Effective Radiance Research Articles

CO retrievals based on MOPITT near‐infrared observations

We report the first retrieval results of tropospheric carbon monoxide (CO) exclusively using near‐infrared (NIR) radiances in the 2.3 μm CO overtone band observed by the Measurements of Pollution in the Troposphere (MOPITT) satellite instrument. For daytime overpasses over land, such observations complement MOPITT's thermal infrared (TIR) observations in the 4.7 μm CO fundamental band, especially for constraining the CO total column. Retrievals are performed in an optimal estimation framework in which effective radiance errors due to geophysical sources are estimated empirically. The new NIR‐based retrievals are evaluated through comparisons with both the standard TIR‐based MOPITT CO product and CO profile measurements.

Radiative Properties of Blackbody Calibration Sources:Recent Advances in Computer Modeling

The radiative characteristics (spectral effective emissivity, spectral radiance, and radiance temperature) of blackbody calibration sources widely used in radiation thermometry are an important subject for advanced computer modeling by the Monte Carlo method. An algorithm and code for stochastic modeling of the radiant heat transfer inside cavities has been developed on the basis of the reciprocity principle and backward ray tracing. The importance sampling technique has been applied to generate the reflected rays according to the surface reflection model that can be a linear combination of the following primary models: Lambertian, Specular, and TETRA (a microfacet model of random tetrahedral pits that mimics reflections from a rough surface). A wide range of axisymmetrical cavities, cylindrical cavities with an inclined flat bottom, and a rectilinear grooved radiator of polygonal profile have been implemented. Various conditions of observation can be modeled to compute appropriate radiation characteristics. A number of different temperature distributions can be assigned to the same node set on the cavity surface, so several related tasks can be modeled in a single run. The results obtained for the radiative properties of isothermal and non-isothermal non-diffuse blackbodies used for the calibration of infrared radiation thermometers are presented and discussed.

Solar Irradiance and Effective Brightness Temperature for SWIR Channels of AVHRR/NOAA and GOES Imagers

Abstract Satellite observations in the shortwave infrared (SWIR) part of spectrum between 3.5 and 4.0 μm deliver critically important information for many applications. The satellite signal in this spectral band consists of solar-reflected radiation and thermal radiation emitted by surface, clouds, and atmosphere. Accurate retrievals require precise knowledge of solar irradiance values within a channel's bandwidth. The magnitudes of solar irradiance for shortwave infrared channels (3.7–3.9 μm) for the Advanced Very High Resolution Radiometer (AVHRR) on board the National Oceanic and Atmospheric Administration-7 (NOAA-7) to NOAA-18 satellites and the Geostationary Operational Environmental Satellite-8 (GOES-8) to GOES-12 are considered in this paper. Four recent solar reference spectra [those of Kurucz, Gueymard, the American Society for Testing and Materials (ASTM), and Wehrli] are analyzed to determine uncertainties in the knowledge of solar irradiance values for SWIR channels of the listed sensors. Because thermal radiation is frequently converted to effective blackbody temperature for analysis, computations, and calibration purposes, it is proposed here to express band-limited solar irradiance values in terms of brightness temperature as well. It is shown that band-limited solar irradiance for AVHRR radiometers expressed in terms of blackbody equivalent brightness temperature correspond to the range 355–360 K, and vary around 345 K for the SWIR channels of the GOES imagers. The values of band-limited solar irradiance and brightness temperatures are provided for various reference solar spectra. The relative differences in band-limited solar irradiance computed for the considered reference solar spectra are between 0% and 2.5%. Differences expressed in terms of brightness temperatures may reach 0.8 K. The results for the ASTM and the Kurucz reference spectra agree within 0.1% relative difference. Parameters of linear fits relating effective brightness temperatures and spectral radiance equivalent temperatures are also determined for all sensors. They are required for precise radiance–temperature and temperature–radiance conversion through Planck's functions in the case of the finite spectral response of real sensors.

Estimation of safe exposure time from an ophthalmic operating microscope with regard to ultraviolet radiation and blue-light hazards to the eye.

Hazards from the optical radiation of an operating microscope that cause damage at the corneal, lenticular, and retinal levels were investigated; we considered, in particular, ultraviolet radiation (UVR) and blue light. The spectral irradiance from a Zeiss operation microscope OPMI VISU 200 was measured in the corneal plane between 300 and 1100 nm. Effective irradiance and radiance were calculated with relative spectral effectiveness data from the American Conference for Governmental and Industrial Hygienists. Safe exposure time to avoid UVR injury to the lens and cornea was found to be 2 h without a filter, 4 h with a UVR filter, 200 a yellow filter, and 400 h with a filter combination. Safe exposure time to avoid retinal photochemical injury was found to be 3 min without a filter and with a UVR filter, 10 min with a yellow filter, and 49 min with a filter combination. The effective radiance limit for retinal thermal injury was not exceeded. The hazard due to the UVR component from the operating microscope is not critical, and operation time can be safely prolonged with the use of appropriate filters. The retinal photochemical hazard appears critical without appropriate filters, permitting only some minutes of safe exposure time. The calculated safe exposure times are for worst-case conditions and maximal light output and include a safety factor.

Correction of Three-Dimensional Effects for Passive Microwave Remote Sensing of Convective Clouds

Abstract This paper presents a simple approach to adjust microwave brightness temperature distributions obtained from slant-path measurements for projection effects. Horizontal displacement in the direction of sight is caused by signal contributions from other than near-surface layers that are projected to the footpoint of observation. In particular at frequencies sensitive to ice particle scattering the horizontal projection effect can amount to values as big as the vertical cloud extent. Based on cloud model–generating, three-dimensional hydrometeor distributions at subsequent model time steps and a modified one-dimensional radiative transfer model, the high correlation of effective radiance contribution altitudes and brightness temperatures at 37.0 and 85.5 GHz is demonstrated. For these altitudes, described by the centers of gravity of the spectral weighting functions, regression equations are derived with standard errors below 0.61 km at 85.5 GHz and 0.22 km at 37.0 GHz for both the Special Sensor Mi...

Linear model of infrared line scanner imaging using wave optics approach

Infrared line scanners (IRLS) have been modelled on the basis of linear filter theory. Imaging through a scanner objective is described with wave optics. Effective wavelength and effective radiance are introduced to reduce the dependence on wavelength. Our theoretical predictions of IRLS images agree well with experimental IRLS images.

Evaluation of model clouds and radiation at 100 km scale using GOES data

Model simulated cloud properties such as cloud fraction, cloud top height, and infrared (IR) radiance are compared to those from GOES-7 satellite observations over a monthly period from an ensemble of thirty-one forecasts launched each day of October 1993. The aim is to evaluate both the above mentioned parameters and the cloud optical properties (emissivity, optical depth) in a global model that carries cloud water (ice and water phases combined) as a prognostic variable. The results show that the model simulates well the cloud fraction, cloud top height, and IR radiance in the middle and high latitude regions, especially in the Northern Hemisphere. Most obvious deficiencies are found in the tropics and subtropics over the ocean: underestimation of cloud fraction and cloud top height in the inter-tropical convergence zone, and overestimation of low clouds in the subtropics. The causes of these deficiencies are discussed. The cloud emissivity parameterization is tested and its sensitivity to effective cloud fraction and outgoing radiance is assessed from comparisons with cases that assume an emissivity of unity. The resulting global infrared budget is realistic with a net loss at the top of the atmosphere of 243 W/m2 and 55 W/m2 at the surface. Although error compensation between cloud height and fraction occurs in some areas to yield good IR radiances, the outgoing radiation is excellent in regions where clouds are best, thus validating the chosen emissivity parameterization. After some adjustment of the parameterized visible cloud optical depth, the global shortwave budget is also reasonable compared to available climatology with a net surface flux of 157 W/m2 and a planetary albedo of 30%. The sensitivity to various parameters affecting the radiation budget is examined.

Modelling of the effective emissivity of a graphite black body

Measurements of the effective radiance temperature of a high-temperature graphite tube black-body furnace are compared with those calculated from two theoretical models. The first model extends the conventional annular zone model by including effects of vignetting and temperature gradients. Because the black-body surface is not a perfect diffuse reflector, this is found to be insufficiently accurate and a Monte-Carlo integration with a diffuse/Fresnel-specular surface model is presented, which gives good agreement with the measured data.

Spatial variability of land surface emissivity in the thermal infrared band: Spectral signature and effective surface temperature

Experimental access to effective surface parameters, surface temperature, and emissivity, relevant to satellite investigations in the thermal infrared band (TIR), is addressed and discussed. The spatial variability of the broad (8–14μ) emissivity was measured over an overall homogeneous area with a movable measuring system. Spectral ground radiances were also measured at many places, the spatial average of which is assumed to represent the effective spectral ground radiance. The effective surface temperature is obtained through renormalization of the effective emissivity signature over the RMS broad band emissivity previously measured. The method is shown to produce the effective surface temperature with an accuracy better than 1 K, without any direct temperature measurement. From the effective signature, the channel emissivities pertaining to a given instrument can be calculated. This has been done for NOAA-11/AVHRR Channels 4 and 5. A preliminary test was made to try and validate nighttime satellite-derived surface temperature by comparison with field effective temperature. The image was acquired during the field experiment. The first result of this attempt is encouraging, although only qualitative because of standard atmospheric corrections. Orientation of future work for better accuracy and reliability is discussed as well.

Measurement of blue-light effective radiance of welding arcs.

Measurement of blue-light effective radiance of welding arcs.

Vicarious satellite calibration in the solar spectral range by means of calculated radiances and its application to Meteosat

The method of vicarious calibration by means of calculated radiances allows absolute calibration of satellite radiometers in orbit. It works by comparing counts from the radiometer to be calibrated with corresponding absolute radiances, calculated from actual values of the relevant optically acting parameters of the atmosphere and the earth's surface. The method is applied to the VIS-channel (it measures in the visible and near IR) of the European geostationary satellite Meteosat-1. To minimize uncertainties, the procedure is carried out over different surfaces, at different atmospheric conditions, and at different sun and satellite angles. The ratio between the effective radiances (the radiances at the satellite weighted with its spectral response) and the measured 6-bit counts of the Meteosat-1-VIS-channel is the calibration constant c(sat) = 2.66 W . m(-2) . sr(-1)/count. The accuracy of the calibration is +/-6%. The inaccuracy is mainly due to the broad digitization steps of the channel. Conversion factors are presented which allow one to calculate from the effective radiance the radiance at the satellite (the radiance leaving the atmosphere).

Martian isotopic ratios and upper limits for possible minor constituents as derived from Mariner 9 infrared spectrometer data

Abstract The Mariner 9 infrared spectrometer obtained data over a large part of Mars for almost a year beginning late in 1971. Mars' infrared emission spectrum was measured from 200 to 2000 cm −1 with an apodized resolution of 2.4 cm −1 . No significant deviation from terrestrial ratios of carbon ( 12 C/ 13 C) or oxygen ( 16 O/ 18 O; 16 O/ 17 O) isotopes was observed on Mars. The 12 C/ 13 C isotopic ratio was found to be terrestrial with an uncertainty of 15%. Upper limits have been calculated for several minor constituents. With an effective noise equivalent radiance of 1.2 × 10 −9 W cm −2 sr −1 /cm −1 , new upper limits in centimeter-atmospheres of 2 × 10 −5 for C 2 H 2 , 4 × 10 −3 for C 2 H 4 , 3 × 10 −3 for C 2 H 6 , 2 × 10 −4 for CH 4 , 1 × 10 −3 for N 2 O, 1 × 10 −4 for NO 2 , 4 × 10 −5 for NH 3 , 1 × 10 −3 for PH 3 , 7 × 10 −4 for SO 2 , and 1 × 10 −4 for OCS have been derived.

Earth Radiance and Reflectivities Relative to the Response of a Silicon Detector

An experiment to measure earth radiance and reflectivity is described. The primary purpose was to determine the minimum effective radiance levels that could be anticipated for horizon sensors employing silicon detectors. The data and results are discussed in terms of minimum radiance levels, limb brightening, and some spectral characteristics.

Variation of the Infrared Spectral Radiance near the Limb of the Earth

The infrared spectral radiance has been calculated near the limb of the earth, using a curved refracting atmosphere. Radiance was calculated in narrow spectral intervals for four atmospheric models representing a wide range of meteorological conditions. Comparison is made of the radiance across the limb for each model atmosphere in several narrow spectral intervals; two hypothetical filters are then convoluted with spectral radiance to show the effective radiance across the limb, as it might be viewed by a horizon sensor. The results indicate that the 15-μ carbon dioxide band and the rotational vapor band are best suited for horizon sensing and that neither is decisively superior on the basis of radiance. It is also shown that radiative flux calculated using the exact radiance values near the limb is only about 1% more precise than that calculated from an extrapolation to the limb of radiance values determined at zenith angles where the plane parallel approximation is valid.