LED Intensity Decay Particle Tracking Velocimetry (PTV)

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. Particle tracking velocimetry (PTV) is a common technique used for measuring fluid flow by tracking the motion of small, micron-sized particles seeded in a fluid medium. Traditionally, PTV and its related technique, particle image velocimetry (PIV), rely on high-powered pulsed lasers that generate thin, intense light sheets to illuminate the particles. A high-speed camera captures sequential images, and advanced processing algorithms determine velocity vectors based on the displacement of the particles over time. While effective, these laser-based systems require expensive components, precise optical setups, and complex synchronization between the laser pulses and the camera, making them costly and challenging to implement. Innovators at NASA’s Langley Research Center (LaRC) have developed LED Intensity Decay Particle Tracking Velocimetry (LED-ID PTV), an alternative to conventional PTV that uses light-emitting diodes (LEDs) for flow illumination. LED-ID PTV leverages the natural capacitive intensity decay of LEDs to encode velocity and directional information passively. This approach eliminates the need for expensive cameras capable of double-pulsing exposures, lasers, and intricate timing mechanisms while still delivering high-resolution flow field data. The system is cost-effective, eye-safe, and adaptable to various experimental conditions, making it ideal for aerospace, industrial, and research applications.