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NASA’s LED-ID PTV system illuminates a seeded flow with an LED rather than a laser. Instead of using double-pulsed laser flashes to capture two separate images of particle positions, the system relies on the inherent intensity decay of an LED pulse to encode velocity information directly into a single long-exposure image. The LED’s light intensity decreases over time due to capacitor discharge characteristics of the driving circuit. This controlled decay serves as a built-in intensity marker, allowing for precise determination of particle velocity and directionality without requiring an actively modulated light source. In a single-color configuration, a monochrome camera captures a long- exposure image of particle streaks as they move through the illuminated region. Because the light intensity is continuously decreasing, the recorded streaks naturally encode velocity information based on their brightness gradient. Faster-moving particles create longer streaks, while slower particles form shorter ones. The intensity variation across the streak provides additional data about directionality, enabling flow field analysis with a minimal hardware setup. For more complex flow analysis, a two-color configuration can be employed to track three- dimensional motion. In this setup, two LEDs of different colors are positioned adjacent to each other to create overlapping light sheets. A color camera, or two monochrome cameras with a dichroic mirror, captures the streaks of particles as they move between these sheets. The color transition within a particle’s streak indicates its movement between the planes of illumination, allowing users to resolve out-of- plane velocity components. Image processing techniques (e.g., advanced algorithms, high-pass filtering methods, sub-interval streak segmentation) further enhance the system's accuracy. NASA’s LED-ID PTV system has been prototyped and demonstrated with excellent results, and is available for patent licensing to industry.

The first optic is a 1D Ronchi ruling, where shortpass or longpass filters replace the traditional opaque lines in the grid pattern. The second optic is a 2D Ronchi ruling, where one set of lines is made from shortpass filters and the orthogonal set from longpass filters. By using two colors of light and a color camera in the focusing schlieren system (or a dichroic mirror with two monochrome cameras), the 1D optic enables simultaneous focusing schlieren and other co-linear techniques, while the 2D optic allows for the unambiguous measurement of two orthogonal density gradients in focusing schlieren images. Unlike standard optical filters, which typically cover an entire substrate, these Ronchi rulings feature alternating clear and filtered regions in structured 1D or 2D patterns. By leveraging color filtering and a color camera, the 1D ruling enables simultaneous focusing schlieren and complementary optical diagnostics, such as Particle Image Velocimetry (PIV), Pressure-Sensitive Paint (PSP), and Thermal-Sensitive Paint (TSP). The 2D ruling enables simultaneous and unambiguous measurement of two orthogonal density gradients, a capability not possible with conventional Ronchi rulings. This advancement significantly improves the accuracy and efficiency of schlieren-based flow measurements. The types of filters are not just limited to shortpass and longpass coatings, but could include notch, bandpass, and multiple-bandpass filter coatings as well. This design expands the utility of schlieren imaging in high-speed aerodynamics, combustion diagnostics, and other fluid dynamics applications. This Ronchi ruling methodology is at TRL 4 (component and/or breadboard validation in a lab environment) and is available for patent licensing.

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.