Previously we introduced a novel fin shape light-emitting diode (LED) architecture based on sub-micron n-ZnO/p-GaN heterojunctions that at high current densities demonstrated a droop-free behavior. Efficiency droop, which is the decline in internal quantum efficiency with increasing current density, is one of the significant challenges facing wide bandgap LEDs. In the present work, we discuss performance of fins of wide band gap materials such as AlGaN that are formed on GaN. Discussion also includes electroluminescence and different types of efficiencies of these LEDs for generation of UV wavelengths as well as the significance of the fin shape in improving their performance.
Germicidal irradiation with a dose of UV-C light is an effective method for disinfecting surfaces, water, and air. New commercial devices are abandoning mercury gas-arc lamps in favor of UV-C LEDs, but UV-C LEDs are less efficient. This requires optical designs that use lenses to deliver UV-C light with high irradiance values to the target. This paper presents metallic optical reflectors (MORs), made with stamping processes, as a new lens for UV-C LEDs. Designs are described, tested, and validated for applications in SMD packages and chip-on- board (COB) modules. Aspherical mirror surface in MORs narrow the beam and achieve viewing angles as small as 15° FWHM with high efficiency. Simulations and tests conducted with a gonio-spectroradiometer agree sufficiently well so that the optical model can be used to design germicidal irradiation systems for surfaces or air in upper-room applications.
We introduce a novel fin LED architecture that eliminates “efficiency droop”. Efficiency droop, which is the decline in internal quantum efficiency with increasing current density, is one of the significant challenges facing wide bandgap LED. In this presentation, we present data on how nanofins as quasi one-dimensional nanoLEDs would operate at low and high injection current densities and how electron-hole recombination is influenced because of the shape and size of the recombination region. Results include temperature-dependent electroluminescence of fin LEDs as well as dependence of their total radiant flux (optical output power) on injected current. We show unique examples of droop-free operation of fin LEDs at extremely high injection current densities beyond 1 million A/cm2. A three-parameter model based on the radiative and nonradiative processes (ABC) including the SRH and Auger recombination is presented that provides insight on how the non-radiative Auger recombination proces
New commercial-off-the-shelf imaging spectrometers promise the combination of high spatial and spectral resolution needed to retrieve solar induced fluorescence (SIF). Imaging at multiple wavelengths for individual plants and even individual leaves from low-altitude airborne or ground-based platforms has applications in agriculture and carbon-cycle science. Data from these instruments could provide insight into the status of the photosynthetic apparatus at scales of space and time not observable with tools based on gas exchange, and could support the calibration and validation activities of current and forthcoming space missions to quantify SIF. High-spectral resolution enables SIF retrieval from regions of strong telluric absorption by molecular oxygen, and also within numerous solar Fraunhofer lines in atmospheric windows not obscured by oxygen or water absorptions. Because the SIF signal can be < 5 % of background reflectance, rigorous instrument characterization and reduction of systematic error is necessary. Here we develop a spectral stray-light correction algorithm for a commercial off-the-shelf imaging spectrometer designed to quantify SIF. We use measurements from an optical parametric oscillator laser at 44 wavelengths to generate the spectral line-spread function and develop a spectral stray-light correction matrix using a novel exposure-bracketing method. The magnitude of spectral stray light in this instrument is small, but spectral stray light is detectable at all measured wavelengths. Examination of corrected line-spread functions indicates that the correction algorithm reduced spectral stray-light by 1 to 2 orders of magnitude.
High power LEDs (HP-LEDs) are key building blocks of solid-state lighting products, therefore, it is important for LED manufacturers, lamp/luminaire manufactures, and testing/calibration laboratories to measure their optical and electrical properties with high accuracy. Measuring HP-LEDs has been difficult because they are highly sensitive to their junction temperatures, which rise rapidly when they are turned on. Various methods have been proposed and used to measure HP-LEDs, but most of them are only useful for particular applications and unable to produce accurate and reproducible measurement results. To address the measurement need, the Illuminating Engineering Society (IES) recently approved three methods that can be used for the measurement of HP-LEDs, which are the DC method, single-pulse method, and continuous-pulse method [1]. All three measurement methods refer to the junction temperature of an HP-LED as the thermal condition and thus, the measured results are considered to be equivalent as long as the junction temperature is set to be the same. However, our recent study shows that the difference in the measurement results of the three different methods can be significant (e.g., 5 % in total luminous flux) due to significant heating of the junction and/or phosphor material of the HP-LED during the period of a measurement. In this paper, we will describe the measurement of HP-LEDs using the three different methods, compare the measurement results, and discuss the cause that results in the significant difference.
[1] Illuminating Engineering Society, “IES LM-85-14: Approved Method: Electrical and Photometric Measurements of High-Power LEDs.” (2014)
There is a growing interest in the science and user community in the Visible Infrared Imaging Radiometer Suite (VIIRS) Day/Night Band (DNB) low light detection capabilities at night for quantitative applications such as airglow, geophysical retrievals under lunar illumination, light power estimation, search and rescue, energy use, urban expansion and other human activities. Given the growing interest in the use of the DNB data, a pressing need arises for improving the calibration stability and absolute accuracy of the DNB at low radiances. Currently the low light calibration accuracy was estimated at a moderate 15%-100% while the long-term stability has yet to be characterized. This study investigates selected existing night light point sources from Suomi NPP DNB observations and evaluates the feasibility of SI traceable nightlight source at radiance levels near 3 nW·cm−2·sr−1, that potentially can be installed at selected sites for VIIRS DNB calibration/validation. The illumination geometry, surrounding environment, as well as atmospheric effects are also discussed. The uncertainties of the ground based light source are estimated. This study will contribute to the understanding of how the Earth’s atmosphere and surface variability contribute to the stability of the DNB measured radiances, and how to separate them from instrument calibration stability. It presents the need for SI traceable active light sources to monitor the calibration stability, radiometric and geolocation accuracy, and point spread functions of the DNB. Finally, it is also hoped to address whether or not active light sources can be used for detecting environmental changes, such as aerosols.
We describe the characterization of a group of NIST spectral irradiance lamps at longer distances and larger angles than are typically issued by NIST. The spectral irradiances from the FEL lamps were measured from 50 cm to 150 cm at 8 different distances using a cosine-corrected filter radiometer to determine if the lamps adhere to the inverse square law. Using the filter radiometer, the spatial uniformities of the FEL lamps were also mapped over a 20 cm square area at 135 cm, 143 cm and 151 cm. In the NIST gonio-spectroradiometer facility, selected lamps were also mapped for the angular dependences of the spectral irradiances at a distance of 123 cm using a spectrograph which measures from 300 nm to 1100 nm for comparisons to the filter radiometer measurements. Using these measurements, an uncertainty budget for the distance and the angular uniformity correction of the FEL lamps was developed.
The relative spectral radiant flux error caused by phosphor fluorescence during integrating sphere measurements is
investigated both theoretically and experimentally. Integrating sphere and goniophotometer measurements are compared
and used for model validation, while a case study provides additional clarification. Criteria for reducing fluorescence
errors to a degree of negligibility as well as a fluorescence error correction method based on simple matrix algebra are
presented. Only remote phosphor type LED light sources are studied because of their large phosphor surfaces and high
application potential in general lighting.
Alternating-current (AC) driven high-power light-emitting diodes (LEDs) have become available and introduced into
solid-state lighting (SSL) products. AC LEDs operate directly from a mains supply with no need of drivers, and thus can
simplify the design of SSL product and potentially increase product's reliability and lifetime. Similar to direct-current
(DC) LEDs the optical and electrical properties of AC LEDs are strongly dependent on the LED junction temperature. In
addition, the instantaneous junction temperature of an AC LED changes rapidly within an AC power cycle. Accurate
measurement of AC high-power LEDs is required for quality control and product qualifications such as the US Energy
Star. We have developed a simple, robust method for measurement of high-power AC LEDs at any specified junction
temperature under a normal AC operating condition. An active heat sink is used for setting and controlling the junction
temperature of the test AC LED. By using this measurement technique, the measurement of an AC LED also obtains the
thermal resistance between the LED junction and the LED heat sink.
The Moderate Resolution Imaging Spectroradiometer (MODIS) on the Earth Observing System (EOS) Aqua
platform has 9 spectral bands with center wavelengths from 412nm to 870nm that are used to produce the
standard ocean color data products. Ocean scenes usually contain high contrast due to the presence of bright
clouds over dark water. The MODIS has been characterized for straylight effects prelaunch. In this paper, we
derive Point-Spread Functions for the MODIS Aqua ocean bands based on the prelaunch Near-Field Response
measurements. We use Harvey-Shack coefficients derived by the system vendor Santa Barbara Remote Sensing.
The crucial step in the derivation of the Point-Spread Function is the normalization of the Harvey-Shack coefficients relative to the center pixel. The straylight contamination of ocean scenes is evaluated based on artificial
test scenes. Furthermore, the dependence of top-of-atmosphere radiances and ocean color products on proximity
to a cloud is analyzed, and a straylight correction algorithm is proposed.
Improperly imaged, or scattered, optical radiation within an instrument is difficult to properly characterize and is often
the dominant residual source of measurement error. Scattered light can originate from the spectral components of a
"point" source and from spatial elements of an extended source. The spectral and spatial scattered light components are
commonly referred to as stray light and can be described by an instrument's spectral line spread function (SLSF) and
point spread function (PSF), respectively. In this paper, we present approaches that characterize an instrument's
response to scattered light and describe matrices that have been developed to correct an instrument's response for this
scattered light. Examples are given to demonstrate the efficacy of the approach and implications for remote sensing
instruments are discussed.
Portable hyperspectral imagers are becoming commonly available as a commercial product. A liquid crystal tunable
filter based CCD imager was evaluated and characterized for spectral stray light using a tunable laser facility. The
hyperspectral imager is currently being used to investigate the use of hyperspectral imaging in medical applications.
This paper discusses the imager design and performance, the characterization of this system, and medical imaging as a
related application. Imagers of this type may be fundamental to transferring radiometric scales using a hyperspectral
image projector. The use of the hyperspectral imager for use with a hyperspectral image projector is also discussed.
Spectrographs are used in a variety of applications in the field of remote sensing for radiometric measurements due to the benefits of measurement speed, sensitivity, and portability. However, spectrographs are single grating instruments that are susceptible to systematic errors arising from stray radiation within the instrument. In the application of measurements of ocean color, stray light of the spectrographs has led to significant measurement errors. In this work, a simple method to correct stray-light errors in a spectrograph is described. By measuring a set of monochromatic laser sources that cover the instrument's spectral range, the instrument's stray-light property is characterized and a stray-light correction matrix is derived. The matrix is then used to correct the stray-light error in measured raw signals by a simple matrix multiplication, which is fast enough to be implemented in the spectrograph's firmware or software to perform real-time corrections: an important feature for remote sensing applications. The results of corrections on real instruments demonstrated that the stray-light errors were reduced by one to two orders of magnitude, to a level of approximately 10-5 for a broadband source measurement, which is a level less than one count of a 15-bit resolution instrument. As a stray-light correction example, the errors in measurement of solar spectral irradiance using a highquality spectrograph optimized for UV measurements are analyzed; the stray-light correction leads to reduction of errors from a 10 % level to a 1 % level in the UV region. This method is expected to contribute to achieving a 0.1 % level of uncertainty required for future remote-sensing applications.
Various new light-emitting diodes (LEDs) including white LEDs are being actively developed for solid-state lighting and many other applications, and there are great needs for accurate measurement of various optical quantities of LEDs. Traditional lamp standards do not suffice for specific measurement needs for LEDs. The National Institute of Standards and Technology (NIST) has recently established calibration services for photometric quantities (luminous intensity and luminous flux) of LEDs, but the measurement needs are expanding. This paper covers the current capabilities and services NIST provides for calibration of LEDs and discusses the future needs for optical metrology of LEDs. Work is just completed at NIST to provide official color calibrations of LEDs (chromaticity coordinates, peak wavelength, correlated color temperature, etc.). Another urgent need addressed is radiometric calibration of LEDs, particularly the total radiant flux (watt) of ultraviolet (UV) LEDs used to excite phosphors for white LEDs. Also, as spectroradiometers coupled with an integrating sphere are increasingly used total spectral radiant flux standards from NIST are in urgent demand. Presented is the scope of NIST plans to realize these new radiometric calibration capabilities for LEDs in the near future.
There is a need for accurate measurement of flashing lights for the proper maintenance of aircraft anticollision lights. A large variation in the measured intensities of anticollision lights has been a problem, and thus, NIST has undertaken the task to establish flashing-light photometric standards to provide calibration services in this area. A flashing-light photometric unit [lux second, (lx (DOT) s)] has been realized based on the NIST detector-based candela, using four standard photometers equipped with current integrators. Two different approaches have been taken to calibrate these standard photometers: one based on electrical calibration of the current integrator, and the other based on electronic pulsing of a steady-state photometric standard. The units realized using these two independent methods agreed to within 0.2%. The relative expanded uncertainty (k equals 2) of the standard photometers, in the measurement of the white xenon flash, is estimated to be 0.6%. The standard photometers are characterized for temporal response, linearity, and spectral responsivity, to be used for measurement of xenon flash sources of various waveforms and colors. Calibration services have been established at NIST for flashing-light photometers with white and red anticollision lights.
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