Yuqin Zong

Simple Spectral Stray Light Correction Method for Array Spectroradiometers

A simple, practical method has been developed to correct a spectroradiometers response for measurement errors arising from the instruments spectral stray light. By characterizing the instruments response to a set of monochromatic laser sources that cover the instruments spectral range, one obtains a spectral stray light signal distribution matrix that quantifies the magnitude of the spectral stray light signal within the instrument. By use of these data, a spectral stray light correction matrix is derived and the instruments response can be corrected with a simple matrix multiplication. The method has been implemented and validated with a commercial CCD-array spectrograph. Spectral stray light errors after the correction was applied were reduced by 1-2 orders of magnitude to a level of approximately 10(-5) for a broadband source measurement, equivalent to less than one count of the 15-bit-resolution instrument. This method is fast enough to be integrated into an instruments software to perform real-time corrections with minimal effect on acquisition speed. Using instruments that have been corrected for spectral stray light, we expect significant reductions in overall measurement uncertainties in many applications in which spectrometers are commonly used, including radiometry, colorimetry, photometry, and biotechnology.

Stray light correction algorithm for multichannel hyperspectral spectrographs

An algorithm is presented that corrects a multichannel fiber-coupled spectrograph for stray or scattered light within the system. The efficacy of the algorithm is evaluated based on a series of validation measurements of sources with different spectral distributions. This is the first application of a scattered-light correction algorithm to a multichannel hyperspectral spectrograph. The algorithm, based on characterization measurements using a tunable laser system, can be extended to correct for finite point-spread response in imaging systems.

Practical method for measurement of AC-driven LEDs at a given junction temperature by using active heat sinks

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 products 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.

Derivation of the MODIS Aqua Point-Spread Function for Ocean Color Bands

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.

The distance dependences and spatial uniformities of spectral irradiance standard lamps

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.

A new method for spectral irradiance and radiance responsivity calibrations using kilohertz pulsed tunable optical parametric oscillators

Continuous-wave (CW) tunable lasers have been used for detector calibrations, especially for spectral irradiance and radiance responsivity, for many years at the National Institute of Standards and Technology (NIST) and other national metrology institutes. These CW tunable lasers, however, are expensive and difficult to automate. To address these issues, we developed a new method for spectral irradiance and radiance responsivity calibrations using relatively low cost, fully automated kilohertz pulsed tunable optical parametric oscillators (OPOs). The new method is based on measurements of the total energy of a pulsed OPO train using two synchronized current integrators (also called charge amplifiers) to measure the total integrated electric charges from a test detector and a standard detector, respectively. The absolute expanded uncertainty of this method is estimated to be 0.05% (with a coverage factor of k = 2) for spectral irradiance responsivity calibrations, and the dominant uncertainty contribution is from the reference trap detector.

Establishment of the NIST flashing-light photometric unit

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.

Evaluation of a portable hyperspectral imager for medical imaging applications

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.

Correction of stray light in spectrographs: implications for remote sensing

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 instruments spectral range, the instruments 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 spectrographs 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.

Fluorescence errors in integrating sphere measurements of remote phosphor type LED light sources

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.