Frequency-modulated continuous-wave digital holography for outdoor three-dimensional imaging

Abstract

Outdoor imaging suffers from significant challenges due to atmospheric effects such as turbulence and scattering in foggy environments. In active imaging systems, where lasers are used to illuminate a scene, scattering reduces the contrast in an image by attenuating the light that propagates to an object and back to the detector. Additionally, unwanted backscattered light can swamp the detector. Overcoming these challenges is essential for the use of active imaging systems in the real world. Frequency-modulated continuous-wave digital holography can be used to produce three-dimensional (3D) images of objects and to overcome deleterious effects of scattering due to its range-selective capabilities. Range-selective digital holography (RSDH) has been developed to selectively image an object at an extended range, even when other objects are present in the depth of field of the imaging system. In this dissertation, RSDH was demonstrated as a 3D imaging technique used to produce object reconstructions and point clouds of scenes indoors and outdoors. To test this RSDH 3D imaging indoors, a custom range extending tower with an adjustable 8-48 m path length was used to create 3D reconstructions of a complex object with 30 features spaced over an 11 cm depth at a range of 8 m. The technique was then demonstrated outdoors, resulting in a point cloud of a scene 90 m from the system, with seven objects spanning a depth of 1.3 m. RSDH's ability to eliminate foreground images has the potential to enhance imaging through scattering environments, such as fog or smoke, by eliminating the obscuring effects of foreground scatterers in holographic images. To investigate this potential use of RSDH, various techniques for fabricating plates that can be placed in the range-extending tower to emulate a controlled volumetric scattering environment were investigated, enabling future studies with well-characterized experimental parameters to test theories and techniques for imaging through adverse conditions. In conclusion, this dissertation presents RSDH as a non-mechanical scanning method to produce high-resolution 3D object reconstructions and point clouds of indoor and outdoor scenes, which can be used in future studies of imaging in adverse weather conditions, such as fog and turbulence.