Optical Degradation Research Articles

Mitigation of Electrical Failure of Silver Nanowires under Current Flow and the Application for Long Lifetime Organic Light‐Emitting Diodes

The extended lifetime of organic light‐emitting diodes (OLEDs) based on enhanced electrical stability of a silver nanowire (AgNW) transparent conductive electrode is reported. Specifically, in depth investigation is performed on the ability of atomic layer deposition deposited zinc oxide (ZnO) on AgNWs to render the nanowires electrically stable during electrical stressing at the range of operational current density used for OLED lighting. ZnO‐coated AgNWs have been observed to show no electrical, optical, or morphological degradation, while pristine AgNW electrodes have become unusable for optoelectronic devices due to dramatic decreases in conductivity, transparency, and fragmentation of the nanowire network at ≈150 mA cm−2. When fabricated into OLED substrates, resulting OLEDs fabricated on the ZnO‐AgNW platform exhibit a 140% increase in lifetime when compared to OLEDs fabricated on indium tin oxide (ITO)/glass, and ≈20% when compared to OLEDs fabricated on AgNW based substrates. While both ZnO‐coated and pristine AgNW substrates outperform ITO/glass due to the lower current densities required to drive the device, morphological stability in response to current stressing is responsible for the enhancement of lifetime of ZnO‐AgNW based OLEDs compared to pristine AgNW based OLEDs.

Laser Scanning-Assisted Tip-Enhanced Optical Microscopy for Robust Optical Nanospectroscopy.

Laser-scanning-assisted tip-enhanced optical microscopy was developed for robust optical nanospectroscopy. The laser-scanning system was utilized to automatically set and keep the center of a tight laser-focusing spot in the proximity of a metallic tip with around 10 nm precision. This enabled us to efficiently and stably induce plasmon-coupled field enhancement at the apex of the metallic probe tip. The laser-scanning technique was also applied to tracking and compensating the thermal drift of the metallic tip in the spot. This technique is usable for long-term tip-enhanced optical spectroscopy without any optical degradation.

Degradation of receiver tube optical performance after four years of operation

The performance of parabolic trough receiver tubes has a direct impact on concentrated solar power plant production and thus must be maintained at as high a level as possible during the life cycle of the plant. In this paper, in-the-field degradation of commercial plant receiver transmittance and absorptance is studied after four years of operation. Since no records of similar measurements were available regarding the reception of the receivers, the results are compared with both the manufacturer’s technical specifications and with the guaranteed values. For this purpose, the Mini Incus portable spectrophotometer developed by Abengoa is used to measure the optical properties of an 80-receiver sample. This sample is built up considering the two main factors most likely involved in transmittance and absorptance degradation: interactions with external agents and operating temperature. Therefore, receivers from different locations both in the field and within loops are chosen. A statistical study is performed on the data collected in order to obtain representative values for the whole population of evaluated receivers as well as for every group differentiated. The results reveal no significant degradation of the absorptance or transmittance of the receivers, with measured average transmittance and absorptance values of 96.6±0.2% and 95.9±0.2%, respectively, higher than the 96% and 95% indicated in the technical specification for new first generation receivers. Receiver location in the field and position within the loop does not influence optical property degradation, since the appreciated differences fall within the uncertainty of the measurement device. The obtained results are then compared with the technical specification values and with those guaranteed by the manufacturer after 4years of operation, and the suitability of the guarantee in matching the reality experienced by the receivers in the field is discussed.

Effects of compressibility and Knudsen number on the aero optics in hypersonic flow fields

Tsien summarized the similarity in hypersonic flows, and related Knudsen number to Mach number and Reynolds number. Recently, a path-based problem, aero-optical effect, arises in hypersonic flows, and it concerns about the compressibility and the Knudsen number of the gas flows, which differs from the Tsien’s focus to some extent. In this paper, the similarity of hypersonic aero optics is theoretically studied, and both flow fields and induced aero-optical effect after flows pass through a cylinder are predicted by a well-accepted particle-based method, direct simulation Monte-Carlo (DSMC) method. The results show that the optical distortions are inversely proportional to the Knudsen number, while the compressibility plays an important role in the optical degradations. Hence, it is confirmed that the effects of Mach number and Knudsen number on the aero-optical effect induced by hypersonic flows are of great significance. Besides, since the Knudsen number is defined straightforwardly based on the optically active region, the physics is clearer than any other similarity criteria.

An Image-Based Model for Early Visual Processing

Figure 2: Contrast discrimination. Increment threshold vs. pedestal contrast for gratigs of 8.37 cyc/deg and 1497ms presentation time. Early spatial vision was explored extensively over several decades in many psychophysical detection and discrimination experiments. Thus a large body of such data is available. Goris et al. (2013) integrated this psychophysical literature and proposed a model based on maximumlikelihood decoding of a neurophysiologically inspired population of model neurons. Their neural population model (NPM) is unique in its ability to predict several data sets simultaneously, using a single set of parameters. However, the NPM is only one-dimensional, operating on the activity of abstract spatial frequency channels. Thus it cannot be applied to arbitrary images as a generic front-end to explore the influence of early visual processing on midor high-level vision. Bradley et al. (2014), on the other hand, recently presented an early vision model operating on images. Their model is thus able to make predictions for arbitrary images. However, compared to the NPM, their model lacks in nonlinear processing, which is replaced by an effective masking contrast depending on the detection target. Thus while Bradley et al. fit a range of detection data they do not fit nonlinear aspects of early vision like the dipper function. Here we combine both approaches into a model which includes nonlinear processing and statistically efficient decoding and operates on images explicitly calculating channel activities over the image. The model applies optical degradation and retinal processing to the image before the image is passed to a spatial frequency and orientation decomposition followed by divisive inhibition. We tested the predictions of our model against a broad range of early psychophysical experiments and found it predicts some hallmarks of early visual processing like the contrast sensitivity function (Figure 1), the dipper function for contrast discrimination (Figure 2) and oblique masking data (Figure 3). The measured data we plot against stems from standard 2AFC contrast detection and discrimination experiments run for Wichmann’s DPhil thesis (1999). Early vision was extensively explored empirically and there are plenty of models which explain different parts of the empirical findings. These results provide detailed information about the first processing steps in visual perception thought to happen in the retina, LGN and V1, which stimuli can be differentiated or detected using their output and in what format the output is available to any higher visual processing. However, research on higher visual processing rarely employs the information available from early vision psychophysics. The main reasons may be that the models described in the early spatial vision literature typically do not predict behaviour on arbitrary images, because they often do not process images, but only predict the (putative) responses of the early visual system to Gabor patches or simple grating stimuli. We try to overcome these limitations by explaining many psychophysical observations within a model taking images as input.

Electronic and dynamic DFT studies on the substituent effects of aminoalcohol stabilizers in sol–gel ZnO precursor

Eight ethanolamine‐core‐like stabilizers of zinc acetate dihydrate (ZAD) are compared by means of density functional theory simulations (Gaussian optimizations and Car–Parrinello molecular dynamics). HOMO and LUMO are described and located, suggesting that photodegradation implies Zn reduction. Substituent effects on the thermal and optical degradation of the precursor formed by the aminoalcohols with ZAD are described. The Zn‐ligand systems are analyzed in detail, and molecular dynamics allows to establish the classification of the aminoalcohols according to the stability of the ZnN bond. Unless the amount of nitrogen is well controlled, nitrogen‐based additives should be avoided as ZAD stabilizers in the synthesis of pure ZnO.

Optical Degradation of Colloidal Eu-Complex Embedded in Silica Glass Film Using Reprecipitation and Sol-Gel Methods.

A reprecipitation method has been investigated for fabricating colloidal nanoparticles using Eu-complex. Herein, we investigated optical degradation characteristics of (1,10-phenanthroline)tris [4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionato]europium(III) colloidal nanoparticles, which were embedded into a silica glass film fabricated by a conventional sol-gel process. At first, we tried several types of good solvents for the reprecipitation method, and dimethyl sulfoxide (DMSO) is found to be a suitable solvent for realizing the small diameter and the high long-term stability against the ultraviolet irradiation even though the boing point of DMSO is higher than that of water used as a poor solvent. By optimizing the good solvent and the concentration of Eu-complex, the relative photoluminescence intensity of 0.96 was achieved even though the ultraviolet light was continuously irradiated for 90 min. In addition, the average diameter of 106 nm was achieved when DMSO was used as a good solvent, resulting in the high transmittance at a visible wavelength region. Therefore, we can achieve the transparent emissive thin film with a center wavelength of 612 nm, and the optical degradation was drastically reduced by forming nanoparticles.

Nanoscale Investigation of Degradation and Wavelength Fluctuations in InGaN-Based Green Laser Diodes

We present a study of the optical inhomogeneities and degradation of InGaN-based green laser diodes based on high-resolution cathodoluminescence (CL) investigation of the output facets and inner cross-section of the devices. The results indicate that 1) degradation originates from a diffusion process, which causes an increase in the threshold current. 2) Nanoscale-level CL points out the circular symmetry of the degraded area, which is wider than the ridge and includes not only the quantum wells but also the waveguiding and cladding layers. 3) The yellow luminescence decreases within the degraded region, whereas its intensity increases outside of the degraded region. 4) Wavelength fluctuations are found in both quantum wells and waveguides, which are critically analyzed and ascribed to inhomogeneity in indium concentration. Their possible effect in the filamentation of the laser emission is discussed.

Optical Degradation Mechanisms of Indium Gallium Nitride-Based White Light Emitting Diodes by High-Temperature Aging Tests

We have measured five major mechanisms that govern light-emitting diode (LED) degradation, calculated respective percentages, and identified the most influential mechanism during the time evolution of aging. After 480 h, due to the reduction of phosphor-conversion efficiency, the increase of non-radiative centers, the deterioration of thermal properties, the darkening of silicone resin lenses, and the degradation of Ag reflective layers, approximately 42% of the average optical power has decayed. Within this 42%, Ag degradation contributes approximately 28% (with the remaining 14% contributed by the degradation of silicone resin lenses), primarily because other chemical elements have deposited on the surface of Ag reflective layers, thus lowering the reflectivity of these layers. The present work can help draw the community's attention to lowering LED junction temperatures, and to the optimization of packaging designs.

Improved Reliability of InGaN-Based Light-Emitting Diodes by HfO2 Passivation Layer.

We utilized a passivation layer to improve the leakage current and reliability characteristics of GaN-based light-emitting diodes. The electrical and optical characteristics of the fabricated LEDs were characterized by current-voltage and optical power measurements. The HfO2 passivation layer showed no optical power degradation and suppressed leakage current. The low deposition temper- ature of sputtered HfO2 is responsible for the improved reliability of the LEDs because it suppresses the diffusion of hydrogen plasma into GaN to form harmful Mg-H complexes.

Assessment of S-NPP VIIRS On-orbit Radiometric Calibration and Performance.

The VIIRS instrument on board the S-NPP spacecraft has successfully operated for more than four years since its launch in October, 2011. Many VIIRS environmental data records (EDR) have been continuously generated from its sensor data records (SDR) with improved quality, enabling a wide range of applications in support of users in both the operational and research communities. This paper provides a brief review of sensor on-orbit calibration methodologies for both the reflective solar bands (RSB) and the thermal emissive bands (TEB) and an overall assessment of their on-orbit radiometric performance using measurements from instrument on-board calibrators (OBC) as well as regularly scheduled lunar observations. It describes and illustrates changes made and to be made for calibration and data quality improvements. Throughout the mission, all of the OBC have continued to operate and function normally, allowing critical calibration parameters used in the data production systems to be derived and updated. The temperatures of the on-board blackbody (BB) and the cold focal plane assemblies are controlled with excellent stability. Despite large optical throughput degradation discovered shortly after launch in several near and short-wave infrared spectral bands and strong wavelength dependent solar diffuser degradation, the VIIRS overall performance has continued to meet its design requirements. Also discussed in this paper are challenging issues identified and efforts to be made to further enhance the sensor calibration and characterization, thereby maintaining or improving data quality.

Demonstration of long-pulse acceleration of high power positive ion beam with JT-60 positive ion source in Japan–Korea joint experiment

The long-pulse acceleration of the high-power positive ion beam has been demonstrated with the JT-60 positive ion source in the joint experiment among Japan Atomic Energy Agency (JAEA), Korea Atomic Energy Research Institute (KAERI) and National Fusion Research Institute (NFRI) under the collaboration program for the development of plasma heating and current drive systems. In this joint experiment, the increase of the heat load and the breakdowns induced by the degradation of the beam optics due to the gas accumulation was one of the critical issues for the long-pulse acceleration. As a result of development of the long-pulse operation techniques of the ion source and facilities of the neutral beam test stand in KAERI, 2MW 100s beam has been achieved for the first time. The achieved beam performance satisfies the JT-60SA requirement which is designed to be a 1.94MW ion beam power from an ion source corresponding to total neutral beam power of 20MW with 24 ion sources. Therefore, it was found that the JT-60 positive ion sources were applicable in the JT-60SA neutral beam injectors. Moreover, because this ion source is planned to be a backup ion source for KSTAR, the operational region and characteristic has been clarified to apply to the KSTAR neutral beam injector.

Optical properties and radiation stability of submicro- and nanopowders titanium dioxide measured in situ

This study carried out an in situ and external investigation on the reflective spectra of micro- and nanopowders titanium dioxide before and after irradiation by 30 keV electrons. The particle sizes range from 60–240 nm. It was established that the decrease in the particle size leads to an increase in intrinsic defects. The particles with intrinsic defects are then transformed into absorption centers during irradiation as a result of optical degradation of TiO2 powders. High radiation stability has particle sizes range from 80–160 nm.

Influence of temperature-induced copper diffusion on degradation of selective chromium oxy-nitride solar absorber coatings

Temperature-induced copper diffusion process and its influences on optical degradation and long-term stability of solar absorber coatings on copper substrates were investigated at intermediate temperatures of 248–500°C. The studied absorbers were sputtered chromium oxy-nitride absorbers having tin oxide anti-reflection coatings. The absorbers were aged by means of thermal accelerated ageing studies and short-period heat treatments up to 500°C for two hours.Ageing mechanisms and degradation of the absorbers were analysed before and after the ageing studies by optical measurements (solar absorptance with a UV/Vis/NIR spectrophotometer and thermal emittance by FTIR spectrophotometry), microstructural analysis using a field-emission scanning electron microscope (FESEM) equipped with an energy dispersive X-ray spectrometer (EDS) and a transmission electron microscope (TEM) with an EDS, composition by time-of-flight elastic recoil detection analysis (TOF-ERDA) and an X-ray photoelectron spectroscope (XPS), and adhesion by tensile test. The relation between optical degradation and diffusion mechanisms was studied using optical modelling and simulation. The results clearly revealed the mechanism of outward copper diffusion: diffusion of copper substrate atoms into the coating and through the coating to the surface, formation of copper oxide islands on the surface of the coating, and formation of voids in the substrate surface. The relation between the diffusion mechanisms and increase in thermal emittance of the absorber surface was demonstrated.

Pitfalls in the Use of Stereoacuity in the Diagnosis of Nonorganic Visual Loss

The Titmus Stereotest (Stereo Optical Co., Inc., Chicago, IL) has been used to estimate visual acuity (VA) in the evaluation of nonorganic visual loss. Previous predictions were derived from optical degradation of VA in normal subjects and may not account for the variability seen in patients with neuro-ophthalmic pathologies included in the differential diagnosis of nonorganic visual loss. The purpose of this study was to evaluate the relationship between Titmus stereoacuity and minimal VA based on a real-world testing environment. Cross-sectional observational study. All patients treated at the authors' neuro-ophthalmology service between April 25, 2014, and July 31,2014. All subjects underwent routine neuro-ophthalmic examination, including Titmus stereoacuity measurements. A compound Bayesian logit-lognormal model accounting for heteroscedasticity was used to determine 95% and 99% prediction intervals of the worse eye's near VA based on stereoacuity. Logarithm of the minimum angle of resolution VA and log stereoacuity were analyzed. Titmus stereoacuity and worse eye VA. Of 561 patients, 364 subjects 11 to 91 years of age were included. Titmus stereoacuity was associated positively with VA: 9 circles correct (40 seconds of arc) indicated VA of at least 20/40 with 95% confidence and VA of at least 20/79 with 99% confidence; 6 circles correct (80 seconds of arc) indicated VA of at least 20/62 and 20/180, respectively; and 4 circles correct (140 seconds of arc) indicated VA of at least 20/110 and 20/570, respectively. When fully accounting for individual variation and the full spectrum of neuro-ophthalmic diseases affecting VA, stereoacuity remains associated with VA, but previous commonly used VA estimates based on stereoacuity overestimated VA. Our results more accurately predict minimum VA from Titmus stereoacuity and should be used preferentially when evaluating patients with suspected nonorganic visual loss. We demonstrated that Titmus stereoacuity cannot definitively establish normal VA, and therefore can suggest, but not fully establish, the diagnosis of nonorganic visual loss.

Optical degradation mechanisms of mid-power white-light LEDs in LM-80-08 tests

In this paper, the optical degradation mechanisms of mid-power white-light LEDs, were studied by using high temperature operation life tests, according to the IES standard LM-80-08. As a consequence, it is found that, (1) the degradation mechanisms are different for LED packages aged at different case temperatures; (2) the optical degradation is a result of combination of chip-related degradation mechanisms and package-related degradation mechanisms. In details, for LEDs aged at 55°C, the optical degradation was induced by both the deterioration of blue chip and silver-coating lead frames, while for LEDs aged at 85°C and 105°C, only blue chip deterioration was considered as the main degradation mechanism.

Vibration budget for observatory equipment

Vibration from equipment mounted on the telescope and in summit support buildings has been a source of performance degradation at existing astronomical observatories, particularly for adaptive optics performance. Rather than relying only on best practices to minimize vibration, we present here a vibration budget that specifies allowable force levels from each source of vibration in the observatory (e.g., pumps, chillers, cryocoolers, etc.). This design tool helps ensure that the total optical performance degradation due to vibration is less than the corresponding error budget allocation and is also useful in design trade-offs, specifying isolation requirements for equipment, and tightening or widening individual equipment vibration specifications as necessary. The vibration budget relies on model-based analysis of the optical consequences that result from forces applied at different locations and frequencies, including both image jitter and primary mirror segment motion. We develop this tool here for the Thirty Meter Telescope but hope that this approach will be broadly useful to other observatories, not only in the design phase, but for verification and operations as well.

Investigation on Some Parameters Affecting Optical Degradation of LED Packages During High-Temperature Aging

The mechanism of optical degradation for the light emitting diode (LED) packages in the light bars under the high-temperature aging condition, one of the accelerated life tests, is studied experimentally and numerically. Some parameters, including junction temperature, encapsulant materials, delamination, leadframe surface degradation, and encapsulation, are closely investigated. It is found that the encapsulants (with or without phosphors) and interface delamination have no effect on the optical degradation during the high-temperature aging. On the contrary, junction temperature, leadframe surface degradation, and encapsulation play important roles on this degradation. Optical degradation of the LED packages is found to be mainly caused by silver chloride, which is generated on the leadframe surfaces with an exposure of silver finish in light and heating conditions after interface delamination happening during the high-temperature aging. The high junction temperature could speed up the generation of silver chloride. Moreover, the blue LED package with an encapsulant (silicone) surprisingly give a lower lumen maintenance than one without encapsulant during the high-temperature aging, because of more light absorption on the darkening leadframe surface by total light reflection from silicone encapsulant. Some important parameters affecting the thermal reliability of the LED packages have been identified and the corresponding optical degradation mechanism has been clarified.

Solar observation of Ozone Mapping and Profiler Suite nadir system during the first 3 years of on-orbit operation

The Ozone Mapping and Profiler Suite (OMPS) on the National Oceanic and Atmospheric Administration Suomi National Polar-Orbiting Partnership spacecraft has completed three more years of orbital operation since the OMPS opened its nadir door for the scene data collection on January 26, 2012. The sensors’ spectral channels have been calibrated in-flight by a working solar diffuser and a reference solar diffuser. The instruments’ optical degradation is determined through the changes in the instruments’ throughput via orbital solar observations. The observed degradation at the shortest wavelengths is <1% for the sensor optics, and in excess of 2% for the OMPS working solar diffuser. The absolute irradiance calibration uncertainties meet the system requirement of 7% for most of the channels. Unexpected orbital wavelength variation in sensor scan direction is evidenced to cause about 2.6% error, not compliant with a 2% allocation and so some margin will be needed to accommodate the exceedance.

On-orbit calibration of Visible Infrared Imaging Radiometer Suite reflective solar bands and its challenges using a solar diffuser

The reflective solar bands (RSBs) of the Visible Infrared Imaging Radiometer Suite (VIIRS) on board the Suomi National Polar-Orbiting Partnership satellite are calibrated by a solar diffuser (SD) panel whose performance is itself monitored by an accompanying solar diffuser stability monitor (SDSM). In this comprehensive work we describe the SD-based calibration algorithm of the RSBs, analyze the calibration data, and derive the performance results-the RSB calibration coefficients or F-factors-for the current three and a half years of mission. The application of the newly derived product of the SD bidirectional reflectance factor and the vignetting function for the SD screen and the newly derived SD degradation, so-called H-factors, effectively minimizes the artificial seasonal patterns in the RSB calibration coefficients due to the errors of these ingredient inputs. The full illumination region, the "sweet spot," during calibration events for SD view is carefully examined and selected to ensure high data quality and to reduce noise owing to non-fully illuminated samples. A time-dependent relative spectral response (RSR), coming from the large out-of-band contribution and the VIIRS optical system wavelength-dependent degradation, is derived from an iterative approach and applied in the SD calibration for each RSB. The result shows that VIIRS RSBs degrade much faster at near-infrared (NIR) and shortwave-infrared (SWIR) wavelength ranges due to the faster degradation of the rotating telescope assembly against the remaining part of the system. The gains of the VIIRS RSBs have degraded 2.0% (410nm, Band M1), 0.2% (443nm, Band M2), -0.3% (486nm, Band M3), 0.2% (551nm, Band M4), 6.2% (640nm, Band I1), 11.0% (671nm, Band M5), 21.3% (745nm, Band M6), 35.8% (862nm, Band I2), and 35.8% (862nm, Band M7), respectively, since launch and 24.8% (1238nm, Band M8), 18.5% (1378nm, Band M9), 11.5% (1610nm, Band I3), 11.5% (1610, Band M10), and 4.0% (2250nm, Band M11), respectively, since 20 January 2012. It is established that the SD calibration accurately catches the on-orbit RSB degradation according to the instrument design and the calibration algorithm. However, due to the inherent nonuniform degradation of the SD affecting especially the short wavelength bands and the lack of capability of the SDSM calibration to catch degradation beyond 935nm, the direct and the unmitigated application of the SD calibration result will introduce nonnegligible error into the calibration coefficients resulting in long-term drifts in the sensor data records and consequently the high-level products. We explicitly unveil the effect of the nonuniformity in SD degradation in the RSB calibration coefficients but also briefly discuss a critical yet simple mitigation to restore the accuracy of the calibration coefficients based on lunar observations. The methodology presented here thus remains intact as the cornerstone of the RSB calibration, and our derived RSB calibration coefficients represent the optimal result. This work has the most impact on the quality of the ocean color products that sensitively depend on the moderate visible and NIR bands (M1-M7), as well as the SWIR bands (M8, M10, and M11).