Filtered Ronchi Rulings for Enhanced Schlieren Imaging

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. Researchers at NASA Langley Research Center have developed two novel types of Ronchi rulings to improve flow measurement in schlieren imaging systems. A previous self-aligned focusing schlieren (SAFS) system, which uses a single Ronchi ruling as both the source and cutoff grids (LAR-TOPS-348), marked a significant advancement but remains limited to measuring density gradients in only one direction. This limitation can hinder more complex flow measurements, such as those at Mach 10 during shock-wave boundary layer interactions. Traditional systems, including those involving knife-edge cutoffs, may not be sufficient for capturing these dynamic phenomena. The new designs break from traditional methods by replacing opaque markings with optical filters in the ruling lines. The first design is a one- dimensional (1D) Ronchi ruling that uses shortpass or longpass filters, while the second design is a two-dimensional (2D) ruling that incorporates orthogonal shortpass/longpass filter sets. Together, these optics leverage color filtering techniques and color cameras to deliver enhanced schlieren imaging, enabling a broader range of precise measurements.