Scientific publications on "Light & Color in Nature"

Many scientific papers have been published on subjects related to "Meteorological Optics" or "Atmospheric Optics". A significant proportion of these papers have been triggered by a series of international meetings on "Light & Color in Nature" which attracts scientists from around the world who love to observe and explain optical phenomena in their natural environment. It began and continues because these people enjoy getting together to learn from and share with each other. Here is a brief outline of the meeting history:

1) Meteorological Optics, August 28-29, 1978, Keystone, Colorado (coordinated by David Lynch)

2) Atmospheric Optics, January 3-5, 1983, Incline Village, Nevada (coordinated by William Mach & Alistair Fraser)

3) Meteorological Optics, April 2-3, 1986, Honolulu, Hawaii (coordinated by David Lynch)

4) Light & Color in the Open Air, July 11-13, 1990, Washington, D.C.(coordinated by Robert Greenler)

5) Light & Color in the Open Air, June 16-18, 1993, State College, Pennsylvania (coordinated by Craig Bohren)

6) Light & Color in the Open Air, February 10-12, 1997, Santa Fe, New Mexico (coordinated by Ken Sassen)

7) Meteorological Optics, June 6-8, 2001, Boulder, Colorado (coordinated by Stanley Gedzelman)

8) Atmospheric/Meteorological Optics, June 13-17, 2004, Bad Honnef, Germany (coordinated by Michael Vollmer)

9) Light & Color in Nature, June 25-29, 2007, Bozeman, Montana (coordinated by Joseph Shaw)

Following each of these meetings, the Optical Society of America (OSA) has generously published a Feature Issue of one of its peer-reviewed journals. These Feature Issues draw heavily (but not exclusively) on contributions made at these meetings.

Journal of Optical Society of America: August 1979

Journal of Optical Society of America: December 1983

Journal of Optical Society of America A: March 1987

Applied Optics: August 1991

Applied Optics: July 1994

Applied Optics: March 1998

Applied Optics: January 2003

Applied Optics: September 2005

The following very long list shows relevant papers published by the OSA. This list is in reverse chronological order, but you can easily search for key words (e.g "halo" or "rainbow") or for the name of a particular author using the "Find" feature of your web browser.

If you have a subscription to the relevant OSA Journal, you can download a PDF version of each paper by clicking on the link marked OSA Optics InfoBase and then clicking on "View Full Text: PDF". If you do not have such a subscription, you can purchase a PDF version (currently at a price of $15 per paper) by following the same link.

However, the following authors have made their papers freely available via their non-commercial web sites - as indicated by the Free download links.
Javier Hernández-Andrés
Günther Können
Philip Laven
Raymond L. Lee Jr
David K. Lynch
Joe Shaw
Michael Vollmer

Please note that OSA's Copyright Policy states that "Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law".
2007
Selecting algorithms, sensors and linear bases for optimum spectral recovery of skylight Free download
Miguel A. López-Álvarez, Javier Hernández-Andrés, Eva M. Valero, and J. Romero
Journal of the Optical Society of America A, Vol. 24, No. 4, pp. 942-956, April 2007 OSA Optics InfoBase
In a previous work [Appl. Opt. 44, 5688 (2005)] we found the optimum sensors for a planned multispectral system for measuring skylight in the presence of noise by adapting a linear spectral recovery algorithm proposed by Maloney and Wandell [J. Opt. Soc. Am. A 3, 29 (1986)]. Here we continue along these lines by simulating the responses of three to five Gaussian sensors and recovering spectral information from noise-affected sensor data by trying out four different estimation algorithms, three different sizes for the training set of spectra, and various linear bases. We attempt to find the optimum combination of sensors, recovery method, linear basis, and matrix size to recover the best skylight spectral power distributions from colorimetric and spectral (in the visible range) points of view. We show how all these parameters play an important role in the practical design of a real multispectral system and how to obtain several relevant conclusions.

Anomalous celestial polarization caused by forest fire smoke: why do some insects become visually disoriented under smoky skies?
Ramón Hegedüs, Susanne Åkesson, and Gábor Horváth
Applied Optics, Vol. 46, No. 14, p. 2717, May 2007 OSA Optics InfoBase
The effects of forest fire smoke on sky polarization and animal orientation are practically unknown. Using full-sky imaging polarimetry, we therefore measured the celestial polarization pattern under a smoky sky in Fairbanks, Alaska, during the forest fire season in August 2005. It is quantitatively documented here that the celestial polarization, a sky attribute that is necessary for orientation of many polarization sensitive animal species, above Fairbanks on 17 August 2005 was in several aspects anomalous due to the forest fire smoke: (i) The pattern of the degree of linear polarization p of the reddish smoky sky differed considerably from that of the corresponding clear blue sky. (ii) Due to the smoke, p of skylight was drastically reduced (pmax ≤14%, paverage ≤8%). (iii) Depending on wavelength and time, the Arago, Babinet, and Brewster neutral points of sky polarization had anomalous positions. We suggest that the disorientation of certain insects observed by Canadian researchers under smoky skies during the forest fire season in August 2003 in British Columbia was the consequence of the anomalous sky polarization caused by the forest fire smoke.

Recovering fluorescent spectra with an RGB digital camera and color filters using different matrix factorizations Free download
Juan L. Nieves, Eva M. Valero, Javier Hernández-Andrés and Javier Romero
Applied Optics, vol. 46, No. 19, p 4144-4154 (June 2007) OSA Optics InfoBase
The aim of a multispectral system is to recover a spectral function at each image pixel, but when a scene is digitally imaged under a light of unknown spectral power distribution (SPD), the image pixels give incomplete information about the spectral reflectances of objects in the scene. We have analyzed here how accurately the spectra of artificial fluorescent light sources can be recovered with a digital CCD camera. The RGB sensor outputs are modified by the use of successive cut-off color filters. Three algorithms for simplifying the spectra datasets are used: non-negative matrix factorization (NMF), independent component analysis (ICA), a direct pseudo-inverse method, and principal component analysis (PCA). The algorithms are tested using both simulated data and data from a real RGB digital camera. The methods are compared in terms of the minimum rank of factorization and the number of sensors required to derive acceptable spectral and colorimetric SPD estimations; PCA results are also given for the sake of comparison. The results show that all the algorithms surpass PCA when a reduced number of sensors is used. Experimental results suggest a significant loss of quality when more than one color filter is used, which agrees with previous results for reflectances. Nevertheless, an RGB digital camera with or without prefilter is found to provide good spectral and colorimetric recovery of indoor fluorescent lighting and can be used for color correction without need of a telespectroradiometer.
2006
 Influence of crystal tilt on solar irradiance of cirrus clouds
Susann Klotzsche and Andreas Macke
Applied Optics, Vol. 45, No. 5, p. 1034, February 2006 OSA Optics InfoBase
The single and multiple scattering and absorption properties of hexagonal ice columns with different degrees of particle orientation are modeled in the solar spectral range by means of a ray-tracing singlescattering code and a Monte Carlo radiative-transfer code. The scattering properties are most sensitive to particle orientation for the solar zenith angles of 50° (asymmetry parameter) and 90° (single-scattering albedo). Provided that the ice columns are horizontally oriented, the usual assumption of random orientation leads to an overestimation (underestimation) of the reflected (transmitted) solar broadband radiation at high Sun elevation and to an underestimation (overestimation) at medium solar zenith angles. The orientation effect is more (less) pronounced in scattering and transmission (absorption) for smaller ice crystals.

Attenuation and impulse response for multiple scattering of light in atmospheric clouds and aerosols
Adrian C. Selden
Applied Optics, Vol. 45, No. 13, p. 3144, May 2006 OSA Optics InfoBase
Model phase functions for atmospheric clouds and aerosols typically comprise a narrow forward lobe (corona), a broad diffuse background, and a narrow backscattering peak (glory), which can reach relatively high values, especially for polyhedral scattering particles, such as hexagonal ice columns and plates. The influence of these three major components on the asymptotic and transient attenuation of the scattered light is compared for several analytic phase functions to assess the dependence of radiative transfer in clouds and aerosols on the choice of phase function. The impulse response (temporal evolution of the angular intensity distribution) is sensitive to the higher moments of the phase function and could prove to be a useful technique for inferring the optical scattering parameters of clouds and aerosols.

Celestial polarization patterns during twilight
Thomas W. Cronin, Eric J. Warrant, and Birgit Greiner
Applied Optics, Vol. 45, No. 22, p. 5582, August 2006 OSA Optics InfoBase
Scattering of sunlight produces patterns of partially linearly polarized light in the sky throughout the day, and similar patterns appear at night when the Moon is bright. We studied celestial polarization patterns during the period of twilight, when the Sun is below the horizon, determining the degree and orientation of the polarized-light field and its changes before sunrise and after sunset. During twilight, celestial polarized light occurs in a wide band stretching perpendicular to the location of the hidden Sun and reaching typical degrees of polarization near 80% at wavelengths > 600 nm. In the tropics, this pattern appears ∼ 1 h before local sunrise or disappears ∼ 1 h after local sunset (within 10 min after the onset of astronomical twilight at dawn, or before its end at dusk) and extends with little change through the entire twilight period.

Polarization properties of Scarabaeidae
Dennis H. Goldstein
Applied Optics, Vol. 45, No. 30, p. 7944, October 2006 OSA Optics InfoBase
Beetles of the scarab family are known to reflect circularly polarized light from incident unpolarized light. They are unusual in that there are many animals that use polarized light in some form and several that actually create it, but there are few examples of the creation of circularly polarized light by animals. Scarabs have been measured with a spectropolarimetric reflectometer and are found to reflect light that is generally left-hand circularly polarized. Previous work is summarized, and what is believed to be new measurements of several scarab specimens are presented.

Effect of aerosol microphysical properties on polarization of skylight: sensitivity study and measurements
Eyk Boesche, Piet Stammes, Thomas Ruhtz, Réne Preusker, and Juergen Fischer
Applied Optics, Vol. 45, No. 34, p. 8790, December 2006 OSA Optics InfoBase
We analyze the sensitivity of the degree of linear polarization in the Sun’s principal plane as a function of aerosol microphysical parameters: the real and imaginary parts of the refractive index, the median radius and geometric standard deviation of the bimodal size distribution (both fine and coarse modes), and the relative number weight of the fine mode at a wavelength of 675 nm. We use Mie theory for single-scattering simulations and the doubling–adding method with the inclusion of polarization for multiple scattering. It is shown that the behavior of the degree of linear polarization is highly sensitive to both the small mode of the bimodal size distribution and the real part of the refractive index of aerosols, as well as to the aerosol optical thickness; whereas not all parameters influence the polarization equally. A classification of the importance of the input parameters is given. This sensitivity study is applied to an analysis of ground-based polarization measurements. For the passive remote sensing of microphysical and optical properties of aerosols, a ground-based spectral polarization measuring system was built, which aims to measure the Stokes parameters I, Q, and U in the visible (from 410 to 789 nm) and near-infrared (from 674 to 995 nm) spectral range with a spectral resolution of 7 nm in the visible and 2.4 nm in the near infrared. We compare polarization measurements taken with radiative transfer simulations under both clear- and hazy-sky conditions in an urban area (Cabauw, The Netherlands, 51.58° N, 4.56° E). Conclusions about the microphysical properties of aerosol are drawn from the comparison.
2005
Feature issue: Applied Optics, September 2005
Light and color in the open air: introduction to the feature issue
Charles L. Adler and Raymond L. Lee, Jr.
Applied Optics, Volume 44, Issue 27, 5623 September 2005 OSA Optics InfoBase

Atmospheric refraction: a history
Waldemar H. Lehn and Siebren van der Werf
Applied Optics, Volume 44, Issue 27, 5624 September 2005 OSA Optics InfoBase
We trace the history of atmospheric refraction from the ancient Greeks up to the time of Kepler. The concept that the atmosphere could refract light entered Western science in the second century B.C. Ptolemy, 300 years later, produced the first clearly defined atmospheric model, containing air of uniform density up to a sharp upper transition to the ether, at which the refraction occurred. Alhazen and Witelo transmitted his knowledge to medieval Europe. The first accurate measurements were made by Tycho Brahe in the 16th century. Finally, Kepler, who was aware of unusually strong refractions, used the Ptolemaic model to explain the first documented and recognized mirage (the Novaya Zemlya effect).

Speculations on the possible causes of the Whymper apparition Free download
Cedric John Hardwick and Jason C. Knievel
Applied Optics, Volume 44, Issue 27, 5637 September 2005 OSA Optics InfoBase
During the first ascent of the Matterhorn, a remarkable optical effect comprising three crosses surrounded by a great arch was observed by Edward Whymper, the British mountaineer. The authors review previous published explanations of the apparition. There are no photographs, only a woodcut and sketch, so the size of the apparition is not known, and it is not possible to make a definitive conclusion about what caused it. A fogbow and ice crystal arcs could have produced a circle and crosses in a direction consistent with the apparition. Some simulations are presented; one has a form approximating Whymper’s sketch. However, while this simulation used a crystal type that can occur, it required an unusual alignment that would be very rare.

The Toboggan Sun
Wayne P. S. Davidson and Siebren Y. van der Werf
Applied Optics, Volume 44, Issue 27, 5644 September 2005 OSA Optics InfoBase
Special variants of the Novaya Zemlya effect may arise from localized temperature inversions that follow the height profile of hills or mountains. Rather than following its natural path, the rising or setting Sun may, under such circumstances, appear to slide along a distant mountain slope. We found early observations of this effect in the literature by Willem Barents (1597) and by Captain Scott and H. G. Ponting (1911). We show recent photographic material of the effect and present ray-tracing calculations to explain its essentials.

Variability of observed low-altitude astronomical refraction (LAAR) from different geographic locations: progress toward a global map of LAAR variability
Russell D. Sampson, Edward P. Lozowski, and Hans G. Machel
Applied Optics, Volume 44, Issue 27, 5652 September 2005 OSA Optics InfoBase22:36 03/11/2007
The variability of the astronomical refraction of the setting Sun as measured from Holetown, Barbados, West Indies, is compared to sunset refraction measured from Edmonton, Alberta, during the same time of year. At about 13 °N latitude, Holetown experiences a marine tropical climate, while Edmonton (53 °N) has a subarctic continental climate. The 17 sunsets recorded from Holetown between 27 December 2003, and 21 February 2004, and between 25 and 30 December 2004, show a mean astronomical refraction of 0°.475 and standard deviation of 0°.012. The 26 sunsets recorded from Edmonton between 31 December 1992, and 17 February 1993, show a mean astronomical refraction of 0°.699 and standard deviation of 0°.118. The Barbados mean is significantly less than the Edmonton mean, while the variability of the Barbados data is an order of magnitude less than the Edmonton data. The variability of refraction appears to be strongly correlated with the variability in the surface vertical temperature gradient recorded on the same day as the sunset observations. This suggests that mapping of the geographic distribution of low-altitude astronomical refraction variability could be based on climatology of the surface vertical temperature gradient.

Effects of absorbing particles on coronas and glories Free download
Michael Vollmer
Applied Optics, Volume 44, Issue 27, 5658 September 2005 OSA Optics InfoBase
Light scattering from small particles changes if the particles are absorbing. Whereas the effect is small for coronas and Bishop’s ring, glories show pronounced attenuation with increasing absorption. Results indicate suitable wavelength regions for studies of glory scattering from cloud tops. The behavior of core–shell particles could have applications for studying the atmosphere of Venus; in addition it provides more insight into the simple ray-path model of the glory.

Atmospheric glories: simulations and observations Free download
Philip Laven
Applied Optics, Volume 44, Issue 27, 5667 September 2005 OSA Optics InfoBase
Mie theory can be used to provide full-color simulations of atmospheric glories. Comparison of such simulations with images of real glories suggests that most glories are caused by spherical water droplets with radii between 4 and 25 µm. This paper also examines the appearance of glories taking into account the size of the droplets and the width of the droplet size distributions. Simulations of glories viewed through a linear polarizer compare well with the few available pictures, but they show some features that need corroboration by more observations.

How are glories formed? Free download
Philip Laven
Applied Optics, Volume 44, Issue 27, 5675 September 2005 OSA Optics InfoBase
Mie theory can be used to generate full-color simulations of atmospheric glories, but it offers no explanation for the formation of glories. Simulations using the Debye series indicate that glories are caused by rays that have suffered one internal reflection within spherical droplets of water. In 1947, van de Hulst suggested that backscattering (i.e., scattering angle theta = 180°) could be caused by surface waves, which would generate a toroidal wavefront due to spherical symmetry. Furthermore, he postulated that the glory is the interference pattern corresponding to this toroidal wavefront. Although van de Hulst’s explanation for the glory has been widely accepted, the author offers a slightly different explanation. Noting that surface waves shed radiation continuously around the droplet (not just at theta = 180°), scattering in a specific direction theta = 180° + d can be considered as the vector sum of two surface waves: one deflecting the incident light by 180° + d and the other by 180° - d. The author suggests that the glory is the result of two-ray interference between these two surface waves. Simple calculations indicate that this model produces more accurate results than van de Hulst’s model.

Halos in cirrus clouds: why are classic displays so rare?
Kenneth Sassen
Applied Optics, Volume 44, Issue 27, 5684 September 2005 OSA Optics InfoBase
Upper tropospheric cirrus clouds consist of hexagonal ice crystals, which geometrical ray-tracing-theory predicts should regularly produce a variety of optical phenomena such as vivid 22° and 46° halos. Yet, cirrus inconsistently generate such optical displays, while a class of more exotic displays are reported, albeit rarely. I review current knowledge of the cirrus cloud microphysical factors that control ice crystal shape, and hence haloarc formation, but also appeal to halo enthusiasts to help investigate the causes of unusually complex, brilliant, or rare optical displays. Currently, a wealth of meteorological information can be tapped from the Internet to help advance our knowledge of the basic meteorological factors leading to these rare events

Designing a practical system for spectral imaging of skylight Free download
Miguel A. López-Álvarez, Javier Hernández-Andrés, Javier Romero, and Raymond L. Lee, Jr.
Applied Optics, Volume 44, Issue 27, 5688 September 2005 OSA Optics InfoBase
In earlier work [J. Opt. Soc. Am. A 21, 13–23 (2004)], we showed that a combination of linear models and optimum Gaussian sensors obtained by an exhaustive search can recover daylight spectra reliably from broadband sensor data. Thus our algorithm and sensors could be used to design an accurate, relatively inexpensive system for spectral imaging of daylight. Here we improve our simulation of the multispectral system by (1) considering the different kinds of noise inherent in electronic devices such as changecoupled devices (CCDs) or complementary metal-oxide semiconductors (CMOS) and (2) extending our research to a different kind of natural illumination, skylight. Because exhaustive searches are expensive computationally, here we switch to a simulated annealing algorithm to define the optimum sensors for recovering skylight spectra. The annealing algorithm requires us to minimize a single cost function, and so we develop one that calculates both the spectral and colorimetric similarity of any pair of skylight spectra. We show that the simulated annealing algorithm yields results similar to the exhaustive search but with much less computational effort. Our technique lets us study the properties of optimum sensors in the presence of noise, one side effect of which is that adding more sensors may not improve the spectral recovery.

Multispectral synthesis of daylight using a commercial digital CCD camera Free download
Juan L. Nieves, Eva M. Valero, Sérgio M. C. Nascimento, Javier Hernández-Andrés, and Javier Romero
Applied Optics, Volume 44, Issue 27, 5696 September 2005 OSA Optics InfoBase
Performance of multispectral devices in recovering spectral data has been intensively investigated in some applications, as in spectral characterization of art paintings, but has received little attention in the context of spectral characterization of natural illumination. This study investigated the quality of the spectral estimation of daylight-type illuminants using a commercial digital CCD camera and a set of broadband colored filters. Several recovery algorithms that did not need information about spectral sensitivities of the camera sensors nor eigenvectors to describe the spectra were tested. Tests were carried out both with virtual data, using simulated camera responses, and real data obtained from real measurements. It was found that it is possible to recover daylight spectra with high spectral and colorimetricaccuracy with a reduced number of three to nine spectral bands.

Short-term variability of overcast brightness Free download
Raymond L. Lee, Jr. and Javier Hernández-Andrés
Applied Optics, Volume 44, Issue 27, 5704 September 2005 OSA Optics InfoBase
Overcasts seen from below seldom are uniform, unchanging cloud shields, yet little is known about their short-term photometric variability (periods < 2 h). Visible-wavelength spectra of daytime and twilight overcast skies measured at 30-s intervals reveal unexpected temporal variability in horizontal illuminance Ev and zenith luminance Lv. Fourier analysis of these time series shows peak fluctuations at periods of 2–40 min. Factors such as cloud type and optical depth, presence of fog or snow, and instrument field of view can affect overcast brightness variability. Surprisingly, under some circumstances overcasttwilight Ev exceeds clear-sky Ev at the same Sun elevation.

Colors of the daytime overcast sky Free download
Raymond L. Lee, Jr. and Javier Hernández-Andrés
Applied Optics, Volume 44, Issue 27, 5712 September 2005 OSA Optics InfoBase
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).

Simulating colors of clear and partly cloudy skies
Stanley David Gedzelman
Applied Optics, Volume 44, Issue 27, 5723 September 2005 OSA Optics InfoBase
A model (SKYCOLOR) is developed that simulates the light and color of the sky and open cloud decks in the vertical plane including the Sun and the observer and animates the changes as the Sun goes down. Model skylight consists of sunbeams that are scattered toward the observer, but depleted by scattering and absorption in the Chappuis bands of ozone. SKYCOLOR includes the Earth’s curvature, atmospheric refraction, cloud shadows, and solar eclipses. Scattering is given a wavelength (lambda) dependence of lambda-4 for air molecules (Rayleigh scattering), lambda-1 for tropospheric aerosols, and lambda+1 for volcanic aerosol particles. Multiple scattering is calculated directly in clouds but is parameterized in clear air by decreasing the scattering rates of sunlight and of skylight in the Earth’s shadow by 30%.

On the colors of distant objects Free download
David K. Lynch and S. Mazuk Applied Optics, Volume 44, Issue 27, 5737 September 2005 OSA Optics InfoBase
Distant objects like clouds, mountains, and the Sun can appear to have colors that are significantly different from their intrinsic colors: the low Sun is often red, white clouds and snow-capped peaks appear yellow or pink, and dark green or gray mountains can appear blue or purple. The color alteration increases with distance, or alternatively, optical depth. We investigate the perceived colors of distant objects by computing the CIE chromaticity coordinates from their spectra. For sources viewed through significant amounts of atmosphere (e.g., the low Sun), MODTRAN4 radiative-transfer calculations are used to retrieve the spectra. In addition to clouds and mountains, the colors of stars, the Sun, and the sky are presented as a function of solar elevation under a variety of atmospheric conditions.

Experimental simulations of pollen coronas Free download
Werner B. Schneider and Michael Vollmer
Applied Optics, Volume 44, Issue 27, 5746 September 2005 OSA Optics InfoBase
A procedure to experimentally simulate pollen coronas is discussed. Observed coronas are due to pine and birch pollen having different geometries. Using computer simulations, two-dimensional projections of a large number of pollenlike objects with adjustable shapes, with or without preferential orientation and statistical or regular spatial distribution, are generated. The photograph of the printout allows samples with typical sizes between 20 and 200 µm. Their diffraction patterns can closely resemble the ones observed in nature and predicted by theory.

Ice analog halos
Zbigniew Ulanowski
Applied Optics, Volume 44, Issue 27, 5754 September 2005 OSA Optics InfoBase
Crystals of sodium fluorosilicate are used to produce easy to set up visual displays of atmospheric halos, including the 22° halo, the Parry arc, and upper tangent arcs. Scattering phase functions for single ice-analog rosettes, including a rough one, and a column aggregate, measured in randomized orientation, are also given. The phase functions show prominent halo features, with the exception of the rough crystal.

Turbulent ship wakes: further evidence that the Earth is round Free download
David K. Lynch
Applied Optics, Volume 44, Issue 27, 5759 September 2005 OSA Optics InfoBase
When viewed from the stern, a ship’s turbulent wake appears as a narrow strip of bubble-whitened water converging toward the horizon. The wake does not reach a sharp point on the horizon but has a finite angular width, indicating that the Earth is not flat, but rather round. A simple analysis of the geometry of the observations shows that the radius of the Earth can be estimated using only simple instruments and observations.
2004
Spectral-daylight recovery by use of only a few sensors Free download
Javier Hernández-Andrés, Juan L. Nieves, Eva M. Valero, and Javier Romero
J. Opt. Soc. Am. A, Vol. 21, No. 1, p. 13 January 2004 OSA Optics InfoBase
Linear models have already been proved accurate enough to recover spectral functions. We have resorted to such linear models to recover spectral daylight with the response of no more than a few real sensors. We performed an exhaustive search to obtain the best set of Gaussian sensors with a combination of optimum spectral position and bandwidth. We also examined to what extent the accuracy of daylight estimation depends on the number of sensors and their spectral properties. A set of 2600 daylight spectra [J. Opt. Soc. Am. A 18, 1325 (2001)] were used to determine the basis functions in the linear model and also to evaluate the accuracy of the search. The estimated spectra are compared with the original ones for different spectral daylight and skylight sets of data within the visible spectrum. Spectral similarity, colorimetric differences, and integrated spectral irradiance errors were all taken into account. We compare our best results with those obtained by using a commercial CCD, revealing the CCD’s potential as a daylight-estimation device.

Spectral-reflectance linear models for color pattern recognition Free download
Juan L. Nieves, Javier Hernández-Andrés, Eva M. Valero and Javier Romero
Applied Optics, vol. 43, N. 9, pp. 1880-1891, March 2004 OSA Optics InfoBase
We suggest a new method for color pattern recognition using a linear description of spectral reflectance functions and the spectral power distribution of illuminants containing very few parameters. We report on methods in which these spectral functions are derived from linear models based on principal component analysis (PCA). The correlation is made in the subspace spanned by the coefficients that describe each reflectance according to a suitable basis. The method is first illustrated in a control experiment where the scenes are captured under known illuminant conditions. The discrimination capacity of the algorithm improves upon conventional RGB multi-channel decomposition when scenes are captured under different illuminant conditions, and is comparable to color recognition based on CIELab system. Then we tested the coefficient method in situations where the target is captured under a reference illuminant and the scene containing the target under an unknown spectrally different one. We show that the method avoid false alarms under changes in the illuminant. The correlation results are satisfactory even for the reduced dimensional basis used here to represent the surface reflectance function of the image pixels.

How well does the Rayleigh model describe the E-vector distribution of skylight in clear and cloudy conditions? A full-sky polarimetric study
Bence Suhai and Gábor Horváth
J. Opt. Soc. Am. A, Vol. 21, No. 9, p. 1669 September 2004 OSA Optics InfoBase
We present the first high-resolution maps of Rayleigh behavior in clear and cloudy sky conditions measured by full-sky imaging polarimetry at the wavelengths of 650 nm (red), 550 nm (green), and 450 nm (blue) versus the solar elevation angle θs . Our maps display those celestial areas at which the deviation Δα < │αmeas - αRayleigh│ is below the threshold αthres < 5°, where αmeas is the angle of polarization of skylight measured by full-sky imaging polarimetry, and αRayleigh is the celestial angle of polarization calculated on the basis of the single-scattering Rayleigh model. From these maps we derived the proportion r of the full sky for which the single-scattering Rayleigh model describes well (with an accuracy of Δα < 5°) the E-vector alignment of skylight. Depending on θs , r is high for clear skies, especially for low solar elevations (40% < r < 70% for θs ≤ 13°). Depending on the cloud cover and the solar illumination, r decreases more or less under cloudy conditions, but sometimes its value remains remarkably high, especially at low solar elevations (rmax = 69% for θs = 0°). The proportion r of the sky that follows the Rayleigh model is usually higher for shorter wavelengths under clear as well as cloudy sky conditions. This partly explains why the shorter wavelengths are generally preferred by animals navigating by means of the celestial polarization. We found that the celestial E-vector pattern generally follows the Rayleigh pattern well, which is a fundamental hypothesis in the studies of animal orientation and human navigation (e.g. in aircraft flying near the geomagnetic poles and using a polarization sky compass) with the use of the celestial α pattern.
2003
Feature issue: Applied Optics, January 2003

Light and color in the open air: introduction to the feature issue
Charles L. Adler, James A. Lock
Applied Optics, Volume 42, Issue 3, 307-308 January 2003 OSA Optics InfoBase
This special feature of Applied Optics reports the results of new experimental and theoretical research concerning a number of naked-eye optical phenomena, including ice-crystal halo displays, mirages, rainbows, glories, optical caustics, clear-sky phenomena, cloud coronas, cloud iridescence, and the extinction of skylight.

Halo Polarization Profiles and Sampled Ice Crystals: Observations and Interpretation Free download
Gunther P. Können, Herman R. A. Wessels, Jaap Tinbergen
Applied Optics, Volume 42, Issue 3, 309-317 January 2003 OSA Optics InfoBase
Simultaneous two-wavelength polarization and radiance distributions have been obtained for 22 ° parhelia in four Antarctic ice-crystal swarms that extended to ground level. Samples of crystals that produced these parhelia were collected and replicated. The wavelength dependence of the width of the halo polarization peak agrees with Fraunhofer diffraction theory, indicating that the broadening of the halos is caused primarily by diffraction. However, the observed broadening is much more than predicted from the size distribution of the replicated crystals. From one halo display to the other, the ratio of observed /predicted broadening is erratic, suggesting size-dependent collection efficiency in the sampling. This would imply that, for South Pole conditions, halo polarimetry (or even photometry) is a more reliable method for crystal size determination than actual sampling. It also implies that shapes of the sampled crystals need not necessarily be representative for the shapes of the halo-making crystals in the swarm. Our previous hypothesis [Appl. Opt. 33, 4569 (1994) , that a spread of interfacial angles is the dominating cause of halo broadening, has proved untenable.]

Symmetry in Halo Displays and Symmetry in Halo-Making Crystals Free download
Gunther P. Können
Applied Optics, Volume 42, Issue 3, 318-331 January 2003 OSA Optics InfoBase
The relation between the symmetry in halo displays and crystal symmetry is investigated for halo displays that are generated by ensembles of crystals. It is found that, regardless of the symmetry of the constituent crystals, such displays are always left-right (L -R) symmetric if the crystals are formed from the surrounding vapor. L -R symmetry of a halo display implies here that the cross sections for formation of a halo arc on the left-hand side of the solar vertical and its right-hand side mirror image are equal. This property leaves room for two types of halo display only: a full symmetric one (mmm -symmetric), and a partial symmetric one (mm2 -symmetric) in which halo constituents lack their counterparts on the other side of the parhelic circle. A partial symmetric display can occur only for point halos. Its occurrence implies that a number of symmetry elements are not present in the shape of the halo-making crystals. These elements are a center of inversion, any rotatory-inversion axis that is parallel to the crystal spin axis P , a mirror plane perpendicular to the P axis, and a twofold rotation axis perpendicular to the P axis. A simple conceptual method is presented to reconstruct possible shapes of the halo-generating crystals from the halos in the display. The method is illustrated in two examples. Halos that may occur on the Saturnian satellite Titan are discussed. The possibilities for the Huygens probe to detect these halos during its descent through the Titan clouds in 2005 are detailed.

Midlatitude Cirrus Cloud Climatology from the Facility for Atmospheric Remote Sensing. IV. Optical Displays
Kenneth Sassen, Jiang Zhu, Sally Benson
Applied Optics, Volume 42, Issue 3, 332-341 January 2003 OSA Optics InfoBase
In this fourth of a series of papers that describe long-term cloud research at the Facility for Atmospheric Remote Sensing at Salt Lake City, Utah, a ~10-year record of polarization lidar and photographic observations is analyzed to characterize the occurrence of optical displays in our local varieties of midlatitude cirrus clouds. The frequencies of occurrence of various types of halo, arc, and corona displays are evaluated according to their appearance and longevity over nominal 1-h observation periods and to the meteorological source of the cirrus. We find that complex halo-arc displays are rare at our locale and that even the so-called common 22° halo occurs infrequently as a complete long-lived ring. For example, only ~6% of the 1561-h daytime cirrus periods have bright and prolonged 22° halos, although a total of 37.3% have some indications of this halo, even if they are brief and fragmentary. Other fairly frequent features are the 22° upper tangent arc (8.6%), 22° parhelia (8.5%), and solar corona (7.2%). Of the optical displays observed, 83.6% are refraction based, only 1.9% are due to reflection phenomena, and a surprising 15.4% are caused by diffraction. Complex halo-arc displays are disproportionally associated with cirrus formed in tropical or subtropical airflow and also contain more horizontally oriented planar ice crystals. Lidar linear depolarization ratios from a subset of vivid displays show significant differences between halo- and the corona-producing cirrus, reflecting the effects of particle shape. Halos are associated with relatively warm cirrus that contain randomly and horizontally oriented planar ice crystals, whereas the colder corona cirrus produce much stronger depolarization from crystals too small to be uniformly oriented. Comparisons are made with available information from other locales, and we attempt to explain the geographical differences in terms of basic cirrus cloud processes

Comparison of Modeled and Observed Astronomical Refraction of the Setting Sun
Russell D. Sampson, Edward P. Lozowski, Arthur E. Peterson
Applied Optics, Volume 42, Issue 3, 342-353 January 2003 OSA Optics InfoBase
In this study a ray-tracing model that uses atmospheric data from VIZ and Vaisala RS80 rawinsondes is compared with the observed astronomical refraction presented by the setting Sun as seen from Stony Plain, Alberta, Canada. Photogrammetric measurements taken from photographs of the setting Sun show good agreement with the model for the 14 and 22 December 1998 sunsets. The poorer model results for the 8 December sunset appear to be caused by an obsolete and possibly defective VIZ rawinsonde. The results suggest that the ray-tracing model can produce improved refraction values when compared with the Pulkovo tables [Pulkovo Observatory, Refraction Tables of the Pulkovo Observatory , 5th ed. (Nauka, Leningrad, 1985) . However, they also indicate that the inverse solution (i.e., extracting the temperature profile from refraction measurements) may produce no improvement on U.S. Standard Atmosphere adjusted to the surface conditions.]

Ray Tracing and Refraction in the Modified US1976 Atmosphere
Siebren Y. van der Werf
Applied Optics, Volume 42, Issue 3, 354-366 January 2003 OSA Optics InfoBase
A new and flexible ray-tracing procedure for calculating astronomical refraction is outlined and applied to the US1976 standard atmosphere. This atmosphere is generalized to allow for a free choice of the temperature and pressure at sea level, and in this form it has been named the modified US1976 (MUSA76) atmosphere. Analytical expressions and numerical procedures are presented for calculating dry-air refractions and for the water-vapor correction. Results for all apparent altitudes are presented and compared with The Star Almanac for Land Surveyors (1951), The Nautical Almanac (1958), and the Pulkovo tables (Refraction Tables of the Pulkovo Observatory , 1985). Dependences on sea-level pressure, temperature, and temperature gradient and on humidity are discussed.

Novaya Zemlya Effect and Sunsets Free download
Siebren Y. van der Werf, Günther P. Können, Waldemar H. Lehn
Applied Optics, Volume 42, Issue 3, 367-378 January 2003 OSA Optics InfoBase
Systematics of the Novaya Zemlya (NZ) effect are discussed in the context of sunsets. We distinguish full mirages, exhibiting oscillatory light paths and their onsets, the subcritical mirages. Ray-tracing examples and sequences of solar images are shown. We discuss two historical observations by Fridtjof Nansen and by Vivian Fuchs, and we report a recent South Pole observation of the NZ effect for the Moon.

Gerrit de Veer's True and Perfect Description of the Novaya Zemlya Effect, 24 -27 January 1597 Free download
Siebren Y. van der Werf, Günther P. Können, Waldemar H. Lehn, Frits Steenhuisen, Wayne P. S. Davidson
Applied Optics, Volume 42, Issue 3, 379-389 January 2003 OSA Optics InfoBase
The first recordings of the Novaya Zemlya (NZ) effect were made during Willem Barents ’ third Arctic expedition. Ray-tracing analyses of the three key observations, on 24 -27 January 1597, show that all the reported details can be explained by adopting one common and realistic type of temperature inversion. In particular, the Moon-Jupiter conjunction could have been visible over the central mountain ridge of the island. We show that the NZ effect distorts the relative positions of Jupiter and the Moon in such a way that the looked-for fingerprint of the conjunction occurred almost 2 h after the true conjunction. The quoted direction for the apparent Moon-Jupiter conjunction is then found to be accurate to within 1°. This delay of the apparent conjunction largely explains the error of 29° in their longitude determination. The truthfulness of these observations, debated for four centuries, now appears to be beyond doubt.

Bright Superior Mirages
Waldemar H. Lehn
Applied Optics, Volume 42, Issue 3, 390-393 January 2003 OSA Optics InfoBase
Superior mirages of unusual brightness are occasionally observed. Two such cases, photographed over the frozen surface of Lake Winnipeg, Canada, are documented. Visually, these mirages appear as featureless bright barriers far out on the lake. They are just images of the lake ice, yet the luminance in one case was 2.5 times (in the other, 1.7 times) the luminance of the ice surface in front of the mirage. The mirage itself can be modeled by means of a conduction inversion, but a proper explanation of the brightness is not yet available.

Halo and Mirage Demonstrations in Atmospheric Optics Free download
Michael Vollmer, Robert Greenler
Applied Optics, Volume 42, Issue 3, 394-398 January 2003 OSA Optics InfoBase
Some laboratory demonstrations on atmospheric optics are presented. The focus is on dispersion effects in mirages, lateral mirages, and inferior mirages produced with small hot plates. We also show a demonstration of the upper-tangent-arc halo, produced with a hexagonal prism, rotating about two axes.

Imaging Polarimetry of the Rainbow
András Barta, Gábor Horváth, Balázs Bernáth, Viktor Benno Meyer-Rochow
Applied Optics, Volume 42, Issue 3, 399-405 January 2003 OSA Optics InfoBase
Using imaging polarimetry, we measured the polarization patterns of a rainbow on the shore of the Finnish town of Oulu in July 2001. We present here high-resolution color-coded maps of the spatial distributions of the degree and angle of linear polarization of the rainbow in the red (650 +/- 30 nm), green (550 +/- 30 nm), and blue (450 +/- 30 nm) ranges of the spectrum. The measured polarization characteristics of the investigated rainbow support earlier theoretical and computational results and are in accordance with previous qualitative observations. To our knowledge, this is the first imaging polarimetric study of rainbow polarization.

Experimental Observation of Total-Internal-Reflection Rainbows
Charles L. Adler, James A. Lock, Jonathon Mulholland, Brian Keating, Diana Ekelman
Applied Optics, Volume 42, Issue 3, 406-411 January 2003 OSA Optics InfoBase
A new class of rainbows is created when a droplet is illuminated from the inside by a point light source. The position of the rainbow depends on both the index of refraction of the droplet and the position of the light source, and the rainbow vanishes when the point source is too close to the center of the droplet. Here we experimentally measure the position of the transmission and one-internal-reflection total-internal-reflection rainbows, and the standard (primary) rainbow, as a function of light-source position.

Observation of the enhanced backscattering of light by the end of a tilted dielectric cylinder owing to the caustic merging transition
Philip L. Marston, Yibing Zhang, David B. Thiessen
Applied Optics, Volume 42, Issue 3, 412-417 January 2003 OSA Optics InfoBase
The scattering of light by obliquely illuminated circular dielectric cylinders was previously demonstrated to be enhanced by a merger of Airy caustics at a critical tilt angle. [Appl. Opt. 37 , 1534 (1998)]. A related enhancement is demonstrated here for backward and near-backward scattering for cylinders cut with a flat end perpendicular to the cylinder's axis. It is expected that merged caustics will enhance the backscattering by clouds of randomly oriented circular cylinders that have appropriately flat ends.

Analysis of the Shadow-Sausage Effect Caustic
James A. Lock, Charles L. Adler, Diana Ekelman, Jonathan Mulholland, Brian Keating
Applied Optics, Volume 42, Issue 3, 418-428 January 2003 OSA Optics InfoBase
We analyze the optical caustic produced by light refracted at the curved meniscus surrounding a cylindrical rod standing partially out of a liquid-filled container. When the rod is tilted from the vertical or when light is diagonally incident, the caustic is a four-cusped astroid with two of its cusps obscured by the rod's shadow. If a portion of the flat end of the rod is raised above the water level, the caustic evolves into a pattern of five interlocking cusps. The five cusps result from symmetry breaking of a three-cusped surface perturbation caustic.

Simulating glories and cloudbows in color
Stanley D. Gedzelman
Applied Optics, Volume 42, Issue 3, 429-435 January 2003 OSA Optics InfoBase
Glories and cloudbows are simulated in color by use of the Mie scattering theory of light upwelling from small-droplet clouds of finite optical thickness embedded in a Rayleigh scattering atmosphere. Glories are generally more distinct for clouds of droplets of as much as ~10 µm in radius. As droplet radius increases, the glory shrinks and becomes less prominent, whereas the cloudbow becomes more distinct and eventually colorful. Cloudbows typically consist of a broad, almost white band with a slightly orange outer edge and a dark inner band. Multiple light and dark bands that are related to supernumerary rainbows first appear inside the cloudbow as droplet radius increases above ~10 µm and gradually become more prominent when all droplets are the same size. Bright glories with multiple rings and high color purity are simulated when all droplets are the same size and every light beam is scattered just once. Color purity decreases and outer rings fade as the range of droplet sizes widens and when skylight, reflected light from the ground or background, and multiply scattered light from the cloud are included. Consequently, the brightest and most colorful glories and bows are seen when the observer is near a cloud or a rain swath with optical thickness of ~0.25 that consists of uniform-sized drops and when a dark or shaded background lies a short distance behind the cloud.

Simulation of Rainbows, Coronas, and Glories by use of Mie Theory Free download
Philip Laven OSA Optics InfoBase
Applied Optics, Volume 42, Issue 3, 436-444 January 2003
Mie theory offers an exact solution to the problem of scattering of sunlight by spherical drops of water. Until recently, most applications of Mie theory to scattering of light were restricted to a single wavelength. Mie theory can now be used on modern personal computers to produce full-color simulations of atmospheric optical effects, such as rainbows, coronas, and glories. Comparison of such simulations with observations of natural glories and cloudbows is encouraging.

Measuring and Modeling Twilight's Purple Light Free download
Raymond L. Lee, Javier Hernández-Andrés
Applied Optics, Volume 42, Issue 3, 445-457 January 2003 OSA Optics InfoBase
During many clear twilights, much of the solar sky is dominated by pastel purples. This purple light’s red component has long been ascribed to transmission through and scattering by stratospheric dust and other aerosols. Clearly the vivid purples of post-volcanic twilights are related to increased stratospheric aerosol loading. Yet our time-series measurements of purple-light spectra, combined with radiative transfer modeling and satellite soundings, indicate that background stratospheric aerosols by themselves do not redden sunlight enough to cause the purple light's reds. Furthermore, scattering and extinction in both the troposphere and the stratosphere are needed to explain most purple lights.

Color and Luminance Asymmetries in the Clear Sky Free download
Javier Hernández-Andrés, Raymond L. Lee, Javier Romero
Applied Optics, Volume 42, Issue 3, 458-464 January 2003 OSA Optics InfoBase
A long-standing assumption about the clear sky is that its colors and luminances are distributed symmetrically about the principal plane. As useful as this approximation is, our digital-image analyses show that clear-sky color and luminance routinely depart perceptibly from exact symmetry. These analyses reconfirm our earlier measurements with narrow field-of-view spectroradiometers [J. Opt. Soc. Am. A 18, 1325 (2001) , and they do so with much higher temporal and angular resolution across the entire sky dome.

Neutral Points of Skylight Polarization Observed During the Total Eclipse on 11 August 1999
Gábor Horváth, István Pomozi, József Gál
Applied Optics, Volume 42, Issue 3, 465-475 January 2003 OSA Optics InfoBase
We report here on the observation of unpolarized (neutral) points in the sky during the total solar eclipse on 11 August 1999. Near the zenith a neutral point was observed at 450 nm at two different points of time during totality. Around this celestial point the distribution of the angle of polarization was heterogeneous: The electric field vectors on the one side were approximately perpendicular to those on the other side. At another moment of totality, near the zenith a local minimum of the degree of linear polarization occurred at 550 nm. Near the antisolar meridian, at a low elevation another two neutral points occurred at 450 nm at a certain moment during totality. Approximately at the position of these neutral points, at another moment of totality a local minimum of the degree of polarization occurred at 550 nm, whereas at 450 nm a neutral point was observed, around which the angle-of-polarization pattern was homogeneous: The electric field vectors were approximately horizontal on both sides of the neutral point.

Coronas and iridescence in mountain wave clouds Free download
Joseph A. Shaw, Paul J. Neiman
Applied Optics, Volume 42, Issue 3, 476-485 January 2003 OSA Optics InfoBase
We use Fraunhofer diffraction theory and meterological data to determine the nature of cloud-particle distributions and the mean particle sizes required for interpreting photographs of coronas and iridescence in mountain wave clouds. Traditional descriptions of coronas and iridescence usually explain these optical phenomena as diffraction by droplets of liquid water. Our analysis shows that the photographed displays have mean particle sizes from 7.6 to 24.3 µm, with over half the cases requiring diffraction by small ( ~20 µm) quasispherical ice particles rather than liquid water droplets. Previous documentation of coronas produced by ice particles are limited to observations in cirrus clouds that appear to be composed of small ice crystals, whereas our observations suggest that coronas and iridescence quite often can be created by tiny quasispherical ice particles that might be unique to mountain wave clouds. Furthermore, we see that the dominant colors in mountain wave-cloud coronas are red and blue, rather than the traditionally described red and green.

Cirrus cloud iridescence: a rare case study
Kenneth Sassen
Applied Optics, Volume 42, Issue 3, 486-491 January 2003 OSA Optics InfoBase
On the evening of 25 November 1998, a cirrus cloud revealing the pastel colors of the iride