Reflection-Reducing Imaging System for Machine Vision Applications
optics
Reflection-Reducing Imaging System for Machine Vision Applications (LAR-TOPS-347)
Compact system that leverages a co-linear, high-intensity LED unit to minimize window reflections
Overview
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. 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, NASAs technology could be deployed for use in BOS, Tomo BOS, photogrammetric, and general machine vision applications.
The Technology
NASAs 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 cameras 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. This effect is demonstrated in the image on the bottom left of the page.
Eight imaging systems were fabricated and used for capturing background oriented schlieren (BOS) measurements of flow from a heat gun in the 11-by-11-foot test section of the NASA Ames Unitary Plan Wind Tunnel (see test setup on right). Two additional camera systems (not pictured) captured photogrammetry measurements.
Benefits
- Reduce or eliminate reflections/shadows: The system improves measurement capabilities by reducing or eliminating severe back-reflections from wind tunnel viewing port windows and unwanted shadows from variations in the refractive index of the imaged volume
- Compact design: Compactness is key, as it enables the systems to sit in the limited space behind wind tunnel viewing port windows and be easily manipulated (e.g., rotated to reduce glare)
- Cost-effective: Use of the CMOS camera and other cost-effective components enables a low Bill of Materials cost
Applications
- Wind tunnel testing: Background-oriented schlieren (BOS), Tomographic BOS, and photogrammetric measurements with retroreflective targets
- Other testing leveraging similar measurement techniques: these could include thermal systems management, gas flow imaging, heat transfer measurements, biomedical R&D, and others
Technology Details
optics
LAR-TOPS-347
LAR-19678-1
Preparation for Tomographic Background-Oriented Schlieren Measurements in the 11-by 11-Foot Transonic Wind Tunnel. June 2020. AIAA Aviation 2020 Forum.
