Digital Projection Focusing Schlieren System

NASA’s digital projection focusing Schlieren system is attached to a commercial-off-the-shelf camera. For focusing Schlieren measurements, it directs light from the light source through a condenser lens and linear polarizer towards a beam-splitter where linear, vertically-polarized component of light is reflected onto the optical axis of the instrument. The light passes through the patterned LCD element, a polarizing prism, and a quarter-wave plate prior to projection from the assembly as left- or right-circularly polarized light. The grid-patterned light (having passed through the LCD element) is directed past the density object onto a retroreflective background (RBG) that serves as the source grid. Upon reflection off the RBG, the polarization state of light is mirrored. It passes the density object a second time and is then reimaged by the system. Upon encountering the polarizing prism the second time, the light is slightly offset. This refracted light passes through the LCD element, now serving as the cutoff grid, for a second time before being imaged by the camera. The LCD element can be programmed to display a variety of grid patterns to enable sensitivity to different density gradients. The color properties of the LCD can be leveraged in combination with multiple colored light sources to enable simultaneous multi-color, multi-technique data collection. This system is ready for integration into commercial flow visualization and diagnostic equipment, offering manufacturers and research facilities an efficient, cost-effective solution for multi-technique imaging. The Schlieren system is currently available for patent licensing. Typically, multiple imaging systems are required to collect focusing schlieren and other image-based measurement data. Additionally, conventional focusing schlieren imaging systems are only sensitive to a single density gradient. Researchers at NASA's Langley Research Center have developed a compact LCD-integrated optical assembly that enables a research camera to simultaneously collect focusing schlieren and other image-based measurement data (e.g., particle tracking velocimetry (PTV), particle imaging velocimetry (PIV), temperature or pressure sensitive paint measurements (TSP, PSP), or photogrammetry). The assembly was designed to enhance the capabilities of NASA Langley's patented self-aligned, single-grid projection focusing schlieren imaging system described in LAR-TOPS-348, and reduce the complexity and time required to perform multiple image-based measurement experiments. NASA's digital single-grid system leverages a programmable LCD as the grid, enabling on-the-fly grid adjustments (or grid deactivation) to enable a substantial amount of experimental flexibility for image-based measurements.