Optical Dome Research Articles

Combined influence of aero-optical flow field and optical dome on imaging deviation in blunt-nose front-window flight vehicle

Combined influence of aero-optical flow field and optical dome on imaging deviation in blunt-nose front-window flight vehicle

Line-of-sight optical Dome Turbulence Monitor

ABSTRACT Optical turbulence in the enclosure of a ground-based telescope can be a major contributor to the total optical turbulence strength and can therefore limit the precision of astronomical observations, in terms of angular resolution, or signal to noise ratio, depending on the instrumentation used. Here we propose a new Dome Turbulence Monitor (DTM) technique. This DTM is based on the well known Scintillation Detection And Ranging (SCIDAR) concept with a few major differences. By designing the instrument specifically for the dome turbulence, we can use a small (<0.2 m telescope), and observe single bright stars (as opposed to the SCIDAR using double stars). This enables a dedicated instrument with enough targets to operate continuously. Operationally, the DTM could be mounted somewhere on the main astronomical telescope and track stars that are visible through the dome aperture. By measuring through the dome aperture we obtain an optical measure of the strength of the turbulence along a similar line of sight as the main telescope itself. We demonstrate the new technique through numerical Monte Carlo modelling and present results from a proof-of-concept demonstration at the European Space Agency Optical Ground Station on Tenerife, Spain.

Experimental investigation on supersonic film cooling of hypersonic optical dome under different nozzle pressure ratios

When an aircraft flies at hypersonic speeds in the atmosphere, its optical window is subjected to severe aerodynamic heating. To investigate the supersonic film cooling laws of hypersonic optical dome, three supersonic (Mach 2.35) film generation devices with different jet outlet heights (1 mm, 2 mm, 3 mm) were designed. The experiments were conducted in a hypersonic (Mach 7.1) gun wind tunnel. The NPR (nozzle pressure ratio = jet outlet static pressure/nearby mainstream static pressure) ranged from 0 to 2.3. The heat flux on the window surface was measured by the thin film resistance thermometer. Experimental results indicate that, under different jet outlet heights, the film cooling effectiveness increased gradually as the NPR increased, corresponding to an increase in effective cooling length. The existing similarity parameters for film cooling could effectively predict the effective cooling length of supersonic film when NPR = 1. However, the prediction effect deviated significantly when NPR ≠ 1. Moreover, the effective cooling length corresponding to the unit mass flow rate of the coolant first increased and then decreased with the increase of NPR, and there existed an optimal solution. Furthermore, with the same coolant mass flow rate, higher jet Mach number lead to better supersonic film cooling performance.

Conformal antireflection coatings for optical dome covers by atomic layer deposition.

Complex 3D-shaped optics are difficult to coat with conventional technologies. In this research, large top-open optical glass cubes with a 100mm side length were functionalized to simulate large dome-shaped optics. Antireflection coatings for the visible range (420-670nm) and for a single wavelength (550nm) were applied by atomic layer deposition simultaneously on two and six demonstrators, respectively. Reflectance measurements on both the inner and outer glass surfaces confirm a conformal AR coating with a residual reflectance significantly below 0.3% for visible wavelengths and 0.2% for single wavelengths on nearly the entire surface of the cubes.

Fluid-Thermal Interaction Simulation of a Hypersonic Aircraft Optical Dome

Hypersonic aircraft design is an enabling technology. However, many problems are encountered, including the design of the hood. The aircraft optical dome can become heated due to aerodynamic effects. Since the optical dome of a hypersonic aircraft should satisfy optical imaging requirements, a conventional ablative coating cannot be adopted. The aerodynamic heating characteristics during the whole flight must be studied. In this study, a numerical simulation method for the aerodynamic heat of hypersonic aircraft under long-term variable working conditions is proposed. In addition, the numerical simulation of the external flow field and structure coupling of the aerodynamic heat problem is performed. The dynamic parameters of temperature and pressure are obtained, and the thermal protection basis of the internal equipment is obtained. Numerical results indicate that the average temperature and maximum temperature of the optical dome for inner and outer walls exhibit an “M” shape with time, with two high-temperature cusps and one low-temperature cusp. The time of average temperature coincides with that of maximum wall temperature. During the flight, the wall pressure changes with time, exhibiting the characteristics of higher temperature at both ends of the flight and lower temperature in the middle. The structural temperature of the hypersonic aircraft is higher than that of the external flow behind the shock wave after 310 s. Therefore, this study provides a reliable reference for the preliminary design and parameter research of optical domes of hypersonic aircraft.

The correction of dynamic aero-optical aberration of optical dome at different altitudes using wavefront coding

In our previous published paper [Opt. Commun. 490, 126876 (2021)], we proposed a method to correct the optical dome’s aero-optical aberration using wavefront coding (WFC). The simulation results were carried out under a fixed flight altitude. However, because the missile tracking the target is a dynamic process, the image quality deterioration of the aero-dynamically heated dome is diverse under different altitude. Therefore, it is of great significance to study the correction effect of WFC on dynamic aero-optical aberration of optical dome at different altitudes, which is performed in our new manuscript. The simulation results demonstrate that the WFC could effectively correct the dynamic aero-optical aberration and improve the accuracy of imaging guidance. These results confirm the possibility of the application of WFC imaging technology in missile-borne optical detection systems.

Shaping an optical dome: The size and shape of the insect compound eye

The insect compound eye is the most abundant eye architecture on earth. It comes in a wide variety of shapes and sizes, which are exquisitely adapted to specific ecosystems. Here, we explore the organisational principles and pathways, from molecular to tissular, that underpin the building of this organ and highlight why it is an excellent model system to investigate the relationship between genes and tissue form. The compound eye offers wide fields of view, high sensitivity in motion detection and infinite depth of field. It is made of an array of visual units called ommatidia, which are precisely tiled in 3D to shape the retinal tissue as a dome-like structure. The eye starts off as a 2D epithelium, and it acquires its 3D organisation as ommatidia get into shape. Each ommatidium is made of a complement of retinal cells, including light-detecting photoreceptors and lens-secreting cells. The lens cells generate the typical hexagonal facet lens that lies atop the photoreceptors so that the eye surface consists of a quasi-crystalline array of these hexagonal facet-lenses. This array is curved to various degree, depending on the size and shape of the eye, and on the region of the retina. This curvature sets the resolution and visual field of the eye and is determined by i) the number and size of the facet lens – large ommatidial lenses can be used to generate flat, higher resolution areas, while smaller facets allow for stronger curvature of the eye, and ii) precise control of the inter facet-lens angle, which determines the optical axis of the each ommatidium. In this review we discuss how combinatorial variation in eye primordium shape, ommatidial number, facet lens size and inter facet-lens angle underpins the wide variety of insect eye shapes, and we explore what is known about the mechanisms that might control these parameters.

Conformal dome aberration correction based on the integrated design of inner surface and fixed corrector

The ellipsoidal conformal dome is commonly used at present, which is a balanced choice between the aero-dynamic performance and optical imaging performance of the optical dome. The paraboloidal dome has superior aerodynamic performance, but the dynamic aberration correction is far more difficult than that of the ellipsoidal dome which has also become a bottleneck restricting on its application. This paper proposed a dynamic aberration correction method based on integrated design of inner surface and fixed corrector, for which realized the design of the dynamic aberration correction of the paraboloidal dome. Based on the analysis of the Zernike aberration characteristics of the paraboloidal dome, we focused on controlling of the Zernike aberration coefficients Z4, Z5, Z8 and Z9 to execute the dynamic aberration correction of the inner surface. Then, after deriving the Wassermann-Wolf equation (W-W equation) of the paraboloidal conformal dome, the initial surface shape of the fixed corrector was numerically solved combining with the ray tracing method, and the following dynamic aberration correction of the fixed corrector was carried out. Finally, with a specific design example of mid-wave infrared optical system with the paraboloidal dome, the dynamic aberration correction method for the paraboloidal conformal dome optical system was verified. These results show that the design method is feasible to solve the dynamic aberration correction difficulties of the paraboloidal dome, and is of great significance to promote the applications of the paraboloidal conformal dome.

Cellular automata ray tracing in flow field near the optical window of the optical dome

For the supersonic flow field with large density fluctuation, an effective method to obtain the beam path is solving ray equation. Generally, when the refractive index distribution is arbitrary, obtaining the ray equation analytic solution is difficult. Cellular Automata (CA) ray tracing algorithm is proposed in this paper for aero-optical calculation in flow field near the optical window of the optical dome. Different from numerically solving the ray equation (NSRE), the coordinate value and the offset angle are calculated according to the position and direction transformation rules in CA algorithm. Then the refractive index field is integrated along the beam path to obtain the optical path length (OPL). The optical path difference (OPD) is calculated from OPL. The OPDrms results quantitatively verify the calculation accuracy. Then the time complexity verifies high efficiency of CA. The results show that CA algorithm is applicable to ray tracing and can simplify the calculation in 3D aero-optical flow field.

S2778 The Dangers of Seeing the Light: Importance of Forward-Facing Orientation for Endoscopic Capsule Deployment

INTRODUCTION: Since 2001 video capsule endoscopy (VCE) has been widely applied to assess small bowel bleeding, iron deficiency anemia (IDA), inflammatory bowel disease, refractory celiac disease and polyposis syndromes. Limitations include the inability to intervene, lack of familiarity with reading video and handling software, as well as time required to interpret findings. The aim of this case series is to present 4 cases of lens damage to the VCE from the VCE capsule holder used during endoscopic placement. CASE DESCRIPTION/METHODS: 86 VCE studies were reviewed; 4 showed structural VCE lens damage after endoscopic placement. These were all placed with lens towards the releasing device rather than forward-facing. No signs of lens damage were observed in the remaining 82 capsule studies. The first case was a 60-year-old woman with IDA and normal VCE study. The second patient was an 84-year-old man with IDA, with small ulcerations throughout the small bowel on VCE. Third was a 30-year-old woman with iron deficiency and normal hemoglobin level, with a non-diagnostic VCE. Last was an 89-year-old man with IDA, with VCE showing duodenal polyp. DISCUSSION: Appropriate endoscopic deployment technique involves the use of a catheter-based device to which a cup is attached to hold the capsule, with a moveable metal core that pushes it out of the cup (US Endoscopy, Mentor, Ohio), allowing deployment. The optical dome of the video capsule should be placed in this holder forward-facing with the lens pointing away from the endoscope. Some physicians insert the capsule with the opposite orientation to confirm deployment by visualizing light source after release. This practice can result in damage to the lens when the metal core is fired, thereby affecting obtained images and decreasing study quality. In this series, each VCE showed a linear scratch on the lens. This scratch can be misinterpreted as a lesion, as well as interfere with the reading of the study. Summary: It is important to take into consideration orientation of the VCE with a forward-facing lens when loading into the endoscopic deployment device. This will prevent lens damage and ensure that the highest quality images possible are obtained during study, optimizing VCE efficacy. Physician awareness is needed to prevent this potential pitfall.Figure 1.: Proper orientation of the capsule device into the endoscopic deployment device.Figure 2.: Capsule images with evidence of lens damage, causing linear scratches on images.Figure 3.: Capsule images with evidence of lens damage, causing linear scratches on images.

Experimental investigation on aero-optical effects of a hypersonic optical dome under different exposure times.

The aero-optical effects induced by the complex flow structure around a hypersonic optical dome is highly unsteady, which leads to significant differences in the imaging quality under different exposure times. It is of great significance to study the influence of exposure time on imaging quality for guiding the design of imaging guidance seekers and improving imaging guidance accuracy. Based on the hypersonic gun wind tunnel, an aero-optical effect measurement platform was built to measure the wavefront from transient exposure to long exposure. With the increase of exposure time, the accuracy of high-order optical path difference (OPD) reconstruction by low-order Zernike polynomials increased from 62.2% to 88.6%. The increase of exposure time was helpful to reduce the complexity of the wavefront spatial distribution structure. In principle, it could reduce the difficulty of wavefront adaptive correction systems. With the increase of exposure time, the ${{\rm OPD}_{\rm rms}}$OPDrms corresponding to ${{\rm OPD}_{\rm high-order}}$OPDhigh-order increased gradually, the amplitude decreased gradually, and the difference of ${{\rm OPD}_{\rm rms}}$OPDrms decreased gradually at different times. Under different exposure times, the large-aperture approximation principle could achieve a better prediction of Strehl ratio values. With the increase of exposure time, the imaging integral resolution, $R$R, decreased obviously, and it was stable at about ${1.43}{R_0}$1.43R0. Compared with that, $R$R was improved by about 30% when the exposure time was 6 ns.

Evaluating imaging quality of optical dome affected by aero-optical transmission effect and aero-thermal radiation effect.

The imaging quality of the aerodynamically heated optical dome was evaluated under the comprehensive influence of aero-optical transmission effect and aero-thermal radiation effect. The ray propagating algorithm based on the fourth order Runge-Kutta method was used to trace the target ray and the thermal radiation ray of the optical dome. Three imaging quality evaluation parameters were proposed to evaluate aero-optical effect: Modulation transfer function (MTF), irradiance, peak signal-to-noise ratio (PSNR) of distorted images. The results show that: as the flight speed increased, the MTF decreased observably compared with the diffraction-limit MTF, the irradiance on the photosensitive surface of the detector increased gradually, and the distorted imaging quality under the influence of the comprehensive aero-optical effect gradually deteriorated. However, as the thickness of the optical dome increased, the MTF decreased sharply and the irradiance decreased gradually, that indicated the aero-optical transmission effect was reinforced and the aero-thermal radiation effect was weakened. The imaging quality improved with thickness increasing. The influence of aero-thermal radiation effect on the PSNR of the image was more serious than that of the aero-optical transmission effect.

Influence of cooling film pressure on the imaging quality of a hypersonic optical dome

The flow-field-caused aero-optical effect is becoming a crucial issue in hypersonic flight. The degraded imaging results can seriously affect the aircraft’s ability to capture targets. Experiments were carried out in a Mach 6-gun tunnel, and the imaging quality of a hypersonic optical dome was investigated under six different values of pressure ratio of jet. Background oriented schlieren was used to obtain the point spread functions (PSFs), which were then used to evaluate the imaging quality of the optical dome. The experimental results showed that cooling film with higher pressure will reduce the support field of the PSF and will have a more severe influence on the imaging quality.

Cellular automata ray tracing in two-dimensional aero-optical flow fields

<sec>For the supersonic flow field with large density fluctuation produced by the unsteady flow and turbulent large-scale structures, an effective method to obtain the beam path is to solve the ray equation. Then the optical path difference (OPD), Strehl ratio (SR), optical transmission function (OTF), etc. can be obtained to analyze the optical distortion, and the correction of aero optics effects can be realized to improve the optical system performance. Generally, when the refractive index distribution is arbitrary, the ray equation analytic solution is difficult to obtain.</sec> <sec>Cellular automata (CA) ray tracing algorithm is proposed in this paper for aero-optical calculation in the 2D discrete flow fields. Unlike numerically solving the ray equation (NSRE), the coordinate value and the offset angle are calculated according to the position and direction transformation rules in CA algorithm. The position transformation rule is used to obtain the end point of the beam vector and determine whether the offset angle needs calculating at each iteration, the direction transformation rule is to calculate the offset angles. Then the refractive index field is integrated along the beam path to obtain the optical path length (OPL). The OPD is calculated from OPL. In this paper, aero-optical calculation is based on two types of flow fields. The supersonic shear layer including supersonic mixing layer and boundary layer 2D density distribution is measured by the nano-tracer-based planar laser scattering (NPLS) technique. The supersonic flow field surrounding the optical dome is simulated based on detached-eddy simulation (DES).</sec><sec>The OPD<sub>rms</sub> and program running time quantitatively verify the calculation accuracy and high efficiency of CA. The results show that for the 2D supersonic NPLS flow field and the 2D supersonic flow field surrounding the optical dome, the calculation accuracy of CA is approximately equal to NSRE. Moreover, we find that the program running time of NSRE is about four times that of the CA algorithm, so the efficiency of ray tracing is effectively improved. Thus, the CA algorithm provides a new scheme for aero-optical calculation in the 2D supersonic flow field. Meanwhile, it also provides a guideline for the research on the ray tracing in 3D discrete aero-optical flow field.</sec>

Experimental study on the influence of attitude angle on the aero-optical effects of a hypersonic optical dome

The aero-optical effects of a hypersonic dome with cooling jet device can be mitigated by micro vortex generators (MVGs) at 0°angle of attack (AoA) and 0°angle of sideslip (AoS). However, the attitude angle of an aircraft is constantly changing during actual flight. In this research, experiment was carried out at the KD-01 hypersonic gun tunnel at Mach 6.0. Using the schlieren technique, the flow field density distribution of a hypersonic optical dome under different pressure ratios of jet (PRJ) was observed. The optical path difference (OPD) was obtained by background-oriented schlieren (BOS). Experimental results showed that a 4°AoA could increase the OPD value more obviously with or without MVGs; and that the use of MVGs under 4°AoS would aggravate the aero-optical effects. These findings place new demands on the design and installation of MVGs.

Influence of altitude on aero-optic imaging quality degradation of the hemispherical optical dome.

We investigated the influence of altitude on aero-optic imaging quality degradation of the hemispherical optical dome. Boundary conditions for the aerodynamic heating effect of the optical dome were calculated by solving the Reynolds-averaged Navier-Stokes equations provided by FLUENT. The finite element model and the thermal-structure simulation results of the optical dome were obtained using ANSYS. The 3D nonuniform refractive index field of the optical dome was obtained according to the thermal-optical effect. The optical tracking method based on the fourth-order Runge-Kutta algorithm was adopted to simulate the optical transmission through the optical dome. The Strehl ratio (SR), encircled energy, distorted target images, and peak signal-to-noise ratio were presented for imaging quality evaluation. The variation rules of these imaging quality evaluation parameters were obtained in the altitude range of 0-45km. The results showed that, in the same flight conditions, with the increase of altitude, peak signal-to-noise ratio (PSNR) of the distorted image, and SR result were increased, and radiuses of dispersion spots, including 80% energy, were decreased; therefore, the influence of aero-optics effect on imaging quality degradation was gradually weakened.

Numerical analysis of thermal radiation noise of shock layer over an infrared optical dome at near-ground altitudes

To examine the effect of atmospheric trace species on the infrared thermal radiation of a supersonic dome, a series of radiation characteristics with different geometry sizes at near-ground altitudes were investigated numerically. The conjugate heat transfer method was applied to build the heat transfer model of the optical dome. Three major radiating species of H2O, CO2, and CO were taken into account in the shock layer. A line-by-line (LBL) method was used for evaluating the radiative properties of species. A line-of-sight (LOS) approach was applied to solve the radiative transfer equation (RTE). The simulated and measured results of the dome were also proposed to validate the numerical method. The effects of the dome geometry size, the dome material and the time-varying altitude on the infrared radiation noise were studied in detail. The results show that the altitude-varying radiation intensity along the LOS is related to the ambient density and velocity. The variation of the dome radius is proportional to the total radiation received on the dome surface. It is observed that the maximum radiation intensity along the LOS does not occur in the normal direction of the receiving point, but it is determined by both the flow field parameters and the path length. Also, the radiation increment corresponding to different dome sizes approximately obeys a Gaussian distribution related to the product of density and velocity.

Reduction in EMI with BaTiO3 and Fe3O4 Thin Film grown by UBM Sputtering

The imaging and surveying ground areas with airborne sensors has important application for disaster monitoring, military application and rescue operation. The proposed solution seeks to explore a new approach in microwave shielding design through coating of high permittivity, hydrophilic oxide materials. To achieve this, multi-stacked metal oxides, Titanium Dioxide (TiO2), Barium Titanium Trioxide (BaTiO3) and Iron Ferrite (Fe) are deposited on the Sapphire ring and optical dome via Unbalanced Magnetron (UBM) sputtering.The complex permittivity (Ɛ’ and Ɛ”) and permeability (μ’ and μ”) of oxide-coated substrates are verified through measurement of their Scattering (S-) parameters (S11 and S21) via Vector Network Analyzer (VNA) between 300MHz to 18GHz regime by airline measurement technique for oxide sputtered on sapphire ring substrate. The optimal layers were sputtered on acrylic substrate as well as acrylic domes with arch reflectance result obtained using Pasco Kit measurement kit at 10.5GHz.

Investigation of thermal radiation effect on optical dome of sapphire coated yttrium oxide

Compared to traditional optical domes, domes of sapphire coated with films can effectively reduce emissivity and increase transmittance. The purpose of this work is to investigate the thermal radiation effect on sapphire optical dome coated with yttrium oxide by a radio frequency magnetron sputtering method. The emissivity of sapphire coated with Y2O3 films is studied by both numerical and experimental methods. The results indicate that the emissivity of sapphire substrate is reduced effectively with increasing the thickness of the Y2O3 film. In addition, a finite element model is developed to simulate the radiation intensity of the optical dome. The thermal responses indicate that the maximum temperature is reduced apparently compared with the uncoated sapphire as Y2O3 film thicknesses increase. The average irradiance distribution at different film thicknesses with time shows that the self-thermal radiation disturbance of sapphire optical dome delays 0.93s when the thickness of Y2O3 film is 200μm, which can guarantee the dome works properly and effectively even in a harsh environment.

红外共形整流罩热噪声分析与修正

When the optical dome of aerobat is initially subjected to aerodynamic flight conditions, the temperature of front surface rises rapidly. The thermal optical dome produces a sharp increase in background radiance on the focal plane detector, which can mask the target signal and decrease the ability of target tracking. Infrared conformal dome has demonstrated its capabilities as a prominent technique for supersonic aero thermal radiation effect. Aero thermal radiation effect of conformal infrared dome is described. The thermal noises of conformal dome are measured by infrared blackbody when dome temperature decreases from 460 K to 320 K. Quadratic surface as noise correction model decided by dualistic nolinear regression analysis is introduced, and the coefficient of determination of quadratic surface fitting is 0.8683. The mean signal noise ratio of the image is increased to 13.7 dB after noise correction by noise correction model. The signal noise ratio of infrared image increases 6.5 dB after noise correction. The result can meet the precision requirement of target tracking system and provide partial evidence for optics system optimization.