NASA researchers have developed a compact, cost-effective imaging system using a co-linear, high-intensity LED illumination unit to minimize window reflections for background-oriented schlieren (BOS) and machine vision measurements. The imaging system tested in NASA wind tunnels can reduce or eliminate shadows that occur when using many existing BOS and photogrammetric measurement systems; these shadows occur in existing systems for a variety of reasons, including the severe back-reflections from wind tunnel viewing port windows and variations in the refractive index of the imaged volume.
The imaging system is comprised of a small CMOS camera fitted with a C-mount lens affixed to a 3D-printed mount. Light from the high-intensity LED is passed through a lens that both diffuses and collimates the LED output, and this light is coupled onto the cameras optical axis using a 50:50 beam-splitting prism.
Use of the collimating/diffusing lens to condition the LED output provides for an illumination source that is of similar diameter to the cameras imaging lens. This is the feature that reduces or eliminates shadows that would otherwise be projected onto the subject plane as a result of refractive index variations in the imaged volume.
By coupling the light from the LED unit onto the camera’s optical axis, reflections from windows, which are often present in wind tunnel facilities to allow for direct views of a test section, can be minimized or eliminated when the camera is placed at a small angle of incidence relative to the windows surface.
Due to its compact size, the system can easily fit in the space behind a typical wind tunnels view port. As a cost-effective, compact imaging system, NASA’s technology could be deployed for use in BOS, Tomo BOS, photogrammetric, and general machine vision applications. It could also be applied for testing leveraging similar measurement techniques: These could include thermal systems management, gas flow imaging, heat transfer measurements, biomedical R&D, and others.