US-20260126383-A1 - One- and Two-Dimensional Spectral-Spatial Filters

US20260126383A1US 20260126383 A1US20260126383 A1US 20260126383A1US-20260126383-A1

Abstract

Light is projected through a grid (e.g., a Ronchi ruling having a shortpass and/or a longpass filter lines) and onto a background, where the light is spectrally and spatially filtered by the grid to form an image of the grid by a camera. The light from this projected image is then reflected back onto the same grid, with a polarizing refractor (e.g., a Rochon polarizing prism) imparting a small offset between the projected and reflected light, where the light is spectrally and spatially filtered by the grid to form an image of the grid by the camera. Thus, a separate source and cut-off grid is not needed, resulting in a focusing schlieren system that is compact and easy to construct and align.

Inventors

Joshua M. Weisberger
Brett F. Bathel

Assignees

UNITED STATES OF AMERICA AS REPRESENTED BY THE ADMINISTRATOR OF NASA

Dates

Publication Date: 20260507
Application Date: 20241106

Claims (20)

1 . A method of schlieren imaging a density object, comprising: projecting light rays having a first linear polarization along an optical axis in a projected direction; spatially and spectrally filtering the first linear polarization light rays through a grid a first time in the projected direction; passing the filtered light rays through the density object a first time in the projected direction; reflecting the filtered light rays back along the optical axis in a reflected direction opposite the projected direction; passing the reflected light rays through the density object a second time in the reflected direction; converting the reflected light rays from the first linear polarization to a second linear polarization that is offset by 90 degrees from the first linear polarization; spatially and spectrally filtering the 90-degree offset liner polarization light rays through the grid a second time in the reflected direction; and imaging the twice-filtered light rays, wherein when the light rays with the first linear polarization are spatially and spectrally filtered through the grid the first time, the grid functions as a source grid, wherein when the light rays with the second 90-degree offset linear polarization are spatially and spectrally filtered through the grid the second time, the grid functions as a cut-off grid, and wherein filtering the light rays through the same grid twice results in self-alignment without any multi-grid alignment step.
2 . The schlieren imaging method of claim 1 , wherein spatially and spectrally filtering the first linear polarization light rays through the grid, and spatially and spectrally filtering the second 90-degree offset linear polarization light rays through the grid, each include filtering the respective light rays through a Ronchi ruling grid having shortpass, longpass, bandpass, or notch filter lines.
3 . The schlieren imaging method of claim 2 , wherein the Ronchi ruling grid is formed by imaging at least one grid pattern onto a color photographic film using at least one colored light source and developing the photographic film to result in a fabricated spectral-spatial filter comprising the Ronchi ruling grid having the shortpass, longpass, bandpass, or notch filter lines.
4 . The schlieren imaging method of claim 1 , further comprising simultaneously making schlieren measurements and another type of measurement using the grid, wherein the grid comprises a one-dimensional spectral-spatial filter.
5 . The schlieren imaging method of claim 4 , wherein the other type of measurement utilizes particle image velocimetry (PIV), pressure sensitive paint (PSP), temperature sensitive paint (TSP), or photogrammetry.
6 . The schlieren imaging method of claim 1 , wherein the grid comprises a two-dimensional spectral-spatial filter that is simultaneously sensitive to density gradients in two orthogonal directions.
7 . The schlieren imaging method of claim 6 , further comprising simultaneously making schlieren measurements and another type of measurement using the grid.
8 . The schlieren imaging method of claim 7 , wherein the other type of measurement utilizes particle image velocimetry (PIV), pressure sensitive paint (PSP), temperature sensitive paint (TSP), or photogrammetry.
9 . The schlieren imaging method of claim 1 , wherein the imaging the twice-filtered light rays is performed using a color camera.
10 . The schlieren imaging method of claim 1 , wherein the imaging the twice-filtered light rays is performed using multiple monochrome cameras with a color filter positioned in front of the monochrome cameras.
11 . The schlieren imaging method of claim 1 , wherein projecting light rays with the first linear polarization includes projecting light rays from a light source and linearly polarizing the light rays with the first linear polarization, wherein the light source projects the light rays transverse to the optical axis, and the method further comprising projecting the light rays from the light source through a beam-splitter on the optical axis that redirects the light rays onto the optical axis.
12 . A system for schlieren imaging a density object, comprising: a light source assembly that projects light rays having a first linear polarization; a grid that spectrally and spatially filters the light rays and is positioned on an optical axis; a background that reflects the light rays and is positioned on the optical axis; one or more optical elements that convert the light rays from the first linear polarization to a second linear polarization that is offset by 90 degrees from the first linear polarization, wherein the one or more linear polarization 90-degree offsetting optical elements are positioned on the optical axis between the grid and the background; and a camera system that images the light rays and is positioned on the optical axis, wherein in use the light rays are projected along the optical axis in a projected direction, spectrally and spatially filtered through the grid a first time, passed through the density object a first time, reflected off the background back along the optical axis in a reflected direction opposite the projected direction, passed through the density object a second time, spectrally and spatially filtered through the grid a second time, and incident on the camera system, wherein in use the one or more linear polarization 90-degree offsetting optical elements convert the light rays from the first linear polarization to the second 90-degree offset linear polarization after the light rays are spectrally and spatially filtered through the grid the first time and before the light rays are spectrally and spatially filtered through the grid the second time so that the grid functions as a source grid and as a cut-off grid.
13 . The schlieren imaging system of claim 12 , wherein the grid comprises a one-dimensional spectral-spatial filter.
14 . The schlieren imaging system of claim 12 , wherein the grid is a Ronchi ruling grid having shortpass, longpass, bandpass, or notch filter lines.
15 . The schlieren imaging system of claim 13 , wherein the grid comprises a color photographic film material with an exposed image of the Ronchi ruling grid having one or two sets of shortpass, longpass, bandpass, or notch filter lines.
16 . The schlieren imaging system of claim 12 , wherein the grid comprises a two-dimensional spectral-spatial filter that is simultaneously sensitive to density gradients in two orthogonal directions.
17 . The schlieren imaging system of claim 12 , wherein the camera system comprises a color camera.
18 . The schlieren imaging system of claim 12 , wherein the camera system comprises multiple monochrome cameras with a color filter positioned in front of the monochrome cameras.
19 . The schlieren imaging system of claim 12 , wherein the light source projects the light rays transverse to the optical axis, the system further comprising a beam-splitter that is positioned on the optical axis and that redirects the light rays onto the optical axis.
20 . The schlieren imaging system of claim 12 , wherein the background is a retroreflective background.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore. BACKGROUND OF THE INVENTION Schlieren imaging is routinely used in wind tunnel experiments as a flow visualization tool that is sensitive to density gradients. A typical conventional schlieren system collimates light from a small, point-like source using a field focusing optic (either a high-quality parabolic mirror or a lens) and passes this collimated light through a flow-field of interest. A second field focusing optic placed on the opposite side of the flow-field images the original light source at a point. A knife-edge located at this point is placed such that it blocks a portion of the image of the light source, with the remainder of the light passing through to a camera sensor. Any gradient in density that exists between the two field optics results in a refractive index gradient that diverts some of the rays of light in the light column. These diverted rays ultimately either terminate on, or pass by, the knife-edge. The resulting image captured by the camera sensor then consists of light and dark regions that correspond to structures of varying density in the flow-field. While the qualitative images schlieren provides are useful for flow characterization, they represent the entire path-integrated density gradient field that exists between the two field focusing optics. Thus, every density gradient feature present in the field-of-view is captured in the resulting image, including features that are not pertinent to the flow of interest. Examples include non-relevant flow features, wind tunnel window scratches or chips, wind tunnel plenum or HVAC thermals, and wind tunnel wall boundary layer turbulent structures. Another drawback of the conventional schlieren visualization technique is its limited field-of-view, which is bound by the clear aperture/diameter of the field focusing optics. The focusing schlieren technique was developed to address the limiting characteristics of the conventional schlieren system; it can significantly reduce the influence of non-pertinent flow features and can provide larger fields-of-view. Typical focusing schlieren systems use a source grid placed on one side of the density object, which is then imaged with a lens onto a matching cut-off grid on the other side of the density object. Source grids usually consist of either a one-dimensional pattern of spaced parallel line pairs or a two-dimensional regular pattern or shape. The cut-off grid must consist of a scaled duplicate of the source grid and can be challenging to create. By adjusting the offset of the cut-off grid relative to the image of the source grid, the sensitivity of the instrument to density gradients present between the source grid and imaging lens can be tuned (similar to adjusting the knife-edge insertion in a conventional system). For this type of setup, the numerous high-intensity/bright regions of the source grid effectively serve as the light sources for a number of conventional schlieren systems whose paths all intersect a common region that contains the flow feature of interest. This method of operation, in effect, defocuses the contribution of features that occur away from this common region in the final image. The most common design is the modern large-field focusing schlieren system, which includes a light source that back-illuminates the source grid, with a Fresnel lens placed between the two in order to better direct light into the camera lens and improve brightness. The source grid is imaged onto the cut-off grid with a field lens, and the resulting focused schlieren image captured at the image plane by a camera. Placement of the source and cut-off grids relative to the field lens are readily determined using the thin lens equation, as is the placement of the schlieren object and image plane. Another design is the retroreflective focusing schlieren system. This system includes an alternative source grid consisting of patterned retroreflective material, with illumination provided by coupling light onto the optical axis via a beam-splitting plate, and with the resulting image of the source grid incident on a matched cut-off grid. This system is useful when cross-tunnel optical access is not available, and when larger fields-of-view are required than provided by a Fresnel lens. In another retroreflective system, the source grid is instead projected onto the screen and imaged onto a separate matched cut-off grid. Newer systems include a digital projection system used to project an image of a digital source grid onto a screen and include recent advancements in digital display technology (e.g., backlit and self-illuminating LCD and LED monitors) that enable the source grid to be tail