Absolute Radiance Calibration Research Articles

Absolute radiance calibration in the UV and visible spectral range using atmospheric observations during twilight

Abstract. We present an improved radiance calibration method for UV–Vis spectroscopic instruments with a narrow field of view (up to a few degrees) based on the calibration method by Wagner et al. (2015). The updated method uses only measurements during the twilight period instead of several hours as for the original method. The calibration is based on the comparison of measurements and simulations of the radiance of zenith-scattered sunlight. The main advantage of our method compared to radiance calibration methods in the laboratory is that the calibration can be directly applied in the field. This allows routine radiance calibrations whenever the sky is clear during twilight. The calibration can also be performed retrospectively and will thus be applicable for the large number of existing data sets. Also, potential changes in the instrument properties during transport from the laboratory to the field are avoided. The new version of the calibration method presented here has two main advantages. First, the required measurement period can be rather short (only a few minutes during twilight for cloud-free conditions). Second, even without knowledge of the aerosol optical depth (AOD), the errors in the calibration method are rather small, especially in the UV spectral range where they range from about 4 % at 340 nm to 8 % at 420 nm. If the AOD is known, the uncertainties are even smaller (about 3 % at 340 nm to 4 % at 420 nm). For visible wavelengths, good accuracy is only obtained if the AOD is approximately known with uncertainties from about 4 % at 420 nm to 10 % between about 550 and 700 nm (generally the AOD is nevertheless smaller in the visible than in the UV spectral range). One shortcoming of the method is that it is not possible to determine the AOD exactly at the time of the (twilight) measurements because AOD observations from sun photometer measurements or the MAX-DOAS (Multi-AXis Differential Optical Absorption Spectroscopy) measurements are usually not meaningful for such high solar zenith angle (SZA). But the related uncertainty can be minimised by repeating the radiance calibrations during the twilight periods of several days.

Pre-Launch Radiometric Characterization of EMI-2 on the GaoFen-5 Series of Satellites

The environmental trace gas monitoring instrument (EMI) is a space-borne imaging spectrometer onboard GaoFen-5, which was launched in May 2018, covering wavelengths in the range of 240–710 nm to measure NO2, O3, HCHO, and SO2. An advanced EMI-2 instrument with a higher spatial resolution and sufficient signal-to-noise is currently planned for launch on the GaoFen-5(02) satellite in 2021. The EMI-2 instrument bidirectional scattering distribution function (BSDF) is obtained from the absolute irradiance and radiance calibration on-ground. Based on EMI-2 earth and sun optical paths, the key factors of BSDF parameters are introduced. An NIST-calibrated 1000 W FEL quartz tungsten halogen lamp and a 2D turntable are adopted for the absolute irradiance calibration. A large aperture integrating sphere system is used for the absolute radiance calibration. Based on absolute irradiance and radiance calibration functions, the BSDF parameters are obtained, with accuracy of 4.9% for UV1, 4.3% for UV2, 4.1% for VIS1, and 4.2% for VIS2. The on-ground measurement results show that the reflectance spectrum can be calculated from BSDF parameters. On-orbit application of the EMI-2 instrument BSDF are also discussed.

Design and investigation of absolute radiance calibration primary radiometer

In order to satisfy the requirement of calibration precision and traceability for space remote sensor, the absolute radiance calibration primary radiometer (ARCPR) is investigated for establishing the irradiance calibration standard in orbit. ARCPR is a mechanical cryogenic absolute radiometer for space application, and works at 20 K. The cryogenic environment is obtained by the Stryn-type pulse tube cryocooler. The environment temperature is stable at 23.9658 K by high-precision temperature controllers. The detector of ARCPR is designed and investigated based on the experimental prototype. The experimental results of detector characteristics measurement illustrate that the sensitivity is 3.9526 K/mW, and the thermal time constant is 134 s. The procedure of optical power measurement is optimised for satisfying the requirements of in orbit measurement. ARCPR achieves a relative standard uncertainty of 210 ppm at an optical power level of 0.4 mW. Therefore, ARCPR could be the radiometric standard for optical measurement. Furthermore, the investigation result provides the theoretical and experimental basis for the establishment of radiometric calibration standard in orbit.

Correction of cavity absorptance measure method for cryogenic radiometer

As the reference detector of absolute radiance calibration primary radiometer (ARCPR) for space remote sensing, total solar irradiance (TSI) cavity is investigated to ensure the measurement accuracy. TSI cavity adopts a sloping bottom cavity. The geometrical parameters and paint material of sloping bottom cavity are both optimised to improve the invariance of absorptance in the environment of ground and space. The optimisation of substitution method is proposed to measure the average absorptance by two-dimensional scan. The uniformity is evaluated by absorptance map. Experimental results illustrate that sloping bottom cavity has a super-high absorptance of 0.999928 (wavelength is 632 nm), and the relative measure uncertainty is approximated to 6 ppm. Meanwhile, compared with the absorptance map of conical cavity, the sloping bottom design improves the uniformity of absorptance. Therefore, the sloping bottom cavity can be adopted as the reference detector of ARCPR. Furthermore, the optimised substitution method is suitable for the investigation of the blackbody cavity with super-high absorptance.

A new method for the absolute radiance calibration for UV–vis measurements of scattered sunlight

Abstract. Absolute radiometric calibrations are important for measurements of the atmospheric spectral radiance. Such measurements can be used to determine actinic fluxes, the properties of aerosols and clouds, and the shortwave energy budget. Conventional calibration methods in the laboratory are based on calibrated light sources and reflectors and are expensive, time consuming and subject to relatively large uncertainties. Also, the calibrated instruments might change during transport from the laboratory to the measurement sites. Here we present a new calibration method for UV–vis instruments that measure the spectrally resolved sky radiance, for example zenith sky differential optical absorption spectroscopy (DOAS) instruments or multi-axis (MAX)-DOAS instruments. Our method is based on the comparison of the solar zenith angle dependence of the measured zenith sky radiance with radiative transfer simulations. For the application of our method, clear-sky measurements during periods with almost constant aerosol optical depth are needed. The radiative transfer simulations have to take polarisation into account. We show that the calibration results are almost independent from the knowledge of the aerosol optical properties and surface albedo, which causes a rather small uncertainty of about < 7 %. For wavelengths below about 330 nm it is essential that the ozone column density during the measurements be constant and known.

A new look at photometry of the Moon

We use ROLO photometry (Kieffer, H.H., Stone, T.C. [2005]. Astron. J. 129, 2887–2901) to characterize the before and after full Moon radiance variation for a typical highlands site and a typical mare site. Focusing on the phase angle range 45° < α < 50°, we test two different physical models, macroscopic roughness and multiple scattering between regolith particles, for their ability to quantitatively reproduce the measured radiance difference. Our method for estimating the rms slope angle is unique and model-independent in the sense that the measured radiance factor I/ F at small incidence angles (high Sun) is used as an estimate of I/ F for zero roughness regolith. The roughness is determined from the change in I/ F at larger incidence angles. We determine the roughness for 23 wavelengths from 350 to 939 nm. There is no significant wavelength dependence. The average rms slope angle is 22.2° ± 1.3° for the mare site and 34.1° ± 2.6° for the highland site. These large slopes, which are similar to previous “photometric roughness” estimates, require that sub-mm scale “micro-topography” dominates roughness measurements based on photometry, consistent with the conclusions of Helfenstein and Shepard (Helfenstein, P., Shepard, M.K. [1999]. Icarus 141, 107–131). We then tested an alternative and very different model for the before and after full Moon I/ F variation: multiple scattering within a flat layer of realistic regolith particles. This model consists of a log normal size distribution of spheres that match the measured distribution of particles in a typical mature lunar soil 72141,1 (McKay, D.S., Fruland, R.M., Heiken, G.H. [1974]. Proc. Lunar Sci. Conf. 5, Geochim. Cosmochim. Acta 1 (5), 887–906). The model particles have a complex index of refraction 1.65–0.003 i, where 1.65 is typical of impact-generated lunar glasses. Of the four model parameters, three were fixed at values determined from Apollo lunar soils: the mean radius and width of the log normal size distribution and the real part of the refraction index. We used FORTRAN programs from Mishchenko et al. (Mishchenko, M.I., Dlugach, J.M., Yanovitskij, E.G., Zakharova, N.T. [1999]. J. Quant. Spectrosc. Radiat. Trans. 63, 409–432; Mishchenko, M.I., Travis, L.D., Lacis, A.A. [2002]. Scattering, Absorption and Emission of Light by Small Particles. Cambridge Univ. Press, New York. < http://www.giss.nasa.gov/staff/mmishchenko/books.html>) to calculate the scattering matrix and solve the radiative transfer equation for I/ F. The mean single scattering albedo is ω = 0.808, the asymmetry parameter is 〈cos Θ〉 = 0.77 and the phase function is very strongly peaked in both the forward and backward scattering directions. The fit to the observations for the highland site is excellent and multiply scattered photons contribute ⩾80% of I/ F. We conclude that either model, roughness or multiple scattering, can match the observations, but that the strongly anisotropic phase functions of realistic particles require rigorous calculation of many orders of scattering or spurious photometric roughness estimates are guaranteed. Our multiple scattering calculation is the first to combine: (1) a regolith model matched to the measured particle size distribution and index of refraction of the lunar soil, (2) a rigorous calculation of the particle phase function and solution of the radiative transfer equation, and (3) application to lunar photometry with absolute radiance calibration.

The Hayabusa Spacecraft Asteroid Multi-band Imaging Camera (AMICA)

The Hayabusa Spacecraft Asteroid Multi-band Imaging Camera (AMICA) has acquired more than 1400 multispectral and high-resolution images of its target asteroid, 25143 Itokawa, since late August 2005. In this paper, we summarize the design and performance of AMICA. In addition, we describe the calibration methods, assumptions, and models, based on measurements. Major calibration steps include corrections for linearity and modeling and subtraction of bias, dark current, read-out smear, and pixel-to-pixel responsivity variations. AMICA v-band data were calibrated to radiance using in-flight stellar observations. The other band data were calibrated to reflectance by comparing them to ground-based observations to avoid the uncertainty of the solar irradiation in those bands. We found that the AMICA signal was linear with respect to the input signal to an accuracy of ≪1% when the signal level was <3800 DN. We verified that the absolute radiance calibration of the AMICA v-band (0.55 μm) was accurate to 4% or less, the accuracy of the disk-integrated spectra with respect to the AMICA v-band was about 1%, and the pixel-to-pixel responsivity (flat-field) variation was 3% or less. The uncertainty in background zero level was 5 DN. From wide-band observations of star clusters, we found that the AMICA optics have an effective focal length of 120.80 ± 0.03 mm, yielding a field-of-view (FOV) of 5.83° × 5.69°. The resulting geometric distortion model was accurate to within a third of a pixel. We demonstrated an image-restoration technique using the point-spread functions of stars, and confirmed that the technique functions well in all loss-less images. An artifact not corrected by this calibration is scattered light associated with bright disks in the FOV.

In-Orbit Radiometric Calibration of the FORMOSAT-2 Remote Sensing Instrument

This principle focus of this study is the absolute radiometric calibrations of FORMOSAT-2 RSI imagery in orbit. There are two principal parts for achieving this calibration. The first is the assessment of the calibration site by examining atmospheric observations from ground stations and field measurements via ground-based radiometric instruments. After careful consideration based on the essential requirements for a suitable calibration site i.e., prevailing clear and clean atmosphere conditions over a wide, flat and near lambertian surface with high reflectance, the airport on Dongsha Island was considered to be an suitable site. The next phase is to design a scheme for the field campaign at the calibration site for radiometric calibration. Thus a synchronous experiment acquiring simultaneous measurements from the FORMOSAT-2 Remote Sensing Instrument (RSI) sensor and ground-based instruments was proposed and implemented for the period 16 to 19 September 2004. As a result, a set of reasonable radiometric coefficients for the absolute radiance calibration of the RSI was successfully constructed via the radiative transfer code associated with the synchronous measurements in this study.

In‐flight calibration and performance of the Mars Exploration Rover Panoramic Camera (Pancam) instruments

The Mars Exploration Rover Panoramic Camera (MER/Pancam) instruments have acquired more than 60,000 high‐resolution, multispectral, stereoscopic images of soil, rocks, and sky at the Gusev crater and Meridiani Planum landing sites since January 2004. These images, combined with other MER data sets, have enabled new discoveries about the composition, mineralogy, and geologic/geochemical evolution of both sites. One key to the success of Pancam in contributing to the overall success of MER has been the development of a calibration pipeline that can quickly remove instrumental artifacts and generate both absolute radiance and relative reflectance images with high accuracy and precision in order to influence tactical rover driving and in situ sampling decisions. This paper describes in detail the methods, assumptions, and models/algorithms in the calibration pipeline developed for Pancam images, based on new measurements and refinements performed primarily from flight data acquired on Mars. Major calibration steps include modeling and removal of detector bias signal, active and readout region dark current, electronic “shutter smear,” and pixel‐to‐pixel responsivity (flatfield) variations. Pancam images are calibrated to radiance (W/m2/nm/sr) using refined preflight‐derived calibration coefficients, or radiance factor (I/F) using near‐in‐time measurements of the Pancam calibration target and a model of aeolian dust deposition on the target as a function of time. We are able to verify that the absolute radiance calibration of most Pancam images is accurate to within about 10% or less and that the filter‐to‐filter and pixel‐to‐pixel precision of the calibrated relative reflectance data (both based on measurements of the Pancam calibration target) are typically about 3% and 1% or less, respectively. Examples are also presented of scientific applications made possible by the high fidelity of the calibrated Pancam data. These include 11‐color visible to near‐IR spectral analysis, calculation of “true color” and chromaticity values, and generation of “super resolution” image data products. This work represents a follow‐on and enhancement to the Pancam preflight calibration process described by Bell et al. (2003).

Derivation of immersion factors for the hyperspectral TriOS radiance sensor

The new radiance sensor RAMSES ARC of the TriOS system is becoming of increasing importance in the ocean optics community due to its price–performance ratio. The company only provides an absolute radiance calibration for use in air. For absolute calibrated radiance measurements in water, immersion coefficients are important, considering the changes in the wavelength-dependent refractive index in the transition from the air–glass to the water–glass interface. For the first time, immersion factors were directly and indirectly determined for the real material of the optical window over the whole spectral range. With the derived correction factors, the sensor is applicable for the determination of the water-leaving reflectance from underwater measurements of radiance profiles in combination with above-water irradiance measurements, to validate ocean colour satellite sensors.

A Rapid-Scan Spectrometer That Sweeps Corner Mirrors Through the Spectrum

By sweeping a sequence of corner mirrors through an intermediate focal plane, a grating spectrometer can be converted to rapid scanning without loss of optical quality. We have developed an instrument using this principle to scan the first order spectrum of a diffraction grating in 1 msec, at rates up to 800 scans/sec. In our design, twenty-four corner mirrors are mounted on the periphery of a dynamically balanced rotating scan wheel, tangential to the intermediate focal plane of a double-pass CzernyTurner monochromator. Specific features include readout linear in time and wavelength, foreoptics focusable from 28 cm to infinity, absolute radiance calibration, and interchangeable gratings and detectors for the region 2500 A to 9 micro. Performance is limited only by the detector signal-to-noise ratio determined by the scan time and the electrical bandwidth required, and is not limited by the optical scanning technique. Spectra illustrating instrument performance are presented.