•Physical derivation of laser-stimulated arbitrary volumetric ultrasonic source.•Uniting pressure (normal and lateral) with thermal expansion generation mechanisms.•Waveform modeling based on ...statistically enhanced Green’s function formalism.•Spatial and directional distributions of source and sensor are incorporated.•Source enlargement reveals limit-to-limit transition of displacement waveforms.
Ultrasound modeling, being an established practice, is used to study the fundamentals of light-matter interactions. Although much has been published on the matter, pressure and thermal expansion induction mechanisms in laser ultrasonics have rarely been combined, as they should, in a single ultrasonic source while the effects of its size variation have only been shown to a limited extent. In the paper, we unite these light-matter interaction mechanisms, with inclusion of lateral optical forces, into a single laser-stimulated source as it is observed in nature. With a laser pulse as a manipulable source, we simulate the multifaceted workings of light-matter interactions by exposing the distinct transients originating from different source localities as generated by different induction mechanisms. We also present a transition of simulated ultrasonic waveforms in the epicentral point on the surface of a solid plate opposite from the source while it is expanded from a point to a quasi-limitless extent for pressure and thermal expansion generation regimes. The model utilizes geometric probability theory together with Huygens’ superposition principle and temporal convolutions to construct the desired waveforms out of individual Green’s functions. We show how the ultrasound generation regimes stem out of a single source and how its size together with energy and momentum transfers during the light-matter interactions affect the induced ultrasonic transients.
We present an optical study of elastic wave propagation inside skin phantoms consisting of agar gel as induced by an Er:YAG (wavelength of 2.94 μm) laser pulse. A laser-beam-deflection probe is used ...to measure ultrasonic propagation and a high-speed camera is used to record displacements in ablation-induced elastic transients. These measurements are further analyzed with a custom developed image recognition algorithm utilizing the methods of particle image velocimetry and spline interpolation to determine point trajectories, material displacement and strain during the passing of the transients. The results indicate that the ablation-induced elastic waves propagate with a velocity of 1 m/s and amplitudes of 0.1 mm. Compared to them, the measured velocities of ultrasonic waves are much higher, within the range of 1.42-1.51 km/s, while their amplitudes are three orders of magnitude smaller. This proves that the agar gel may be used as a rudimental skin and soft tissue substitute in biomedical research, since its polymeric structure reproduces adequate soft-solid properties and its transparency for visible light makes it convenient to study with optical instruments. The results presented provide an insight into the distribution of laser-induced elastic transients in soft tissue phantoms, while the experimental approach serves as a foundation for further research of laser-induced mechanical effects deeper in the tissue.
•Modeling using Green’s function formalism and streamlined Huygens’ principle.•The model incorporates spatial distributions of source and sensor sensitivity.•A statistically simplified area-to-area ...ultrasound transfer function was developed.•Uniform, Gaussian and annular source ultrasonic waveforms are simulated.•Thus-simulated waveforms match the measurements more closely than the point model.
The near-field, surface-displacement waveforms in plates are modeled using interwoven concepts of Green’s function formalism and streamlined Huygens’ principle. Green’s functions resemble the building blocks of the sought displacement waveform, superimposed and weighted according to the simplified distribution. The approach incorporates an arbitrary circular spatial source distribution and an arbitrary circular spatial sensitivity in the area probed by the sensor. The displacement histories for uniform, Gaussian and annular normal-force source distributions and the uniform spatial sensor sensitivity are calculated, and the corresponding weight distributions are compared. To demonstrate the applicability of the developed scheme, measurements of laser ultrasound induced solely by the radiation pressure are compared with the calculated waveforms. The ultrasound is induced by laser pulse reflection from the mirror-surface of a glass plate. The measurements show excellent agreement not only with respect to various wave-arrivals but also in the shape of each arrival. Their shape depends on the beam profile of the excitation laser pulse and its corresponding spatial normal-force distribution.
Optodynamic interaction between a laser pulse and the surface of an opaque, solid elastic object produces transient waves that propagate and reverberate within the object. They can be, in general, ...categorized into three distinctive types which are all formed through different mechanisms: ablation-induced waves, light-pressure-induced waves, and thermoelastic waves. In this paper, out-of-plane displacements of such waves are simulated at the epicentral position on the opposite side of an extended plane-parallel elastic plate. Wave propagation is mathematically described by Green’s transfer functions convolved with suitable time profiles of the incoming laser pulses. The simulated size of the circularly symmetric laser-illuminated area on the plate surface is varied to show the limit-to-limit transition of the displacement waveforms: from a 2D point source to an infinite 1D source.
Electromagnetic momentum carried by light is observable through the mechanical effects radiation pressure exerts on illuminated objects. Momentum conversion from electromagnetic fields to elastic ...waves within a solid object proceeds through a string of electrodynamic and elastodynamic phenomena, collectively bound by momentum and energy continuity. The details of this conversion predicted by theory have yet to be validated by experiments, as it is difficult to distinguish displacements driven by momentum from those driven by heating due to light absorption. Here, we have measured temporal variations of the surface displacements induced by laser pulses reflected from a solid dielectric mirror. Ab initio modelling of momentum flow describes the transfer of momentum from the electromagnetic field to the dielectric mirror, with subsequent creation/propagation of multicomponent elastic waves. Complete consistency between predictions and absolute measurements of surface displacements offers compelling evidence of elastic transients driven predominantly by the momentum of light.
In medicine and cosmetics a need has arisen for an accurate, fast, simple and broadly applicable three-dimensional skin surface measuring system. A new development of such handheld optical device ...that uses white light Fourier transform profilometry for skin surface topography measurement is presented here. The device is based on a commercially available DSLR camera and a newly designed optical system which projects a Ronchi grating pattern on the measured surface and records its image. The 3D surface is reconstructed from a single image. The topography is described using four surface roughness parameters: mean absolute deviation R.sub.a, root-mean-square deviation R.sub.q skewness R.sub.sk and kurtosis R.sub.ku Experimental measurements were conducted on five facial skin areas, together belonging to three people, before and after the laser skin treatment therapy, to evaluate its effects on skin surface roughness. The results show the viability and usefulness of the developed optical system for skin topography measurement. Keywords: Fourier transform profilometry, optical triangulation, skin topography, surface roughness parameters Highlights V clanku je predstavljena nova rocna opticna naprava, ki uporablja Fourierjevo transformacijsko profilometrijo z belo svetlobo za merjenje topografije koze. V medicini in kozmetiki se pojavlja potreba po natancnem, hitrem, enostavnem in siroko uporabnem sistemu za tridimenzionalno in-vivo merjenje povrsine koze. Taka opticna naprava bi dajala natancne in konsistentne informacije o topografiji in barvi koze, pri cemer bi bilo brezdoticno in neinvazivno merjenje globine njena najpomembnejsa novost. Najbolj prakticne in zato tudi najbolj razsirjene metode za in-vivo analizo povrsine koze spadajo v kategorijo tako imenovanih projekcijskih povrsinskih topografij--ena izmed takih merskih metod je Fourierjevo transformacijska profilometrija. Pri tej metodi se na merjeno povrsino projicira droben crtast vzorec, nato se posname sliko te projekcije, naposled pa se z ustreznim racunalniskim programom posneto sliko obdela. Posneta slika je sestavljena iz mnozice merskih tock in se interpretira kot mnozica vrstic, pri cemer vsaka vrstica predstavlja signal intenzitete svetlobe, ki je moduliran v fazi in amplitudi, in ima neko prostorsko nosilno frekvenco. Signal iz vsake vrstice se razvije v Fourierjevo vrsto, prostorsko Fourierjevo transformira, filtrira okoli nosilne frekvence ter inverzno Fourierjevo transformira v preciscen kompleksni signal. V argumentu tega signala se skriva fazni zamik, ki vsebuje informacijo o globinskem polozaju merskih tock. Informacijo o ravninskem polozaju merskih tock pa se enostavno prebere iz njihovih polozajev na sliki. Tridimenzionalna oblika povrsja se tako rekonstruira iz ene same slike. Naprava je bila razvita predvsem za natancno in objektivno ocenjevanje izgleda povrsine koze pred raznimi laserskimi terapijami ter po njih. V iskanju enostavne, dostopne in cenovne ugodne zasnove je bila ta naprava povecini razvita in sestavljena iz trzno dostopnih produktov. Tako temelji na digitalnem zrcalnorefleksnem (DSLR) fotoaparatu Nikon D90 z vgrajeno bliskavico kot izvorom svetlobe. Fotoaparat dopolnjuje opticni sistem s projektorjem, ki projicira Ronchijev crtni vzorec na merjeno povrsino, in objektivom, ki preslika sliko vzorca na senzor fotoaparata. Meritve z novo napravo so bile izvedene na petih vzorcih (ki so skupno pripadali trem ljudem) koze na obrazu --pred lasersko terapijo koze in po njej. Smisel te terapije je glajenje koze; njeni ucinki na hrapavost vzorcev pa so bili eksperimentalno merjeni in ocenjevani. Ker je koza polprosojna in nehomogena, je bil na vzorce nanesen kozni puder, ki je bistveno izboljsal njene opticne lastnoti za tako meritev. Topografija koze je v tej analizi opisana s stirimi parametri hrapavosti povrsine: s povprecnim absolutnim odstopanjem R.sub.a, s povprecnim kvadratnim odstopanjem R.sub.q, z asimetricnostjo R.sub.sk in s sploscenostjo R.sub.ku. Za njihovo dolocitev je bilo najprej potrebno izracunti idealizirano referencno povrsino koze, ki je povsem gladka. Izracunani parametri hrapavosti povrsine koze so pokazali, da ima vecina vzorcev povprecno absolutno odstopanje R.sub.a med 45 µm in 80 µm ter povprecno kvadratno odstopanje R.sub.q med 55 µm in 110 µm. Parametri tudi kazejo, da imajo vzorci koze vecinoma nizke vrhove in globoke vdolbine (R.sub.sk < 0) ter da so ti vrhovi in vdolbine relativno konicasti (R.sub.ku > 0). Kar se tice laserske terapije in njenih ucinkov na hrapavost povrsine koze, je jasno razvidno, da imajo vzorci po terapiji izrazito manjse vrednosti parametrov R.sub.a in R.sub.q kot pred njo. To nakazuje, da ima laserska terapija zelen ucinek in resnicno zmanjsuje hrapavost koze. Pri primerjavi ostalih dveh parametrov, R.sub.sk in R.sub.ku, pa se ne more povedati nic gotovega. Rezultati so tako pokazali zmoznosti in predvsem uporabnost razvitega opticnega sistema za merjenje topografije koze. Kljucne besede: Fourierjevo transformacijska profilometrija, opticna triangulacija, topografija koze, parametri hrapavosti povrsine
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