Labsphere Inc.

The objective of the Ground to Space CALibration Experiment (G-SCALE) is to demonstrate the use of convex mirrors as a radiometric and spatial calibration and validation technology for Earth Observation assets, operating at multiple altitudes and spatial scales. Specifically, point sources with NIST-traceable absolute radiance signal are evaluated for simultaneous vicarious calibration of multi- and hyperspectral sensors in the VNIR/SWIR range, aboard Unmanned Aerial Vehicles (UAVs), manned aircraft, and satellite platforms. We introduce the experimental process, field site, instrumentation, and preliminary results of the G-SCALE, providing context for forthcoming papers that will detail the results of intercomparison between sensor technologies and remote sensing applications utilizing the mirror-based calibration approach, which is scalable across a wide range of pixel sizes with appropriate facilities. The experiment was carried out at the Rochester Institute of Technology’s Tait Preserve in Penfield, NY, USA on 23 July 2021. The G-SCALE represents a unique, international collaboration between commercial, academic, and government entities for the purpose of evaluating a novel method to improve vicarious calibration and validation for Earth Observation.

Many existing and emerging remote sensing applications in the UV, Visible, NIR, SWIR, MWIR and LWIR regions are challenging the conventional thinking of radiance and temperature calibration techniques. While the relationship between blackbody temperature and optical radiation is well understood, often there is an “invisible” dividing line between treatments of these values as either optical radiance or temperature. It is difficult to perform seamless temperature and radiance calibrations across the point of 2.5um. Spectrum above 2.5um is typically related in temperature terms and below 2.5um may be either spoken of in terms of temperature or optical radiance. There is also a natural unit “convergence” issue at 2.5um, due to the crossover of significant levels of emissivity, reflectance and temperature at this point. NMI traceability in the spectral region of 2.5-14.0um can also be a problem especially for spectral radiance. This paper will outline a possible turn-key test bench solution that provides traceable solutions for both temperature and radiance value in these regimes. The intent of this paper is to offer a possible solution and challenge the infrastructure that exists today over the 0.3-14um range in order to obtain a valid spectral radiance or temperature value, or both, to support emerging sensor fusion technology.

Sintered polytetrafluoroethylene (PTFE) is an extremely stable, near-perfect Lambertian reflecting diffuser and calibration standard material that has been used by national labs, space, aerospace and commercial sectors for over two decades. New uncertainty targets of 2 % on-orbit absolute validation in the Earth Observing Systems community have challenged the industry to improve is characterization and knowledge of almost every aspect of radiometric performance (space and ground). Assuming "near perfect" reflectance for angular dependent measurements is no longer going to suffice for many program needs. The total hemispherical spectral reflectance provides a good mark of general performance; but, without the angular characterization of bidirectional reflectance distribution function (BRDF) measurements, critical data is missing from many applications and uncertainty budgets. Therefore, traceable BRDF measurement capability is needed to characterize sintered PTFE's angular response and provide a full uncertainty profile to users. This paper presents preliminary comparison measurements of the BRDF of sintered PTFE from several laboratories to better quantify the BRDF of sintered PTFE, assess the BRDF measurement comparability between laboratories, and improve estimates of measurement uncertainties under laboratory conditions.

An in-vessel calibration light source (ICLS) has been implemented for remote use during extended shutdown periods of the Joint European Torus (JET). The ICLS facilitated the in situ calibration of optical diagnostics, which previously were performed when the diagnostics were removed from JET. Since the ICLS is used to calibrate diagnostics over the entire, exact optical path as used when plasma discharge data are measured, the ICLS calibration implicitly accounts for any vignetting losses in the JET vessel viewports in addition to the vacuum window transmission. At least ten diagnostic systems have benefited from the ICLS during the extended ITER-like wall shutdown of 2009-2011. Examples of the use of the ICLS in JET are given.

Historically, the tools used at NREL to compensate for the difference between a reference spectrum and a simulator spectrum have been well-matched reference cells and the application of a calculated spectral mismatch correction factor, M. This paper describes the algorithm for adjusting the spectrum of a 9-channel fiber-optic-based solar simulator with a uniform beam size of 9 cm square at 1-sun. The combination of this algorithm and the One-Sun Multi-Source Simulator (OSMSS) hardware reduces NREL's current vs. voltage measurement time for a typical three-junction device from man-days to man-minutes. These time savings may be significantly greater for devices with more junctions.

This article describes a method of measuring the absolute outdoor longwave irradiance using an absolute cavity pyrgeometer (ACP), U.S. Patent application no. 13/049, 275. The ACP consists of domeless thermopile pyrgeometer, gold-plated concentrator, temperature controller, and data acquisition. The dome was removed from the pyrgeometer to remove errors associated with dome transmittance and the dome correction factor. To avoid thermal convection and wind effect errors resulting from using a domeless thermopile, the gold-plated concentrator was placed above the thermopile. The concentrator is a dual compound parabolic concentrator (CPC) with 180{sup o} view angle to measure the outdoor incoming longwave irradiance from the atmosphere. The incoming irradiance is reflected from the specular gold surface of the CPC and concentrated on the 11 mm diameter of the pyrgeometer's blackened thermopile. The CPC's interior surface design and the resulting cavitation result in a throughput value that was characterized by the National Institute of Standards and Technology. The ACP was installed horizontally outdoor on an aluminum plate connected to the temperature controller to control the pyrgeometer's case temperature. The responsivity of the pyrgeometer's thermopile detector was determined by lowering the case temperature and calculating the rate of change of the thermopile output voltage versus the changing net irradiance. The responsivity is then used to calculate the absolute atmospheric longwave irradiance with an uncertainty estimate (Uââ) of {+-}3.96 W m°² with traceability to the International System of Units, SI. The measured irradiance was compared with the irradiance measured by two pyrgeometers calibrated by the World Radiation Center with traceability to the Interim World Infrared Standard Group, WISG. A total of 408 readings were collected over three different nights. The calculated irradiance measured by the ACP was 1.5 W/m² lower than that measured by the two pyrgeometers that are traceable to WISG, with a standard deviation of {+-}0.7 W m⁻². These results suggest that the ACP design might be used for addressing the need to improve the international reference for broadband outdoor longwave irradiance measurements.

Application-specific integrating sphere-based, integral veiling glare measurement systems are described. The sources use the integral method for measuring the veiling glare (VG) index of various lens-based imaging systems. The calibration source has provisions in the form of a collimating lens holder to simulate a situation where the black target and bright surround are at a sufficiently great distance to give measurements of VG index which are the same as that which would result if the distance where infinite. The design criteria for the integral VG test source are presented. Included is a summary of the end-user specifications in regards to spectral radiance, levels of attenuation, irradiance stability, and aperture uniformity and contrast. Spectral radiometric predictions and actual output levels are compared.

A laser scanning system has been developed by the National Renewable Energy Laboratory for the rapid characterization of crystal defects in single- and poly-crystalline semiconductors. The scanning defect mapping system has been commercialized by Labsphere, Inc. as the PVScan 5000. In the unprocessed material, the system produces digital color maps of the spatial distributions of dislocations and grain boundaries simultaneously. After device fabrication, the PVScan 5000 is used to produce photoresponsivity maps of the light beam induced current (LBIC) on a photovoltaic device such as a solar cell or a photodetector. An additional feature is that it also measures the spatial distributions of optical reflectance, both specular and diffuse, which can be applied to the LBIC maps to determine the internal responsivity of the device. The internal responsivity is proportional to the minority carrier diffusion length of silicon devices. It may be possible, therefore, to determine the diffusion length for certain devices.

Multi-spectral laser imaging is a technique that can offer a combination of the laser capability of accurate spectral sensing with the desirable features of passive multispectral imaging. The technique can be used for detection, discrimination, and identification of objects by their spectral signature. This article describes and reviews the development and evaluation of semiconductor multi-spectral laser imaging systems. Although the method is certainly not specific to any laser technology, the use of semiconductor lasers is significant with respect to practicality and affordability. More relevantly, semiconductor lasers have their own characteristics; they offer excellent wavelength diversity but usually with modest power. Thus, system design and engineering issues are analyzed for approaches and trade-offs that can make the best use of semiconductor laser capabilities in multispectral imaging. A few systems were developed and the technique was tested and evaluated on a variety of natural and man-made objects. It was shown capable of high spectral resolution imaging which, unlike non-imaging point sensing, allows detecting and discriminating objects of interest even without a priori spectroscopic knowledge of the targets. Examples include material and chemical discrimination. It was also shown capable of dealing with the complexity of interpreting diffuse scattered spectral images and produced results that could otherwise be ambiguous with conventional imaging. Examples with glucose and spectral imaging of drug pills were discussed. Lastly, the technique was shown with conventional laser spectroscopy such as wavelength modulation spectroscopy to image a gas (CO). These results suggest the versatility and power of multi-spectral laser imaging, which can be practical with the use of semiconductor lasers.

An application-specific uniform calibration source is described. The biggest challenge in developing the system is to achieve 25% higher spectral radiance values than the Earth's spectral radiance, with the lowest wavelength being the hardest to meet. This pre-flight test equipment will be used for characterization and calibration of imaging radiometers which will be used as satellite-borne remote sensors for KOPMSAT-3. The integrating sphere-based system will be used as a spectral radiance standard. Included are the end user's requirements in regards to spectral radiance levels, radiance stability, radiance uniformity and spectral radiance monitoring. Detailed design challenges, approach and modeling information is discussed.

Spectrometers that include extended-range linear InGaAs arrays make it possible to measure optical signals to 2500 nm. Available arrays, however, have more than 100 times the dark current as that of conventional arrays, which are limited to 1700 nm. This behavior leads to non-linearity in a short-wave infrared spectroradiometer used to monitor spectral radiance of an integrating sphere uniform source. A method of improving linearity in an extended-range InGaAs array is presented. The non-linearity is corrected using a multi-point calibration at a number of lamp power levels whereby the calibration factor for each wavelength point depends on the lamp power in the integrating sphere. An algorithm in the spectroradiometer software chooses the correct calibration factors and reports the system spectral radiance values accordingly. This method reduced error by more than a factor of two.

LCD backlighting applications require diffuse illumination over an extended area of a display unit while maintaining high luminance levels. Since such applications involve multiple reflections within a reflective cavity, the efficiency of the cavity can be affected significantly by relatively small changes in the reflectance of the cavity material. Materials with diffuse rather than specular (or mirror-like) reflectance scatter light, averaging out hot spots and providing a uniform field of illumination. Reflectors with specular components tend to propagate non-uniformities in the illuminator system. The result is a spatial variation in brightness visible to the viewer of the display. While the undesirability of specular materials for such applications has been widely recognized, some diffuse materials in common use exhibit a significant specular component. This paper describes a method for measuring the specular component of such materials, and presents a simple approach to evaluating the effect of such secondary specular behavior on the performance of a backlight cavity. It is demonstrated that significant differences exist among available diffuse reflectance materials, and that these differences can lead to significant differences in the performance of the displays in which these materials are used.

An application-specific contracted integrating sphere source of uniform spectral radiance is described. The source is used for pre-launch test and calibration of imaging radiometers which will be used as satellite borne earth remote sensors. The calibration source is primarily intended to serve as a transfer standard of radiance. Design criteria for the uniform radiance source are presented. Included is a summary of the end-user specifications in regards to spectral radiance, radiance levels of attenuation, radiance stability, and aperture uniformity. Radiometric theory used to predict the source radiance for a specific spectral flux input is reviewed. Reasoning for the use of an integrating sphere platform for this application and characteristic features of the source are discussed. Calibration methods and instrumentation are described. The resultant data presented include the modeled data compared with the measured performance. Methods of data reduction and uncertainty are addressed where applicable.

The demand for progressively more powerful lasers has caused those employing side-pumped laser designs to become acutely aware of pumping efficiency and performance. Additionally, precision applications demand beam stability and uniformity for the lifetime of the laser flash lamp. The use of highly diffuse, high reflectance pump chamber reflectors such as Spectralon(R)‡ have been shown to amplify overall power and performance. Spectralon is used in a wide range of side-pumped applications for its superior optical characteristics and design flexibility but stated damage thresholds of approximately 4 J/cm2 have limited it to lower power applications. To increase energy tolerances, initial damage thresholds are defined through mathematical simulation. A general form of the heat equation is studied numerically to develop a theoretical model of Spectralon's damage threshold. The heat equation is discretized using the Euler method. Secondly, process modifications are performed to test for increased material durability and to physically reproduce initially defined theoretical parameters.

The measurement of sunscreens using an in vitro technique that correlates to in vivo measurements has been proposed for many years. In vivo testing, where human volunteers are subjected to potentially damaging and carcinogenic doses of ultraviolet radiation, has been the method of choice by regulatory agencies for determining the efficacy of sunscreens to protect humans from both sunburn (solar erythema) and potential skin cancers related to high UV doses. The problems with in vitro measurements are many fold. A normal spectrophotometer cannot accurately capture the transmitted light from a sunscreen, since both the media and the sunscreen may scatter the incident and transmitted radiation. Secondly, a suitable substrate for dispersing the potential sun protective agent must be found – a material that transmits sufficiently over the range of interest of measurement but also has a texture similar to the human epidermis to allow for proper dispersion of the sunscreen.This paper will discuss the theory of measurement of diffuse transmittance measurements, including the various instrument geometries used to make such measurements. It will also address the calculations required to convert transmittance values to that of sun protection factor. Finally, there is a discussion of substrates for in vitro measurements.

Fluorescent pigments and dyes add brightness and color to our lives, but brightness and light to the consumer can spell difficulties in color matching and formulation for a manufacturer. While fluorescent pigments have been used for many years, the proper formulation and quality control of colored materials containing them has been a challenge, primarily due to lack of proper instrumentation to make the necessary measurements. Another difficulty has been a general lack of understanding of how such materials achieve fluorescence. This paper, and those also published in this section, are intended to present an overview of the general principles associated with measuring fluorescent color. It addresses the chemistry and physics of what causes fluorescent pigments to work and gives some examples of applications for the pigments. Considerations as to why standard spectrophotometers and colorimeters fail to give completely accurate results in measuring such materials are discussed. After an overview of the effects of different illuminants on fluorescent pigments, I will discuss how geometry of measurement can affect results, and how several national laboratories are making such measurements.

As part of the Intersociety Colour Council (ISCC) effort to update ISCC Guide 89-1, a survey of manufacturers and national measurement laboratories was undertaken. This survey was to determine availability of artifact standards and measurement services related to colour. The results were presented as a poster paper at the Third Oxford Conference on Spectrometry in July, 1998.

A laser scanning system, originally developed by NREL for the rapid characterization of crystal defects in single- and poly-crystalline semiconductors, has been commercialized by Labsphere, Inc. as the PVScan 5000. In the unprocessed material, the system produces digital color maps of the spatial distributions of dislocations and grain boundaries simultaneously. After device fabrication, the PVScan 5000 is used to produce photoresponsivity maps of the light beam induced current (LBIC) on a photovoltaic device such as a solar cell or a photodetector. It also measures the spatial distributions of optical reflectance, both specular and diffuse, which can be applied to the LBIC maps to determine the internal responsivity of the device. It may be possible in the future to determine the diffusion length for certain devices.