Daylight Sky Research Articles

Flow of Light: Balancing Directionality and CCT in the Office Environment

ABSTRACT Human perception and vision have evolved in response to dynamic daylight, a combination of radiation from direct sunlight and diffuse skylight, which has created a flow of variations in light, in terms of direct:diffuse distribution, intensities and spectrum. This study investigates the qualities of the flow of light in an office after adding ceiling-mounted spotlights (32° tilt angle) to traditional diffuse ceiling panels. The intention is to create a flow of task light – a light-zone at each work-plane – complementing the directionality of the natural daylight inflow from the windows. An experiment was carried out in an office, in two parts. Four ratios of direct:diffuse light were tested by 30 people. Then one ratio was tested in five combinations of high, neutral and low color temperatures by 15 people in two daylight situations: overcast and clear sky. The visual light quality and perceived atmosphere of the office environment was tested through questionnaires, reaction cards and semi-structured interviews. The direct flow of light is recommended to be more than 15% of the total illuminance at the work-plane to provide the distinct visual appearance of modeling and a cozier atmosphere, which is preferable for socializing, and less than 45% to avoid glare and high contrast for visual tasks. Direct warm and diffuse cool lighting were perceived as the most natural but were not always preferred. There is an indication of slight preference for cooler ambient lighting in clear sky situations and warmer ambient lighting in overcast situations. Especially the preference in relation to sky conditions needs to be further investigated. A field study will implement these findings in a double dynamic lighting concept responding to daylight level and sky character. Strong individual preferences for combinations of color temperatures was identified, this open up new research areas for personalized flows of light in future dynamic lighting designs.

On the feasibility of using a laser guide star adaptive optics system in the daytime

We investigate the use of ultra-narrow band interference filters to enable daytime use of sodium laser guide star (LGS) adaptive optics systems. Filter performance is explored using theoretical and vendor supplied filter transmission profiles, a modeled daylight sky background, broadband measurements of the daytime sky brightness on Maunakea, and an assumed photon return from the sodium LGS and read noise for the wavefront sensor (WFS) detector. The critical parameters are the bandpass of the filter, the out-of-band rejection, and the peak throughput at the wavelength of the LGS light. Importantly, a systematic trade between these parameters leads to potentially simple solutions enabling daytime observations. Finally, we simulated the Mid-Infrared Adaptive Optics (MIRAO) system planned for the Thirty Meter Telescope with an end-to-end simulation, folding in daytime sky counts. We find that MIRAO with five sodium LGSs, commercial off-the-shelf filters to suppress the sky background in the LGS WFSs, and a near-infrared natural guide star (K ≤ 13) tip/tilt/focus WFS can attain daytime Strehl ratio values comparable to those at night.

Reconstructing atmospheric cloud particles from multiple fisheye cameras

In this article we address the problem of accurately reconstructing the 3D position and short-term motion of atmospheric cloud using passive instruments. The proposed system is based on multiple fish-eye cameras that capture daylight sky images in a synchronous manner. After introducing the inherent difficulties of the task, we propose novel methods for improving robustness and accuracy in the given application scenario. We emphasize the effectiveness of a multi-view (in our example a trinocular) camera system over a stereo camera pair with respect to reliability and precision of both 3D position and motion reconstructions. During the evaluation of the methods we address limitations and possible effective parametrization of the system, including the positioning of the cameras.

Daylight operation of a sodium laser guide star for adaptive optics wavefront sensing

We report contrast measurements of a sodium resonance guide star against the daylight sky when observed through a tuned magneto-optical filter (MOF). The guide star was created by projection of a laser beam at 589.16 nm into the mesospheric sodium layer and the observations were made with a collocated 1.5-m telescope. While MOFs are used with sodium light detecting and ranging systems during the day to improve the signal-to-noise ratio of the measurements, they have not so far been employed with laser guide stars to drive adaptive optics (AO) systems to correct atmospherically induced image blur. We interpret our results in terms of the performance of AO systems for astronomy, with particular emphasis on thermal infrared observations at the next generation of extremely large telescopes now being built.

The daylight sky and Avogadro’s number

Two methods for estimating Avogadro’s number from the observation of the daylight sky are presented, both suitable for undergraduate students. One is very simple and based on simple naked-eye observation, and the other exploits a common digital camera as a photometer.

Coldness Triggers Northward Flight in Remigrant Monarch Butterflies

Each fall, eastern North American monarch butterflies (Danaus plexippus) migrate from their northern range to their overwintering grounds in central Mexico. Fall migrants are in reproductive diapause, and they use a time-compensated sun compass to navigate during the long journey south. Eye-sensed directional cues from the daylight sky (e.g., the horizontal or azimuthal position of the sun) are integrated in the sun compass in the midbrain central complex region. Sun compass output is time compensated by circadian clocks in the antennae so that fall migrants can maintain a fixed flight direction south. In the spring, the same migrants remigrate northward to the southern United States to initiate the northern leg of the migration cycle. Here we show that spring remigrants also use an antenna-dependent time-compensated sun compass to direct their northward flight. Remarkably, fall migrants prematurely exposed to overwintering-like coldness reverse their flight orientation to the north. The temperature microenvironment at the overwintering site is essential for successful completion of the migration cycle, because without cold exposure, aged migrants continue to orient south. Our discovery that coldness triggers the northward flight direction in spring remigrants solves one of the long-standing mysteries of the monarch migration.

The Color of the Sky

The color of the sky in day-time and at twilight is studied by means of spectroscopy, which provides an unambiguous way to understand and quantify why a sky is blue, pink, or red. The colors a daylight sky can take primarily owe to Rayleigh extinction and ozone absorption. Spectra of the sky illuminated by the sun can generally be represented by a generic analytical expression which involves the Rayleigh function , ozone absorption, and, to a lesser extend, aerosol extinction. This study is based on a representative sample of spectra selected from a few hundred observations taken in different places, times, and dates, with a portable fiber spectrometer.

A Study of Solar Radiation Daylight Illuminance and Sky Luminance Data Measurements for Hong Kong

Solar radiation, outdoor illuminance and sky luminance data are important for accurate cooling bad determinations and daylighting evaluations. Systematic data measurements are regarded as the most effective and accurate method of settingup reliable databases. This paper describes solar data measurements made at the City University of Hong Kong and reports some of the general findings. The daily and hourly average values of solar radiation and daylight illuminance are computed and presented using graphical methods. Sky luminance distributions for different typical sky conditions including overcast, clear and intermediate are studied and described. Implications for energy-efficient building designs are discussed

Spectral fine‐structure in the polarisation of skylight

High‐spectral resolution polarisation measurements of the clear daylight sky reveal an unexpected rich spectral fine‐structure in the polarisation super‐imposed on the more familiar broad‐band continuum. This polarisation spectrum shows the spectral fingerprints of scattering and absorption processes which is illustrated with radiative transfer calculations. In particular, radiative transfer calculations suggest the potential of high‐spectral resolution polarimetry within absorption bands for remote sensing of aerosol vertical distribution, which is relevant to the Earth's radiation budget and ozone hole chemistry. In addition, the presence of spectral fine‐structure in the polarisation of skylight also has relevance to radiance measurements performed with polarisation sensitive spectrometers, such as the space‐based GOME instrument.

Improved Acquisition of Targets Seen Against the Daylight Sky

Closed-circuit television equipment can aid an observer in visual acquisition of weak targets seen against a daylight sky. Theoretical and practical factors are reviewed, and results are shown.

Gezeiteneinfluss des mondes auf den tages- und monatsgang der himmelslicht-polarisation

The variations of maximum polarization of the daylight sky have a semi-mensual period which is shown to be real. A semi-diurnal periodic variation is noticeably weaker and found to be fictitious. It can be concluded that these tidal fluctuations of optical polarization occur in a system which either possesses inertia or is situated outside the Earth's atmosphere. Therefore air density may be disregarded in favour of the concentration of dust in the upper atmosphere or in interplanetary space near the Earth.

Vital Staining in Normal Eyes and in Keratoconjunctivitis Sicca

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Detection of high-altitude explosions by observation of air fluorescence

X-rays radiated from a nuclear explosion in space excite fluorescence of air molecules in the upper atmosphere. The fluorescence has a characteristic space-time signature which can be recognized from the ground even against a bright daylight sky. The criteria determining detectability and range are discussed, along with a number of practical systems of detection. False alarms, caused mainly by lightning flashes, can cause excessive triggering of single-station detectors unless discrimination techniques are employed. They also compel reliance on a net of stations for reliable recognition of true, nuclear explosion induced fluorescence.

Astronomy from satellites and space vehicles

Space experiments of interest to astronomy fall naturally into three groups. First, the elimination of the earth's atmosphere as a barrier to observation exposes to view the entire electromagnetic spectrum of radiation from extraterrestrial sources. It also permits the investigation both of the faint outer extensions of the solar atmosphere, which are now obliterated by the bright daylight sky, and of the weak radiations from faint stars and nebulas which are masked by radiation from atoms and molecules in the earth's upper atmosphere. Second, the advent of satellites and space vehicles makes possible a whole series of experiments and observations which are absolutely unique and which can test the foundations of physical theories.

Explanation of the Brightness and Color of the Sky, Particularly the Twilight Sky

By use of the Rayleigh scattering theory for pure air, primary scattering and known upper air densities from rockets, the brightness of the zenith twilight sky was calculated and values were obtained two to four times greater than those observed at Sacramento Peak, New Mexico ( J. Opt. Soc. Am.42, 353 ( 1952). The attenuation of the atmosphere there was observed to be twenty percent above that of the Rayleigh theory, and the ozone thickness was measured to be about 2.2 mm. When the absorption of the Chappuis band of ozone in the visible spectrum was added to the Rayleigh theory, the calculated sky brightness came into agreement with observation for solar depression angles below the horizon from about 0° to 6°. For the sun below 7° the calculated zenith sky brightness fell rapidly below the observed brightness, showing that primary scattering in the atmosphere above about 60 km does not contribute appreciably to the brightness, as has long been known ( J. Opt. Soc. Am.28, 227, 1938).Calculation showed that during the day the clear sky is blue according to Rayleigh, and that ozone has little effect on the color of the daylight sky. But near sunset and throughout twilight ozone affects the sky color profoundly. For example, in the absence of ozone the zenith sky would be a grayish green-blue at sunset becoming yellowish in twilight, but with ozone the zenith sky is blue at sunset and throughout twilight (as is observed), the blue at sunset being due about 13 to Rayleigh and 23 to ozone, and during twilight wholly to ozone.

The Brightness and Polarization of the Daylight Sky at Altitudes of 18,000 to 38,000 Feet above Sea Level

The brightness and polarization of the daylight sky over Arizona and Southern California were measured from B-29 aircraft while flying at altitudes of 18,000, 27,500, 34,000, and 38,000 feet above sea level. Comparison of the results with the Tousey and Hulburt theory of sky brightness showed that the theory agreed fairly well with the experimental data for all altitudes of observation, for all points in the sky more than 30° from the horizon, and for scattering angles greater than about 40° from the sun, when an atmospheric attenuation coefficient of 0.017/km was assumed. The values of sky brightness observed at scattering angles within 30° of the sun indicated that large scattering particles were present in the atmosphere overhead at all altitudes of observation including 38,000 feet.

The Visibility of Stars in the Daylight Sky

From the theory of the passage of light through a telescope and into the eye, formulas are derived for the improvement in the visual threshold of a point source, as a star, viewed in a field of light, as the sky, in terms of telescope parameters, as magnification, transmission, and entrance and exit pupils. For fields of the brightness of the daylight sky the first parameter is the most important and the others are of less importance. Observations of artificial stars in the laboratory and of real stars in the daylight sky are given, which were in fair accord with the theory.Charts and tables are presented of the visual thresholds of stars in various parts of a clear daylight sky in terms of the stellar magnitude, the magnification of the telescope, the use of a polarizer, the altitude of the observer above the surface, and the altitude of the sun above the horizon.New data are given of extra-foveal thresholds of point sources in a field of brightness 360 ft.-c−2.

Brightness and Polarization of the Daylight Sky at Various Altitudes above Sea Level

Brightness and Polarization of the Daylight Sky at Various Altitudes above Sea Level

Temperatures of Stars in Planetary Nebulæ

DURING the past summer, when working at the Dominion Astrophysical Observatory in Victoria, I secured some quantitative data as regards the luminosity of planetary nebulae. I did not have occasion to elaborate the material until this winter, which I spent in Pasadena, on my leave of absence from the University of Washington at Seattle.1 The method consists of taking a slitless spectrogram of the nebula with its central star, and impressing on the same film on which this spectrum is taken a series of comparison spectra of varying intensities and of the same time of exposure as the nebula. The comparison spectra are obtained from a sensitometer having the daylight sky as a source, a real image of the sensitometer patches being formed at the slit of the spectrograph. The photographs yield the total intensity of each monochromatic picture of the nebular envelope in terms of the intensity per frequency unit of the adjoining star spectrum of approximately the same wave-length.