Optically Projected Length Scale for Use in Photogrammetry
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Abstract
Photogrammetry is a measurement technique where 3D coordinate data and feature information can be extracted from 2D photographs or images. Length scale objects are calibrated artifacts placed into a photogrammetry scene which have precisely defined dimensions and are used to scale coordinate data. Scale artifacts can be cumbersome to use due to their length - often multiple meters. In this research, a novel technique has been explored whereby a low cost and portable module projects a structured light pattern into a photogrammetry scene to provide scale. A holographic diffraction grating is used in the module to create and project an 11 × 11 square grid of laser spots into the field, forming a structured light pattern. This grid is then duplicated using a pellicle beamsplitter and fold mirror such that two patterns are projected into the field. Two novel algorithms were constructed to realize the optically projected length scale with the module. The first calibrates the pointing directions of beams in each structured light pattern from the module and finds the separation distance between the two projection origins. Photogrammetry is combined with a transformation fitting algorithm to solve for the projection source’s position and pose within a space. Once these parameters are known, the dataset is transformed such that the projection origin lies at a global coordinate system origin, allowing classification of beam pointing directions in spherical coordinates. This calibration process requires a length scale artifact which is readily detected in a photogrammetry scene and whose length is precisely defined. A photogrammetry scale artifact was designed and fabricated for use in the calibration phase. Its length was characterized to 1677.66 ± 0.01 mm on a coordinate measuring machine. The second algorithm uses the calibration information to trace a projected pattern in a scene back to its projection origin for a single photogrammetry measurement. By projecting two calibrated patterns into a photogrammetry scene and solving the distance between the two unscaled pattern origins, this unscaled distance can then be compared to the known calibrated separation length to obtain the scale factor by which all photogrammetry coordinates can be scaled. A prototype dual pattern projection module was designed, fabricated, and tested. Using a novel method, the spherical coordinate pointing directions for the 121 beam pattern were calibrated and the projection origin separation distance in the module estimated as 55.16 ± 0.05 mm (coverage factor k=1). Following calibration, the module’s performance was experimentally validated in two measurement trials of 20 measurements each to fractional uncertainty of parts per thousand. Monte Carlo simulations estimated the module’s measurement uncertainty in its length scale to 3.4 parts per thousand, which agreed with the experimental results. Monte Carlo simulations were also used to explore design parameters which limit module performance. Additional simulation data shows the viability of redesigned modules with fractional uncertainty of parts per hundred thousand or beyond, supporting the conclusion that optical pattern modules could offer a portable alternative to traditional scale artifacts.