Pprime]
2 XLIM - XLIM

When performing Tomo-PIV experiments, the calculation of accurate camera models is a key point for successful measurements. In severe configurations, where optical interfaces are involved and refractive index variations occur along the line of sight, analytical models can fail to accurately represent the projection and back-projection functions. This inability can lead to significant inaccuracies in particle location and volume reconstruction, which has a considerable impact on the calculation of velocity fields. In order to overcome these limitations, an innovative camera model based on the combination of an analytical model, such as a pinhole model or a polynomial model, with discrete corrections is proposed. In this method, the analytical projection and back-projection are adjusted with a discrete correction stored in two adaptive grids that save both memory and computation time. These correction grids require calibration which is performed with triangulation procedures similar to those used in misalignment corrections. The calculation and operation of the model are described in this paper. The performance of the camera model is evaluated on simulated and experimental setups based on a large depth-offield calibration performed in a glass water tank. The presence of multiple optical interfaces and fluids, resulting in large light deviations, makes it difficult to compute a high accuracy camera model. In this configuration, the proposed technique successfully reduces the triangulation error from 1 pixel to less than 0.01 pixel. The usefulness of the model is demonstrated in a Tomo-PIV experiment where the deflection of light through the water tunnel walls prevents classical analytical functions from accurately modelling the projection and back-projection functions. This corrected model can also solve discontinuity problems in the projection functions and can be used when there are interfaces in the measurement volume. It opens new perspectives in the study of fluid-structure interaction when transparent solids are involved.

Mots clés: Multiple camera calibration; Optical interface; Triangulation optimization; Discrete camera model; Tomo-PIV

Domaines: Dynamique des Fluides [physics.flu-dyn]

Gwenaël Acher, Lionel Thomas, Benoît Tremblais, Laurent David. A new camera model combining an analytical model and a discrete correction to overcome refractive index variation challenges. Measurement Science and Technology, 2022, 33 (12), pp.125204. ⟨10.1088/1361-6501/ac8b9b⟩. ⟨hal-03758866⟩