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 b...

The issues of realistically rendering naturally illuminated scenes at night are examined. This requires accurate models for moonlight, night skylight, and starlight. In addition, several issues in tone reproduction are discussed: eliminatiing high frequency information invisible to scotopic (night vision) observers; representing the flare lines around stars; determining the dominant hue for the...

Migrating monarch butterflies (Danaus plexippus) use a time-compensated sun compass to navigate from eastern North America to their overwintering grounds in central Mexico. Here we describe the neuronal layout of those aspects of the butterfly's central complex likely to establish part of the internal sun compass and find them highly homologous to those of the desert locust. Intracellular recor...

Neutral points are specific directions in the light field where the three Stokes parameters Q, U, V, and thus the degree of polarization simultaneously go to zero. We have made the first measurement of non-principal-plane neutral points in the upwelling light field in natural waters. These neutral points are located at approximately 40°- 80° nadir angle and between 120° - 160° azimuth to the su...

Microphysical properties and chemical compositions of aerosol particles determine polarized radiance distribution in the atmosphere. In this paper, the influences of different aerosol properties (particle size, shape, real and imaginary parts of refractive index) on Stokes parameters of polarized skylight in the solar principal and almucantar planes are studied by using vector radiative transfe...

To casual observers, haze's visible effects on clear daytime skies may seem mundane: significant scattering by tropospheric aerosols visibly (1) reduces the luminance contrast of distant objects and (2) desaturates sky blueness. However, few published measurements of hazy-sky spectra and chromaticities exist to compare with these naked-eye observations. Hyperspectral imaging along sky meridians...