Real-time, reference-free background oriented schlieren imaging system

Patent metadata

Field	Value
Publication number	US-11796469-B2
Application number	US-202217725693-A
Country	US
Kind code	B2
Filing date	Apr 21, 2022
Priority date	Oct 20, 2021
Publication date	Oct 24, 2023
Grant date	Oct 24, 2023

Abstract

A real-time reference free background oriented schlieren system is provided. One embodiment includes a display device capable of generating an image pattern projected onto a retroreflective background. A beam splitter is used to transmit a portion of the projected image pattern towards a reference image sensor and another portion of the image pattern towards the retroreflective background and past a density object. The retroreflective background reflects the projected pattern back through the beam splitter and onto a signal imaging sensor. Collected data from the reference image sensor and the signal image sensor may be processed in real-time. The image pattern may be altered as necessary without requiring a new reference image, reducing the amount of time required to set up and adjust the system. A display device may be capable of switching between a schlieren visualization capability to a shadowgraph system allowing for the use of two different imaging techniques.

First claim

What is claimed is:

1. A projected background oriented optical system for measuring a density object, comprising:
an image generator configured to generate an incident light having a predetermined image pattern;
a beam splitter configured to:
reflect a first portion of the image pattern along a signal axis of the optical system; and
transmit a second portion of the image pattern along a reference axis of the optical system;

a retroreflective background positioned downstream of the density object to receive the transmitted image pattern and reflect the image pattern back through the density object and towards the beam splitter;
a field lens positioned between the beam splitter and the test object, wherein the field lens is configured to focus the image pattern onto the retroreflective background;
a signal imaging sensor positioned to receive the reflected image pattern transmitted through the beam splitter and collect a set of signal data;
a reference imaging sensor positioned to receive the second portion of the image pattern and collect a set of reference data; and
a processing system in electronic communication with the signal imaging sensor and the reference imaging sensor, the processing system having a processor and a non-transitory computer-readable medium with computer-executable instructions that when executed by a processor, cause the processor to at least:
receive the set of signal data and the set of reference data;
compare the received sets of signal data and reference data; and
generate a set of flow data.

2. A projected background oriented optical system according to claim 1 , wherein the image generator comprises:
light source configured to generate an incident light; and
a transparent slide having a predetermined pattern, wherein the transparent slide is positioned to receive the incident light and project an image pattern towards the beam splitter.

3. A projected background oriented optical system according to claim 1 , wherein the image generator comprises a LCD element configured to:
generate an incident light; and
display a predetermined pattern, wherein the LCD element is positioned to project an image pattern towards the beam splitter.

4. A projected background oriented optical system according to claim 3 , wherein the LCD element is further configured to:
selectively replace the predetermined pattern with a blank display; and
reduce the display to a focused point of light that is projected towards the beam splitter.

5. A projected background oriented optical system according to claim 1 , further comprising:
a half-wave plate positioned between the image generator and the beam splitter; and
a quarter-wave plate positioned between the test object and the retroreflective background.

6. A projected background oriented optical system according to claim 5 , wherein the beam splitter comprises a polarizing beam splitter configured to reflect LVP light along the signal axis of the instrument.

7. A projected background oriented optical system according to claim 1 , further comprising:
a first focusing lens positioned between the beam splitter and the signal imaging sensor; and
a second focusing lens positioned between the beam splitter and the reference imaging sensor.

8. A method of measuring a density object with a projected background oriented optical system, comprising:
directing an incident light source having an image pattern towards a beam splitter configured to:
reflect a first portion of the image pattern along a signal axis of the optical system; and
transmit a second portion of the image pattern along a reference axis of the optical system towards a reference imaging sensor configured to store the transmitted second portion of the image pattern as a set of reference data;

projecting the first portion of the past the test object and towards a retroreflective background, wherein the retroreflective background is configured to:
receive the transmitted image pattern; and
reflect the image pattern back through the test object and towards the beam splitter;

passing the reflected image pattern through the beam splitter and towards a signal imaging sensor configured to store the reflected image patterns as a set of signal data;
processing the stored sets of reference data and signal data with a processing system connected to the reference and signal imaging sensors, the processing system having a processor and a non-transitory computer-readable medium with computer-executable instructions that when executed by a processor, cause the processor to at least:
compare the received sets of signal data and reference data; and
generate a set of flow data.

9. A method of measuring a density object with a projected background oriented optical system according to claim 8 , wherein the image pattern is formed by:
light source configured to generate an incident light; and
a transparent slide having a predetermined pattern, wherein the transparent slide is positioned to receive the incident light and project an image pattern towards the beam splitter.

10. A method of measuring a density object with a projected background oriented optical system according to claim 8 , wherein the image pattern is formed by an LCD element configured to:
generate an incident light; and
display a predetermined pattern, wherein the LCD element is positioned to project an image pattern towards the beam splitter.

11. A method of measuring a density object with a projected background oriented optical system according to claim 10 , wherein the LCD element configured to:
selectively replace the predetermined pattern with a blank display; and
reduce the display to a focused point of light that is projected towards the beam splitter.

12. A method of measuring a density object with a projected background oriented optical system according to claim 8 , further comprising:
positioning a half-wave plate between the image generator and the beam splitter; and
positioning a quarter-wave plate between the test object and the retroreflective background.

13. A method of measuring a density object with a projected background oriented optical system according to claim 12 , wherein the beam splitter comprises a polarizing beam splitter configured to reflect linear vertically polarized light (LVP) light along the signal axis of the instrument.

14. A method of measuring a density object with a projected background oriented optical system according to claim 8 , further comprising:
positioning a first focusing lens between the beam splitter and the signal imaging sensor; and
positioning a second focusing lens between the beam splitter and the reference imaging sensor.

15. A method of measuring a density object in a test section of an instrument with an imaging system, comprising:
directing an incident light source having an image pattern towards a beam splitter configured to
reflect a first portion of the image pattern as linear vertically polarized light (LVP) along a signal axis of the imaging system; and
transmit a second portion of the image pattern along a reference axis to a reference imaging processor configured to store the transmitted image pattern as a set of reference data;

passing the LVP light through a test window and into the test section;
positioning a quarter-wave plate (QWP) between the test window and a retroreflective background;
reflecting the LVP light back through the test section via the retroreflective background, wherein the LVP light is converted to LHP after passing back through the QWP;
passing the LHP light back through the beam splitter towards a s

Assignees

• Nasa

Inventors

• Weisberger Joshua M
• Bathel Brett F

Classifications

• G01N21/455 Primary

  Schlieren methods, e.g. for gradient index determination; Shadowgraph · CPC title

• G01B11/2513

  with several lines being projected in more than one direction, e.g. grids, patterns · CPC title

• G01P5/001 Primary

  Full-field flow measurement, e.g. determining flow velocity and direction in a whole region at the same time, flow visualisation · CPC title