Geometric and Optic Characterization of a Hemispherical Dome Port for Underwater Photogrammetry

Abstract

The popularity of automatic photogrammetric techniques has promoted many experiments in underwater scenarios leading to quite impressive visual results, even by non-experts. Despite these achievements, a deep understanding of camera and lens behaviors as well as optical phenomena involved in underwater operations is fundamental to better plan field campaigns and anticipate the achievable results. The paper presents a geometric investigation of a consumer grade underwater camera housing, manufactured by NiMAR and equipped with a 7′′ dome port. After a review of flat and dome ports, the work analyzes, using simulations and real experiments, the main optical phenomena involved when operating a camera underwater. Specific aspects which deal with photogrammetric acquisitions are considered with some tests in laboratory and in a swimming pool. Results and considerations are shown and commented.

An Under-Ice Hyperspectral and RGB Imaging System to Capture Fine-Scale Biophysical Properties of Sea Ice

“…Such problematics add up to the known effects of the water column on measured light intensity and spectral composition in the visible range [49]. Overall, the increased costs of deploying optical sensors underwater need to be considered together with the additional challenges of geometric and chromatic correction of underwater images associated to the diverse refractive indices across the seawater-glass-air interface [50][51][52]. Such aberrations are not trivial to correct and depend on multiple factors such as the sensors optical parameters and settings, deployment mode (e.g., distance from the ice and field of view (FOV) inclinations), water optical properties and the underwater housing lens design (e.g., flat versus dome), and material (e.g., thickness of the acrylic window).…”

“…Remote Sens. 2019, 11, x FOR PEER REVIEW 6 of 35 diverse refractive indices across the seawater-glass-air interface [50][51][52]. Such aberrations are not trivial to correct and depend on multiple factors such as the sensors optical parameters and settings, deployment mode (e.g., distance from the ice and field of view (FOV) inclinations), water optical properties and the underwater housing lens design (e.g., flat versus dome), and material (e.g., thickness of the acrylic window).…”

“…Since the system travels at a fixed distance from the target, the horizontal and vertical footprint of the sensors can be estimated for the entire transect using standard imaging formulas (e.g., see Appendix B in [24]). Nonetheless, a flat-port causes magnification of images due to the multiple refractions at the air-acrylic-water interfaces, thus reducing the apparent FOV [51]. The amount of magnification is generally ≤1.33 and can be theoretically obtained using Snell's law.…”

“…The same approach would not be possible under highly heterogeneous topographies or would not be as effective for highly dynamic imaging conditions. Under these sub-optimal circumstances, the options could be to retrieve an accurate camera model using underwater calibration targets [39,55], to estimate it through its mathematical formulation and/or to implement the use of dome ports [51]. Another option remains the addition of physical GCPs.…”

Underwater Calibration of Dome Port Pressure Housings

“…In Menna et al (2016), the geometric and optic characterization of the 7" NI320 dome port ( Fig. 5f) produced by the Italian company NiMAR is presented.…”