Stray Light Reduction Research Articles

Low computational demand stray light correction method for hyperspectral imaging spectrometers

Stray light correction in hyperspectral imaging spectrometers has long been restricted by high computational requirements. This paper presents a low computational demand method, based on the matrix operations for spectrometer stray light correction, using an iterative approach to efficiently correct stray light across both spectral and spatial dimensions. The efficacy of this method is demonstrated through its application to simulated and real images, achieving an overall reduction of stray light by over 50%, with significantly reduced computation time and memory usage compared to the method by Zong et al. based on a sparse matrix [Appl. Opt. 45, 1111 (2006)10.1364/AO.45.001111APOPAI2155-3165]. By enabling stray light correction on general-purpose computers, this method enhances affordability and accessibility, promoting broader use and reducing measurement uncertainties in various hyperspectral imaging applications.

Analysis and reduction of back-reflection straylight in laser ranging system

In the coaxial satellite laser ranging system or the other telescope-based measurement systems, the emitted beam produces backward reflection at the center of the secondary mirror, which affects the detector in the system. To reduce the impact of straylight, a new device for reduce the backward reflection of the secondary mirror has been proposed. The specific parameters of the device have been determined through numerical calculation and computer simulation. The device has been implemented in a 1.2 m laser ranging system and has been experimentally proven to reduce the laser straylight by 32.4 % with the single photon detector and there is an increase in the camera's contrast for target imaging, making it more effective to measuring the space target.

Stray light reduction in monolithic GaInN-based μLED arrays for high-definition display realization

Monolithic GaInN-based micro μLEDs arrays are expected to be applied to ultra-high-definition displays. In this study, stray light behavior of them fabricated on sapphire substrates was investigated. Results reveal that strong stray light appears considerably outside the μLED drive region, which is a major obstacle to the realization of high-definition displays. We also explored various techniques to effectively reduce the stray light. Use of flat sapphire substrates and removal of GaN material between μLEDs are effective in reducing stray light and are essential for achieving high definition in monolithic GaInN-based μLED array displays.

Black zeolite coatings for stray light reduction and in-orbit molecular decontamination

Black zeolite coatings were developed for two major contaminations encountered by actors of the space field: in-orbit molecular contamination and stray light contamination. They both affect the performance of on-board equipment of satellites and especially optical devices. A silicone binder and FAU-type zeolite were combined together with different amounts of black pigment (bone char or carbon black) in order to develop various coatings. Binder content was fixed at 20 wt % for all black zeolite coatings after the drying procedure whereas zeolite contents varied from 50 to 75 wt % and from 60 to 79 wt % for coatings developed with bone char and carbon black, respectively. Amounts involved of bone char and carbon black ranged from 5 to 30 wt % and from 1 to 20 wt %, respectively. The aim of increasing black pigment amount was to improve light absorption properties of the coatings. Despite the addition of black pigment, interesting adhesion and cohesion properties were obtained. Total reflection measurements showed a clear improvement of light absorption properties by black coatings. Small amounts of black pigment especially with carbon black (1–5 wt %) were sufficient to increase light absorption compared to white zeolite coating. Nitrogen adsorption-desorption experiments showed a partial clogging of black zeolite coatings porosity from 20 to 25 wt %. Nevertheless, interesting results regarding n-hexane adsorption capacities where values varying from 100 to 130 mg/ganhydrous zeolite were determined. Black zeolite coatings developed in this study show a way to deal with the above-mentioned issues.

Comparison of global UV spectral irradiance measurements between a BTS CCD-array and a Brewer spectroradiometer

Abstract. Spectral measurements of UV irradiance are of great importance for protecting human health as well as for supporting scientific research. To perform these measurements, double monochromator scanning spectroradiometers are the preferred devices thanks to their linearity and stray-light reduction. However, because of their high cost and demanding maintenance, CCD-array-based spectroradiometers are increasingly used for monitoring UV irradiance. Nevertheless, CCD-array spectroradiometers have specific limitations, such as a high detection threshold or stray-light contamination. To overcome these challenges, several manufacturers are striving to develop improved instrumentation. In particular, Gigahertz-Optik GmbH has developed the stray-light-reduced BTS2048-UV-S spectroradiometer series (hereafter “BTS”). In this study, the long-term performance of the BTS and its seasonal behavior, regarding global UV irradiance, was assessed. To carry out the analysis, BTS irradiance measurements were compared against measurements from the Brewer MK-III #150 scanning spectrophotometer during three campaigns. A total of 711 simultaneous spectra, measured under cloud-free conditions and covering a wide range of solar zenith angles (SZAs; from 14 to 70∘) and UV indexes (from 2.4 to 10.6), were used for the comparison. During the three measurement campaigns, the global UV spectral ratio BTS / Brewer was almost constant (at around 0.93) in the 305–360 nm region for SZAs below 70∘. Thus, the BTS calibration was stable during the whole period of study (∼ 1.5 years). Likewise, it showed no significant seasonal or SZA dependence in this wavelength region. Regarding the UV index, a good correlation between the BTS and the Brewer #150 was found, i.e., the dynamic range of the BTS is comparable to that of the Brewer #150. These results confirm the quality of the long-term performance of the BTS array spectroradiometer in measuring global UV irradiance.

Stray Light Correction Algorithm for High Performance Optical Instruments: The Case of Metop-3MI

Stray light is a critical aspect for high performance optical instruments. When stray light control by design is insufficient to reach the performance requirement, correction by post-processing must be considered. This situation is encountered, for example, in the case of the Earth observation instrument 3MI, whose stray light properties are complex due to the presence of many ghosts distributed on the detector array. We implement an iterative correction method and discuss its convergence properties. Spatial and field binning can be employed to reduce the computation time but at the cost of a decreased performance. Interpolation of the stray light properties is required to achieve high performance correction. For that, two methods are proposed and tested. The first interpolate the stray light in the field domain while the second applies a scaling operation based on a local symmetry assumption. Ultimately, the scaling method is selected and a stray light reduction by a factor of 58 is obtained at 2σ (129 at 1σ) for an extended scene illumination.

Stray light analysis by ray tracing simulation for the wide-angle multiband camera OROCHI onboard the Martian Moons eXploration (MMX) spacecraft

• Ray tracing analysis was conducted to reduce stray light of OROCHI onboard the MMX. • OROCHI’s optical system is simulated and its CCD reflectance was measured. • Our techniques are incorporated into the preliminary design of OROCHI. The Martian Moons eXploration (MMX) spacecraft is equipped with two cameras, i.e., the TElescopic Nadir imager for GeOmOrphology (TENGOO) and the Optical Radiometer composed Of CHromatic Imagers (OROCHI), for the scientific observation of the Martian moon Phobos. OROCHI is a wide-angle multiband camera system comprising seven cameras with different bandpass filters and one monochromatic camera in the visible and near-infrared range. Thus, OROCHI can simultaneously obtain multiband images. Previous space observations have revealed that the reflection spectra of Phobos exhibit regional differences, including a red unit and a blue unit with weak redness. Moreover, Phobos shows an absorption band near 650 nm, attributed to the red unit of Phobos. Global observations using OROCHI require the determination of such regional variations with high precision; therefore, we must reduce noise in the optical system design. In this study, to meet this requirement, we focus on stray light reduction. Stray light is a source of noise generated by reflections at surfaces of optical components (e.g., sensors, filters, lenses, and lens barrels). We design simulation models of the OROCHI optical system, measure the reflectance of its charge-coupled device (CCD) image sensor, and conduct stray light analysis via ray tracing. We estimate the ratio of the stray light intensity to the target signal intensity by deriving both the reflected (constituting stray light) and unreflected rays. We show that placing the filter away from the sensor and reducing the reflectances of the filter and lenses effectively reduce stray light. Such approaches are incorporated into the preliminary design of OROCHI.

Improved temporal contrast of streak camera measurements with periodic shadowing.

Periodic shadowing, a concept used in spectroscopy for stray light reduction, has been implemented to improve the temporal contrast of streak camera imaging. The capabilities of this technique are first proven by imaging elastically scattered picosecond laser pulses and are further applied to fluorescence lifetime imaging, where more accurate descriptions of fluorescence decay curves were observed. This all-optical approach can be adapted to various streak camera imaging systems, resulting in a robust technique to minimize space-charge induced temporal dispersion in streak cameras while maintaining temporal coverage and spatial information.

Nanoimprintable super antireflective matte black surface achieved by hierarchical micro and nano architecture

We report an effective antireflective surface structure fabricated by a sequential process comprising colloidal lithography, maskless plasma etching, and inverted nanoimprinting replication. The hierarchical inverse micro–nano structure is composed of randomly positioned microholes of 3–5 μm in diameter and numerous nanoprotrusions of 60–80 nm diameter located at the bottom surface of the microholes. The inverse micro–nano structure behaves as a high-performance light absorber, exhibiting outstanding optical performances of 0.78% for hemispherical reflectance, and 0.011% for specular reflectance at the incident angle 5° off normal, both on average in the visible range (380–780 nm). Additionally, the optical behavior of the inverse micro–nano structure is numerically investigated with the electric field strength in both frequency and time domains by the finite-element method for Maxwell's equation. As the inverse micro–nano structure exhibits significantly low reflectance and black appearance, it can be applied to antiglare/antireflective surfaces, suitable for the reduction of stray light existing inside of a housing of optical instruments.

Fundus image quality assessment for an eye model with cataract using adapted illumination patterns

PurposeIn recent years, different methods have been developed to improve fundus image quality in eyes with cataract. For stray light reduction, we have introduced a segmented LED light source. To analyze the effect of different illumination patterns at different cataract characteristics, we developed an optical cataract eye model. Accordingly, we performed fundus imaging using a fundus camera equipped with a LED light source controlled by a Raspberry Pi.MethodsA customized 24‐fold segmented LED light source was connected to a fundus camera. The eye model consists of a plano‐convex lens, a cover glass, and a test chart with five black‐white test fields. The pupil diameter was set to 7 mm. To test the influence of variable illumination patterns, four different cortical cataract patterns were produced by sand blasting the cover glasses. Three spoke‐like (1Spoke, 4S, 7S) patterns and one crescent‐shaped/CS pattern were realized. To analyze the effect of adapted illumination conditions, the gray‐level co‐occurrence matrix (GLCM) contrasts were calculated. At that, one image was taken at standard annular illumination (all LEDs on) as baseline followed by seven defined adapted configurations, for each type of cataract.ResultsFor the four types of cataracts, the baseline GLCM contrast values were 0.21 (1S), 0.18 (4S), 0.17 (7S), and 0.22 (CS). In each cataract type, the best‐fitting illumination pattern resulted in 0.23 (1S), 0.29 (4S), 0.22 (7S), and 0.23 (CS). Compared to the baseline values, the increases of the GLCM contrasts were 9.5% (1S), 61% (4S), 29% (7S), and 4.5% (CS).ConclusionsWe successfully performed structured LED illumination for different cortical cataract types using an optical eye model. An improvement of fundus image quality could be determined in all investigated cases using the gray‐level co‐occurrence matrix contrast.

Design for stray-light reduction to a Martian ionospheric imager

We demonstrate a new design high-contrast optics suitable for use on a stray-light reduction. Two-dimensional distribution of diffracted light was numerically analyzed and confirmed by a laboratory measurement to determine its characteristics. Laboratory measurements confirm stray-light performance with ∼10-8 rejection viewing as close as 1.0° to a bright-light source. The present improvement replaces the conventional normal-edge vane by an edge with microscopic Gaussian-shaped structures. This permits simplified fabrication, provides even more weight reduction, and delivers improved performance for a given size.

Wide-Angle Spectrally Selective Absorbers and Thermal Emitters Based on Inverse Opals

Engineered optical absorbers are of substantial interest for applications ranging from stray light reduction to energy conversion. We demonstrate a large-area (centimeter-scale) metamaterial that f...

Fabrication of absorbing Nb-Ti suboxide anti-reflective thin film stacks

Optically black anti-reflective coatings that absorb light are potentially useful for color isolation in the flat panel display industries and stray light reduction in imaging technologies. Here we report the design and fabrication of high performance multilayered black anti-reflective coatings. Thin film suboxides NbxTi1−xO2, NbO2 and TiO2−x were prepared using pulsed laser deposition (PLD). Their optical constants were then used to design a six layer coating which has less than 2% reflectivity across the visible range (400–700 nm) even at moderate incidence angles. Furthermore, this six layer coating displays large and constant absorbance across the visible range making it optically black. This optical behavior is due largely to the use of the suboxide NbxTi1−xO2 which is unique because of its spectrally constant absorption coefficient.

Measuring scattering light distributions on high-absorptive surfaces for stray-light reduction in gravitational-wave detectors.

In order to establish scattering measurements in material investigations for gravitational-wave detectors, we have built-up devices for measuring the hemispherical scattering distribution of materials which are planned to be used in those detectors as suppressors of scattered light. The measurement benches we have built, a hemispherical goniometer and a direct back-scatterometer, have a maximum background noise of ∼10-4sr-1 BRDF at 1.064 μm wavelength which is the wavelength of the laser-light for our large interferometer for detecting gravitational waves, KAGRA. With these instruments, we have characterized the surface scattering of, e.g., NiP platings, metals, and different carbonaceous coatings, which are supposed to minimize the amount of scattered light in interferometers. The three most important materials for KAGRA's construction (SiC, "Solblack", and "VantaBlack") are presented in this paper. Furthermore, we will try to explain the scattering distributions with the generalized Harvey-Shack model (smooth-surface approximation) which is a common method for surface-scattering calculations. At the end, we give also some valuations about the vacuum compatibility of the materials, which is important for instruments like KAGRA that work under ultra-high vacuum conditions.

New light trap design for stray light reduction for a polarized scanning nephelometer.

Light scattering is an important tool for gathering information about the structure and origin of atmospheric aerosols. We build a polarized scanning nephelometer to measure the properties of aerosol particles. However, the accuracy of the backward-scattered light measurements is limited by stray forward-scattered light reflected back into the collection optics. We briefly analyze this stray light. A new form of light trap with multiple hollow cones is introduced to suppress backward-scattered stray light. To evaluate the effect of the light trap on suppressing stray light for our nephelometer, a simulation model with and without the light trap was analyzed. Our results show that without the light trap, the percentage of backward-scattered stray light can be more than 50% for some kinds of particles. With the light trap with multiple hollow cones, the percentage of stray light with a backward-scattered angle can be less than 0.7%, which remains stable over different angles. Our results indicate that this structure could be particularly suitable for a light trap with a very large aperture but limited space.

Global spectral irradiance array spectroradiometer validation according to WMO

Solar spectral irradiance measured by two recently developed array spectroradiometers (called UV-BTS and VIS-BTS) are compared to the results of a scanning double monochromator system which is certified as a travelling reference instrument by the Network for the detection of atmospheric composition change (NDACC) and fulfils the specifications of S-2 UV instruments of the world meteorological organization (WMO). The comparison took place between 15 and 18 May 2017 at the Institute of Meteorology and Climatology of the University of Hanover (IMuK) between 4:00 and 17:00UTC. The UV-BTS array spectroradiometer is equipped with special hardware to significantly reduce internal stray light which has been the limiting factor of many array spectroradiometers in the past. It covers a wavelength range of 200 nm–430 nm. The VIS-BTS covers a wider spectral range from 280 nm up to 1050 nm, and stray light reduction is achieved by mathematical methods. For the evaluation, wavelength integrated quantities and spectral global irradiance are compared. The deviation for UV index measured by the UV-BTS, is within ±1% for solar zenith angles (SZA) below 70° and increased to a maximum of ±3% for SZA between 70° and 85° when synchronisation between measurements was possible. The deviation of global spectral irradiance is smaller ±2.5% in the spectral range from 300 nm to 420 nm (evaluated for SZA < 70°). The VIS-BTS achieved the same deviation for blue light hazard as the UV-BTS for the UV index. The evaluations of global spectral irradiance data of the VIS-BTS show a deviation smaller than ±2% in the spectral range from 365 nm to 900 nm (evaluated for SZA < 70°). Below 365 nm, the deviation rises up to ±7% at 305 nm due to remaining stray light. The agreement within the limited time of the intercomparison is considered to be satisfactory for a number of applications and provides a good basis for further investigations.

High resolution Thomson scattering system for steady-state linear plasma sources.

The high resolution Thomson scattering system with 63 points along a 25 mm line measures the radial electron temperature (Te) and its density (ne) in an argon plasma. By using a DC arc source with lanthanum hexaboride (LaB6) electrode, plasmas with electron temperature of over 5 eV and densities of 1.5 × 1019 m-3 have been measured. The system uses a frequency doubled (532 nm) Nd:YAG laser with 0.25 J/pulse at 20 Hz. The scattered light is collected and sent to a triple-grating spectrometer via optical-fibers, where images are recorded by an intensified charge coupled device (ICCD) camera. Although excellent in stray-light reduction, a disadvantage comes with its relatively low optical transmission and in sampling a tiny scattering volume. Thus requires accumulating multitude of images. In order to improve photon statistics, pixel binning in the ICCD camera as well as enlarging the intermediate slit-width inside the triple-grating spectrometer has been exploited. In addition, the ICCD camera capture images at 40 Hz while the laser is at 20 Hz. This operation mode allows us to alternate between background and scattering shot images. By image subtraction, influences from the plasma background are effectively taken out. Maximum likelihood estimation that uses a parameter sweep finds best fitting parameters Te and ne with the incoherent scattering spectrum.

Black silicon integrated aperture

This paper describes the incorporation of nanotextured black silicon as an optical absorbing material into silicon-based micro-optoelectromechanical systems devices to reduce stray light and increase optical contrast during imaging. Black silicon is created through a maskless dry etch process and characterized for two different etch conditions, a cold etch performed at 0°C and a cryogenic etch performed at − 110 ° C . We measure specular reflection at visible wavelengths to be 0.001 % at near-normal incidence for both processes, whereas the total diffuse scatter is 3 % and 1% for the cold and cryogenic processes, respectively. These surfaces exhibit less reflectivity and lower scatter than black velvet paint used to coat optical baffles and compare favorably with other methods to produce black surfaces from nanotextured silicon or using carbon nanotubes. We illustrate the use of this material by integrating a black silicon aperture around the perimeter of a deformable focus-control mirror. Imaging results show a significant improvement in contrast and image fidelity due to the effective reduction in stray light achieved with the self-aligned black aperture.

Low‐Temperature Thermal CVD of Superblack Carbon Nanotube Coatings

The deposition of carbon nanotube (CNT) coatings via thermal chemical vapor deposition (CVD) is intensively reported. The surface acidity, chemical nature of the catalytic nanoparticles, and the carbon precursor are highly inter‐related key parameters. Furthermore, reducing the typical high‐growth temperature requires the implementation of toxic and hazardous organic precursors. In this study, the growth of CNT coatings is demonstrated using a single‐step CVD process in which magnesium oxides, material with enhanced basicity, and nanoparticles of cobalt are codeposited. This deposit catalyzes simultaneously the decomposition of ethanol to spark the growth of CNTs. The deposition is successively performed at 330–500 °C. Grown CNTs below 400 °C feature a high defect concentration and large diameters, 20 nm, relative to those obtained at ≥400 °C with no apparent defects and diameter of 12 nm. In terms of optical properties, films grown at ≥400 °C reflect less than 0.5% of light in the UV–vis–near IR, and exhibit a Lambertian behavior. Furthermore, the bidirectional reflectance distribution function measurements reveal identical optical properties irrespective of the underlying substrate. Therefore, the process holds a great potential for applications involving stray light reduction.

A new dump system design for stray light reduction of Thomson scattering diagnostic system on EAST

Thomson scattering (TS) diagnostic is an important diagnostic for measuring electron temperature and density during plasma discharge. However, the measurement of Thomson scattering signal is disturbed by the stray light easily. The stray light sources in the Experimental Advanced Superconducting Tokamak (EAST) TS diagnostic system were analyzed by a simulation model of the diagnostic system, and simulation results show that the dump system is the primary stray light source. Based on the optics theory and the simulation analysis, a novel dump system including an improved beam trap was proposed and installed. The measurement results indicate that the new dump system can reduce more than 60% of the stray light for the diagnostic system, and the influence of stray light on the error of measured density decreases.