Showing papers on "Sky published in 1986"

Continuous tone representation of three-dimensional objects illuminated by sky light

Abstract: Natural lighting models to date have been limited to calculation of direct sunlight. However, this paper proposes an improved model for natural lighting calculations that adequately considers both direct sunlight and scattered light caused by clouds and other forms of water vapor in the air. Such indirect natural light is termed skylight and can be an important factor when attempting to render realistic looking images as they might appear under overcast skies.In the proposed natural lighting model, the sky is considered to be a hemisphere with a large radius (called the sky dome) that acts as a source of diffuse light with nonuniform intensity. In order to adequately take into account the nonuniform intensity of such skylight, the sky dome is subdivided into bands. The light intensity within individual bands can be assumed to be transversely uniform and longitudinally nonuniform and therefore the total luminance emanating from each band can be calculated more accurately.The proposed method significantly improves the realism of natural lighting effects. Its advantages are particularly apparent when simulating lighting under an overcast sky or when rendering surfaces that fall within a shadow cast by an obstruction lit by direct sunlight.

The Determination of Sky View-Factors in Urban Environments Using Video Imagery

Abstract: A technique is described whereby sky view-factors may be determined using a video camera equipped with a fish-eye lens. The video image is digitized and then analyzed to distinguish between “sky” and “non-sky” pixels. View-factors are calculated for each pixel and then summed for all “sky” pixels to yield a composite sky view-factor for the image. The technique is illustrated by applying it in three urban locations, all of which are characterized by high building densities (and hence complex skylines). The three images processed have sky, view-factors in the range 0.15 to 0.46 (as independently determined). It is shown that the present technique produces values in close agreement with these and appears quite robust when compared with calculations based on the work of Johnson and Watson.

Spectral distribution of global and diffuse solar radiation

Abstract: Spectral distributions of solar radiation were measured over the wavelength range 300 to 800 nm in Ny-Alesund (78"55'N, 1 I"56'E). The apparatus consisted of a grating monochromator with an automatic scanning system, an integrating sphere supposed to give a good picture of the radiation received on a horizontal surface from the whole celestial hemisphere, a photomultiplier tube with S-20 response, and a strip chart recorder. Great importance was attached to the calibration procedure, which was carried out by means of a 1000 W quartz-iodine lamp with known spectral irradiance (calibration traceable to NBS, delivered by Eppley Laboratory). The main part of the discussion is based on relative spectral distributions. Spectra in absolute units were obtained by comparison with simultaneous pyranometer recordings of total solar energy, which by means of filter measurements could be reduced to the same wavelength range as for the spectral scans. In addition, the pyranometer recordings served as a means of judging whether the radiation conditions were sufficiently stable to give reliable spectral curves. Spectral measurements were made of radiation from: (I) clear sky and sun, (2) clear sky alone, a sky totally covered by (3) clouds or (4) fog, (5) zenith clear and (6) zenith overcast sky. The spectral curves of (I), (3), (4) and (6) do not differ very much, but deviate markedly from those of (2) and (5). An overcast sky has relatively higher spectral irradiances towards the short-wave end of the spectrum than the corresponding spectrum for a clear sky. Three groups of solar altitudes are considered, with mean altitudes of about 30" (noon), 18" (evening), and 10" (night). Comparison of the spectra of these groups clearly reveals the effect of increasing absorption by atmospheric gases with decreasing solar altitude. At the same time, due to growing importance of selective scattering processes, the proportion of long-wave to short-wave irradiances increases. The Ny-Alesund noon spectrum for clear-sky global radiation seems to agree quite well with spectra from lower latitudes referring to a correlated colour temperature close to 6500" I(. This also applies to the CIE standard daylight source De5. Considering the spectra in absolute units, it turns out that while the spectral irradiances of global radiation on an overcast day are about 40% of those for a clear day in the ultraviolet part, the corresponding relation decreases to about 30% in the near infrared. Sky radiation on a clear day is responsible for more than half of the ultraviolet content of global radiation, but does not contain more than about 5% ofthe near infrared radiation. Comparable spectra from Nottingham and Pretoria seem to be considerably more influenced by aerosol scattering. Nevertheless Pretoria has as high irradiances in the ultraviolet as Ny-Alesund, in all probability due to a smaller ozone absorption at the former station. For situations with a clear sky daylight illuminances calculated on the basis of absolute spectral irradiances and the spectral sensitivity of the human eye agree well with directly observed illuminances for the same solar altitudes. The chromaticity co-ordinates representing different phases of daylight in Ny-Alesund are slightly on the green side of the black body locus. Somewhat unexpected is the fact that daylight at midnight has a slightly higher correlated colour temperature (is more bluish) than that observed during the middle of the day. Estimates demonstrate the significant part played in this connection by the ozone absorption in the Chappuis bands.

The projected dark cloud distribution of the Milky Way

Abstract: The dark cloud catalogs of Lynds (1962) for the northern hemisphere and Feitzinger and Stuewe (1984) for the southern hemisphere are combined. By calibrating the opacity classes, it is demonstrated that the above data base is complete and uniform. For the total Milky Way band and for the northern, anticenter, and southern sectors, the obscuration percentages of the sky are deduced, as well as average cloud areas, cloud distributions in Galactic longitude and latitude, and mean spatial distances. The differences between the three sectors are discussed and interpreted. The morphological classification of dark clouds according to van den Bergh (1972) fits into the picture of the local Galactic structure. The flattening of the clouds (opacity classes 5 and 6) seems to be somewhat aligned with the Galactic plane, whereas clouds with lower opacities show elongations and alignments with respect to each other and to the galactic plane. By means of the stellar-statistical integral equation, the intrinsic dark cloud area distribution and absolute number densities are calculated. Comparisons are made with the size distribution of molecular clouds. 53 references.

The HEAO A-1 all-sky survey of fast X-ray transients

Abstract: Ten fast X-ray transient sources were discovered in the first complete pass of the sky by the HEAO A-1 Sky Survey Experiment, including 7 relatively faint sources and 3 bright ones. The survey technique and each of the transients are described. Four of the transients are identified with active coronal sources; the remaining ones are unidentified. The statistical properties of the total sample of 10 sources are discussed, including event rates, distribution on the sky, and the number-flux distribution.

A comprehensive sky radiance data base

Abstract: This paper reports the availability and format of an extensive set of sky radiance measurements. These data were collected at the University of Toronto as a step toward satisfying the need for a sound general description of the angular distribution in the sky of the diffuse sky radiance for the full range of sky conditions. The data set includes approximately 90 000 narrow field-of-view sky radiance measurements spanning the 13 month period from February 1983 to February 1984 and covers the full range of sky conditions encountered at Toronto over the year. The data have been subjected to quality control editing and are now available from the authors on magnetic computer tape. These data can be used in the development of analytical models of the sky radiance distributions in time and position. Well founded models of the diffuse radiance distribution are particularly important in solar engineering since up to half the solar radiance incident on a collection surface is diffuse, and since angle of incidence is a significant factor. The authors are anxious to make this data base available to those with interests in developing such models or in using it for other purposes.

Kelvin on an old, celebrated hypothesis

Abstract: Lord Kelvin in 1901 tested an “old and celebrated hypothesis” that if we could see far enough into space the whole sky would be occupied with stellar disks all of perhaps the same brightness as the Sun. Kelvin was the first to solve quantitatively and correctly the riddle of a dark night sky, a riddle that had been previously solved qualitatively by Edgar Allan Poe, and is now known as Olbers' paradox.

The characterization and modelling of the diffuse sky radiance

Abstract: Results of a detailed analysis of a large set of sky radiance measurements taken in 1982 by the SKYSCAN system are presented. The SKYSCAN system, an automated robot system built and operated at the University of Toronto, takes narrow field-of-view sky radiance measurements with a precision and frequency adequate for most analysis and modelling purposes. The analysis of these data has confirmed the supposition that clouds strongly affect the distribution of the diffuse radiance. It has shown that the spatial distribution of the radiance for skies categorized according to cloud type and amount is statistically influenced in an orderly manner by the presence of the clouds. The data appeared to be generally compatible with a model of the form of the Three-Component Continuous Distribution (TCCD) model introduced by Hooper and Brunger, and suggested that successful descriptors of the sky radiance could be developed based on that model, or on similar formulations.

The NASA SETI program at JPL

Abstract: The NASA All Sky Survey strategy for detecting signals of extraterrestrial origin is examined. The All Sky Survey observes the entire celestial sphere, covers a frequency range of 1-10 Ghz, and has spatial direction and frequency coverage. The survey time is dependent on the area of the sky surveyed, the telescope diameter, scan rate, the angular separation of the scans, the bandwidth of the receiver, and the frequency coverage. The distance at which a transmitter can be detected for a given sensitivity is analyzed. The interaction between scan strategy and various accumulation strategies and its effect on the sensitivity of the survey are investigated using field tests. A diagram of the SETI Sky Survey observational system is provided.

An Automated South Pole Stellar Telescope

Abstract: An 8-cm optical telescope is constructed for use at the south pole It is designed to make photoelectric observations of selected stars continuously through an austral winter The automated operation is controlled by a computer The aim is to study the variability of the star γ2 Velorum as well as the condition of the polar sky and the performance of the instrument

The IR emission of the Orion-Monoceros molecular clouds

Abstract: The present investigation is based on both IRAS and Columbia CO observations of a region (200° ≤ 1 ≤ 225° and -25° ≤ b ≤ -7.5°) which includes all molecular clouds observed by Maddalena et al. (1985), except the one around λ Ori. The IR emission of the sky comes from dust in molecular clouds, H II regions, the diffuse atomic interstellar gas, and the solar system. Consequently, to get the IR emission radiated by the Orion-Monoceros molecular clouds and HII regions, the zodiacal and atomic gas contributions must be subtracted from the IRAS data. Since outside the molecular clouds the IR emission correlates well with the integrated H I emission (Boulanger et al. 1986), the atomic gas contribution is modelled from the HI data. The zodiacal light is modelled by a wedge in ecliptic coordinates fitted over all map pixels outside the molecular clouds.