View source for Physics:Schlieren imaging

{{Short description|Method to visualize density variations in transparent media}}
'''Schlieren imaging''' is a method to visualize density variations in transparent media.<ref name="korpel-1987">{{cite book |doi=10.1109/ULTSYM.1987.199011 |chapter=Schlieren Imaging of Sound Fields |title=IEEE 1987 Ultrasonics Symposium |year=1987 |last1=Korpel |first1=A. |last2=Mehrl |first2=D. |last3=Lin |first3=H.H. |pages=515–518 |s2cid=122562535 }}</ref>
[[Image:Schlieren HIFU 1MHz.JPG|thumb|300px|Schlieren imaging of a focusing ultrasonic transducer]]
[[File:Bullet Wiki.ogv|thumb|Schlieren image sequence of a bullet traveling in free-flight, demonstrating the air pressure dynamics surrounding the bullet]]
The term "schlieren imaging" is commonly used as a synonym for [[Schlieren photography|schlieren photography]], though this article particularly treats visualization of the pressure field produced by ultrasonic transducers, generally in water or tissue-mimicking media. The method provides a two-dimensional (2D) projection image of the acoustic beam in real-time ("live video"). 
The unique properties of the method enable the investigation of specific features of the acoustic field (e.g. focal point in HIFU transducers), detection of acoustic beam-profile irregularities (e.g. due to defects in transducer) and on-line identification of time-dependent phenomena <ref name="brown-2009">{{cite journal |last1=Brown |first1=Spencer A. |last2=Greenbaum |first2=Lior |last3=Shtukmaster |first3=Stella |last4=Zadok |first4=Yehuda |last5=Ben-Ezra |first5=Shmuel |last6=Kushkuley |first6=Leonid |title=Characterization of Nonthermal Focused Ultrasound for Noninvasive Selective Fat Cell Disruption (Lysis): Technical and Preclinical Assessment |journal=Plastic and Reconstructive Surgery |date=July 2009 |volume=124 |issue=1 |pages=92–101 |doi=10.1097/PRS.0b013e31819c59c7 |pmid=19346998 |s2cid=205965366 }}</ref> (e.g. in [[Physics:Phased array ultrasonics|phased array transducers]]). Some researchers{{who|date=May 2020}} say that schlieren imaging is equivalent to an [[Physics:Radiography|X-ray radiograph]] of the acoustic field.{{citation needed|date=May 2020}}

== Setup ==
[[Image:Schlieren imaging setup.GIF|thumb|300px|Schlieren imaging system setup: linear lens-based configuration]]
The optical setup of a schlieren imaging system may comprise the following main sections:{{citation needed|date=May 2020}}
Parallel beam, focusing element, stop (sharp edge) and a camera.
The parallel beam may be achieved by a point-like light source (a laser focused into a pinhole is sometimes used) placed in the focal point of a collimating optical element. 
The collimating element may be a lens or a mirror.
The optical stop may be realized by a razor placed horizontally or vertically in the focal point of the focusing element, carefully positioned to block the light spot image on its edge.
The camera is positioned behind the stop and may be equipped with a suitable lens.{{citation needed|date=May 2020}}

== Physics ==

=== Ray optics description ===
A parallel beam is described as a group of straight and parallel 'rays'.{{citation needed|date=May 2020}}
The rays cross through the transparent medium while potentially interacting with the contained acoustic field, and finally reach the focusing element.{{citation needed|date=May 2020}}
Note that the principle of a focusing element is directing (i.e. focusing) rays that are parallel - into a single point on the focal plane of the element. 
Thus, the population of rays crossing the focal plane of the focusing element can be divided into two groups: those that interacted with the acoustic field and those that didn't. The latter group is undisturbed by the acoustic field, so it remains parallel and forms a point in a well-defined position in the focal plane. The optical stop is positioned exactly at that point, so as to prevent all corresponding rays from further propagating through the system and to the camera.{{citation needed|date=May 2020}}
Thus we get rid of the portion of light that crossed the acoustic field without interaction.
However, there are also rays that did interact with the acoustic field in the following manner:
If a ray travels through a region of nonuniform density whose spatial gradient has a component orthogonal to the ray, that ray is deflected from its original orientation, as if it were passing through a [[Physics:Prism (optics)|prism]]. This ray is no longer parallel, so it doesn't intersect the focal point of the focusing element and is not blocked by the knife. In some circumstances the deflected ray escapes the knife-blade and reaches the camera to create a point-like image on the camera-sensor, with a position and intensity related to the inhomogeneity experienced by the ray. An image is formed in this way, exclusively by rays that interacted with the acoustic field, providing a mapping of the acoustic field.{{citation needed|date=May 2020}}

=== Physical optics description ===
The [[Physics:Acousto-optics|acousto-optic]] effect couples the optical [[Physics:Refractive index|refractive index]] of the medium with its density and pressure. Thus, spatial and temporal variations in pressure (e.g., due to ultrasound radiation) induces corresponding variations in refractive index. Optical [[Wavelength|wavelength]] and [[Physics:Wavenumber|wavenumber]] in medium depend on refractive index. The [[Physics:Phase (waves)|phase]] acquired by [[Physics:Electromagnetic radiation|electromagnetic wave]] traveling through the medium is related to the line-integral of the wavenumber along the propagation line.{{citation needed|date=May 2020}}

For a plane-wave electromagnetic radiation traveling parallel to the Z-axis, the XY planes are iso-phase manifolds (regions of constant phase; the phase does not depend on coordinates (x,y)). However, when the wave emerges from the acoustic field, XY planes are not iso-phase manifolds anymore; the information about the accumulated pressure along each (x,y) line resides in the phase of the emerging radiation, forming a phase image (phasor) in the XY plane. The phase information is given by the Raman-Nath parameter:<ref name="raman-1935">{{cite journal |last1=Raman |first1=C. V. |last2=Nagendra Nathe |first2=N. S. |title=The diffraction of light by high frequency sound waves: Part I. |journal=Proceedings of the Indian Academy of Sciences, Section A |date=October 1935 |volume=2 |issue=4 |pages=406–412 |doi=10.1007/BF03035840 |s2cid=198141323 }}</ref>
: <math> v(x,y)= \frac{2\pi\kappa}{\lambda} \int{p(x,y,z)} \, dz</math>
with <math>\kappa</math> - the piezooptic coefficient, <math>\lambda</math> the optical wavelength and 
<math>p(x, y, z)</math> the three-dimensional pressure field.<ref name="cook">{{cite journal |last1=Cook |first1=B.D. |last2=Cavanagh |first2=E. |last3=Dardy |first3=H.D. |title=A Numerical Procedure for Calculating the Integrated Acoustooptic Effect |journal=IEEE Transactions on Sonics and Ultrasonics |date=July 1980 |volume=27 |issue=4 |pages=202–207 |doi=10.1109/T-SU.1980.31173 |s2cid=9320796 }}</ref> The schlieren technique converts the phase information into an intensity image, detectable by a camera or a screen.

== Application ==
The accepted gold-standard for quantitative acoustic measurement is the hydrophone. However, scanning the acoustic field with a hydrophone suffers from several limitations, giving rise to supplementary evaluation methods such as the schlieren imaging. The importance of the schlieren imaging technique is prominent in HIFU research and development.
<ref name="charlebois">{{cite journal |first1=Thomas F. |last1=Charlebois |first2=Roger C. |last2=Pelton |title=Quantitative 2D and 3D Schlieren Imaging for Acoustic Power and Intensity Measurements |journal=Medical Electronics |pages=66–73 |date=June 1995 |url=http://www.ondacorp.com/images/Schlieren_Imaging_For_Acoustic_Power.pdf }}</ref>
Advantages of schlieren imaging include:
* Free field: the investigated acoustic field is not distorted by the measuring probe.
* High intensity measurements: the method is compatible with high acoustic intensities.
* Real time: Schlieren imaging system provides on-line, live video of the acoustic field.

== References ==
{{Reflist}}

==Further reading==
*{{cite journal |last1=Hargather |first1=Michael John |last2=Settles |first2=Gary S. |title=Natural-background-oriented schlieren imaging |journal=Experiments in Fluids |date=9 July 2009 |volume=48 |issue=1 |pages=59–68 |doi=10.1007/s00348-009-0709-3 |s2cid=53590637 }}
*{{cite journal |last1=Atcheson |first1=Bradley |last2=Heidrich |first2=Wolfgang |last3=Ihrke |first3=Ivo |title=An evaluation of optical flow algorithms for background oriented schlieren imaging |journal=Experiments in Fluids |date=5 October 2008 |volume=46 |issue=3 |pages=467–476 |doi=10.1007/s00348-008-0572-7 |s2cid=17713504 }}
*{{cite journal |last1=Skeen |first1=Scott A. |last2=Manin |first2=Julien |last3=Pickett |first3=Lyle M. |title=Simultaneous formaldehyde PLIF and high-speed schlieren imaging for ignition visualization in high-pressure spray flames |journal=Proceedings of the Combustion Institute |date=2015 |volume=35 |issue=3 |pages=3167–3174 |doi=10.1016/j.proci.2014.06.040 |doi-access=free }}
*{{cite journal |last1=Willert |first1=Christian E. |last2=Mitchell |first2=Daniel M. |last3=Soria |first3=Julio |title=An assessment of high-power light-emitting diodes for high frame rate schlieren imaging |journal=Experiments in Fluids |date=5 April 2012 |volume=53 |issue=2 |pages=413–421 |doi=10.1007/s00348-012-1297-1 |bibcode=2012ExFl...53..413W |s2cid=120726611 }}

== External links ==
* [https://www.youtube.com/watch?v=02VCZsd9M58 A presentation of schlieren imaging on YouTube]
* [http://www.ultrasonics.org/Proceedings_2007_UIA/Ben-Ezra_2007_UIA.pdf Acoustic  Field Characteriztation with Schlieren System - a short presentation]

[[Category:Acoustics]]
[[Category:Ultrasound]]
[[Category:Imaging]]

{{Sourceattribution|Schlieren imaging}}