Twilight and daytime colors of the clear sky

Digital image analysis of the cloudless sky's daytime and twilight chromaticities challenges some existing ideas about sky colors. First, although the observed colors of the clear daytime sky do lie near the blackbody locus, their meridional chromaticity curves may resemble it very little. Second, analyses of twilight colors show that their meridional chromaticity curves vary greatly, with some surprising consequences for their calorimetric gamuts.     

Radiative properties of snow for clear sky solar radiation

Spectral and integrated radiative properties (reflection, transmission, and the rate of heating) of finegrained wind-packed snow typical of subpolar regions are studied through a model taking into account surface reflection and volumetric multiple scattering. The surface reflection is modeled by a bidirectional reflectance distribution function applicable to powdered dielectric material. For the volumetric multiple scattering, the radiative transfer equation designed for strongly asymmetric scattering is solved. All multiple scattering parameters (single scattering albedo, various moments of the scattering phase function, and optical depth) are related to measurable physical characteristics (density, grain size, and the absorption coefficient of pure ice).Parameterized atmospheric spectral transmission coefficients for scattering and absorption by aerosols and gases are used to obtain the direct and diffuse components of solar flux, incident on the snow-cover. Calculated values of spectral and integrated visible and near infrared reflection and flux attenuation coefficients of snow are compared with observations. The rate of radiative heating at different depths within the snowcover is calculated from the net flux divergence. It is shown that the conventional method of calculating this rate using measured bulk extinction coefficients grossly underestimates the amount of heating within the top few millimeters. This study provides a better overall understanding of the radiative properties of snow under clear sky conditions in terms of the physical characteristics of the snowcover.     

Comment on “radiative properties of snow for clear sky solar radiation”

Choudhury's (1981) addition of a special “surface reflection” term to a theoretical model for snow albedo is both unnecessary and incorrectly formulated.     

Comparison between measurements and modeling of UV-B irradiance for clear sky: a case study

We compare the TJV-B global and direct irradiances computed with a radiative transfer model (discrete ordinate method) and measured during a European intercomparison campaign in Greece in July 1991, with clear sky. The agreement between the model and the measurements is within 6%. The sensitivity of the model to the accuracy of the input parameters as well as the potential of modeling for instrument calibration is discussed.     

Images from the sky

Space-based imagery has, until recently, been available primarily to high-level government leaders and subordinate agencies operating within the government. Rapid advances in communications technology and satellite remote sensing has made it possible for the individual citizen to purchase remote sensing imagery of virtually any area on the globe. No longer the private domain of governments, imagery is readily available to corporate organizations with huge investments in forestry, minerals, crops, or other natural resources. More importantly, imagery can be obtained without the permission of the sensed nation. For example, imagery produced by SPOT Imaging of France was used extensively by the coalition forces and the United States Air Force before and during Operations Desert Shield and Storm. Corporations and governments are not the only groups eager to take advantage of this new technology. One only needs to watch the evening weather report for a glimpse of how low-to-medium resolution satellite imagery is used by the news media to predict and track weather patterns around the globe. For the media to take full advantage however, satellite resolution must be reduced to five meters or less. This paper reviews the history of commercial satellite remote sensing, as well as the U.S. government's highly classified KH-series satellites, and explores the future impact of “Mediasat” with regard to its use by the news media. The satellite images that play an increasing role in TV news and weather reports have a short but revealing history, and are part of a potentially even more revolutionary news reporting technology. What follows is a brief attempt to assess this history and the possible future in terms of technological developments, commercial applications, and social applications.     

Colors of the daytime overcast sky

Time-series measurements of daylight (skylight plus direct sunlight) spectra beneath overcast skies reveal an unexpectedly wide gamut of pastel colors. Analyses of these spectra indicate that at visible wavelengths, overcasts are far from spectrally neutral transmitters of the daylight incident on their tops. Colorimetric analyses show that overcasts make daylight bluer and that the amount of bluing increases with cloud optical depth. Simulations using the radiative-transfer model MODTRAN4 help explain the observed bluing: multiple scattering within optically thick clouds greatly enhances spectrally selective absorption by water droplets. However, other factors affecting overcast colors seen from below range from minimal (cloud-top heights) to moot (surface colors).     

Atmospheric ozone and colors of the Antarctic twilight sky

Zenith skylight is often distinctly blue during clear civil twilights, and much of this color is due to preferential absorption at longer wavelengths by ozone's Chappuis bands. Because stratospheric ozone is greatly depleted in the austral spring, such decreases could plausibly make Antarctic twilight colors less blue then, including at the zenith. So for several months in 2005, we took digital images of twilight zenith and antisolar skies at Antarctica's Georg von Neumayer Station. Our colorimetric analysis of these images shows only weak correlations between ozone concentration and twilight colors. We also used a spectroradiometer at a midlatitude site to measure zenith twilight spectra and colors. At both locations, spectral extinction by aerosols seems as important as ozone absorption in explaining colors seen throughout the twilight sky.     

谁持彩练当空舞--访浙宁印刷包装机械有限公司总经理於金龙

三伏时节,冒着火辣辣的太阳,我慕名踏进了海宁市浙宁印刷包装机械有限公司的大门,专程拜访了仰慕已久的印刷包装机械行家,公司董事长兼总经理於金龙. 於总的办公室不大,但相邻的两个接待室宽敞舒适,凉爽如秋,表现了主人严己宽人的厚道.於总自驾小轿车,正陪同2位远道而来的客户入厂,在客人品茗休息的片刻,访谈开始了.     

Upper-limit luminance for the surface-color mode appearance

A series of experiments were carried out to reveal determinants for the mode of color appearance by measuring the upper-limit luminance of a color chip for the surface-color mode. We used a CRT color monitor to present test and surround stimuli in the surface-color mode. The stimuli were composed of a three-by-three array of color chips on a gray background with a white frame. The observer increased the luminance of a center test color until it just ceased to appear in the surface-color mode. Our results show that this upper-limit luminance was different among test colors, but their brightnesses, calculated from the luminance and brightness/ luminance values, were almost the same and were slightly below the brightness of the white frame. The existence of the surrounding color chips affected the results, but their sizes and spatial arrangements did not. When all of the luminances of the surrounds changed equally, the upper-limit luminances of the test colors for the surface-color mode appearance changed by the same ratio. This result indicates that the brightness of a target was a determinant for selecting the mode of color appearance and that the brightest surround stimulus acted as a cue for determining the judgment.     

Blur tolerance for luminance and chromatic stimuli

We investigated the blur tolerance of human observers for stimuli modulated along the isoluminant red-green, the isoluminant yellow-blue, and the luminance (black-white) direction in color space. We report the following results: (i) Blur difference thresholds for red-green and luminance stimuli (of equal cone contrast) are very similar and as low as 0.5 min of visual angle; for yellow-blue the lowest blur thresholds are much higher (1.5 min of visual angle). (ii) The smallest blur thresholds are found for slightly blurred square waves (reference blur of 1 arc min) and not for sharp edges. (iii) Blur thresholds for red-green and black-white follow a Weber law for reference (pedestal) blurs greater than the optimum blur. (iv) Using the model proposed by Watt and Morgan [Vision Res. 24, 1387 (1984)] we estimated the internal blur of the visual system for the black-white and the red-green color directions and arrived at the following estimates: 1.2 arc min for black-white stimuli at 10% contrast and 0.9 arc min for red-green stimuli at 10% cone contrast. Blur tolerance for yellow-blue is independent of external blur and cannot be predicted by the model. (v) The contrast dependence of blur sensitivity is similar for red-green and luminance modulations (slopes of -0.15 and -0.16 in log-log coordinates, respectively) and slightly stronger for yellow-blue (slope = -0.75). Blur discrimination thresholds are not predicted by the contrast sensitivity function of the visual system. Our findings are useful for predicting blur tolerance for complex images and provide a spatial frequency cutoff point when Gaussian low-pass filters are used for noise removal in colored images. They are also useful as a baseline for the study of visual disorders such as amblyopia.     

Selective cone suppression by the L-M- and M-L-cone-opponent mechanisms in the luminance pathway.

We investigated how transient changes of background color influence the L- and M- (long- and middle-wavelength-sensitive-) cone signals in the luminance pathway. Motion identification thresholds were measured for a drifting sinusoidal grating (1 cycle/deg) modulated along different vector directions in L- and M-cone contrast space. The color of a central 4-deg-diameter region was briefly altered (500 ms) by incrementing or decrementing either L- or M-cone excitation. Incrementing L-cone and decrementing M-cone excitation produced a field that appeared reddish relative to the yellow surround. Likewise, incrementing M-cone and decrementing L-cone produced a field that appeared greenish. Motion identification thresholds were obtained on the yellow field following the brief color transitions. The results show that the threshold for the L-cone direction was selectively elevated by the background substitution of incrementing L-cone and decrementing M-cone excitation (shift toward reddish color). The same substitution, however, did not affect the threshold in the M-cone direction. Similarly, the threshold for the M-cone direction was selectively elevated by the background substitution of incrementing M-cone, decrementing L-cone excitation (shift toward greenish) without affecting the threshold in the L-cone direction. Experiments using the motion quadrature paradigm confirmed that these effects occur within the luminance mechanisms. These results indicate that the activation of L-on plus M-off signals suppresses the L-cone signal and that the activation of L-off plus M-on signals suppresses the M-cone signals in the luminance pathway. We propose a retinal model based on the experimental results.     

Frequency and phase contributions to the detection of temporal luminance modulation

Observers detected a temporally modulated luminance pattern embedded in dynamic noise. A Gabor function with a carrier frequency, in separate conditions of 0, 1.56, or 3.12 Hz, modulated signal contrast. Classification images were constructed in the time, temporal frequency, and temporal phase domains. As stimulus frequency increased, amplitudes of the phase images decreased and amplitudes of the frequency images increased, indicating a corresponding shift in the observers' criteria. The reduced use of phase attenuated time-domain images from signal-absent trials, but physical interactions between signal and noise components tended to preserve time-domain images from signal-present trials. The results illustrate a frequency-dependent strategy shift in detection that may reflect a degree of stimulus uncertainty in the time domain.     

Modeling of the time characteristics of luminance in thin-film electroluminescent structures

The pattern of luminance waves in thin-film electroluminescent structures was modeled and the duration of transient lighting processes in these systems was determined. A principle of the indicator display control is formulated which provides for a maximum luminance of the light-emitting devices.     

New formulas for computing luminance and color temperatures

Conclusions 1. On the basis of Planck's radiation law precise formulas have been obtained for computing the true temperature of a nonblack body from its luminance temperature and emissivity factor. 2) Precise formulas have also been obtained for calculating the color temperature of a nonblack body from its luminance temperature and color emissivity factor. 3) Approximate formulas have been obtained which relate the color and true temperatures of an actual body and provide the computation of the color and true temperatures with the required precision. 4) Approximate formulas have been obtained which provide the computation of the color temperature of a nonblack body with the required precision from two measured luminance temperatures of this body.Translated from Izmeritel'naya Tekhnika, No. 8, pp. 35–37, August, 1967.     

Flash masking and facilitation by nearby luminance perturbations

Light-adapted foveal luminance increment thresholds were measured for white photopic targets of 1.5-arc min diameter and 220-ms duration. We aimed to learn about the properties of mechanisms that subserve the detection of these targets. To study this subject we developed a noise probe technique that inserts noise close to the site of the stimulus. Threshold is more than doubled when zero-mean luminance noise is placed at a pair of flanking spots in the horizontal meridian centered on the test spot and 1.5 arc min distant. The detection mechanisms thus has a broad field, since noise effects persist at 5-arc min separation. The masking effect increases when the noise is in antiphase at the two flanking spots. Neither even- nor odd-symmetric mechanisms are able to explain these findings, regardless of whether linear or nonlinear processing is employed. The target detection may be mediated in part by a motion-sensitive mechanism.     

Simultaneous masking interactions between chromatic and luminance gratings

Simultaneous masking using test and mask gratings composed of isochromatic luminance variations and isoluminant chromatic variations was studied. Masking of chromatic gratings by chromatic gratings shows less spatial-frequency specificity than does masking of luminance gratings by luminance gratings. Luminance gratings mask chromatic gratings of identical space-average luminance and chromaticity little and only when the spatial frequencies of the test and mask gratings are similar. Chromatic gratings, however, profoundly mask luminance gratings with a degree of spatial-frequency specificity akin to that of luminance-luminance masking. The insensitivity of the luminance-color masking results to the relative phase of the chromatic and luminance gratings indicates that the observed asymmetry is not due to local interactions.     

An anomaly in luminance amplitude near the melting point of platinum

We describe an experimental study of oscillations in the temperature of platinum specimens heated by electron bombardment in a region including the melting point.