Measuring the spectral response of a division-of-focal-plane polarization imager using a grating monochromator.

Spectral characterizations are performed on imagers to obtain a relative spectral response (RSR) curve. This process often utilizes a grating monochromator with an output that changes polarization as a function of wavelength (our monochromator's degree of linear polarization was found to vary from less than 10% to more than 70%). When characterizing a polarization-sensitive imager, this introduces polarization artifacts into the RSR curve. We present a simple method to avoid these polarization artifacts for division-of-focal-plane polarization imagers by directly illuminating the camera with the monochromator output and calculating the S0 Stokes parameter at each super pixel, then we show consistent results from this method for two division-of-focal-plane polarization imagers. We also show that ignoring the monochromator polarization results in order-of-magnitude RSR errors. The recommended method uses an iris to limit the spatial extent of the monochromator output, which was found experimentally to increase the minimum signal-to-noise ratio by more than a factor of 2.

Optical transmittance of 3D printing materials.

The increasing prevalence of three-dimensional (3D) printing of optical housings and mounts necessitates a better understanding of the optical properties of printing materials. This paper describes a method for using multithickness samples of 3D printing materials to measure transmittance spectra at wavelengths from 400 to 2400 nm [visible to short-wave infrared (IR)]. In this method, 3D samples with material thicknesses of 1, 2, 3, and 4 mm were positioned in front of a uniform light source with a spectrometer probe on the opposing side to measure the light transmittance. Transmission depended primarily on the thickness and color of the sample, and multiple scattering prevented the use of a simple exponential model to relate transmittance, extinction, and thickness. A Solidworks file and a 3D printer file are included with the paper to enable measurements of additional materials with the same method.

Lidar remote sensing of the aquatic environment: invited.

This paper is a review of lidar remote sensing of the aquatic environment. The optical properties of seawater relevant to lidar remote sensing are described. The three main theoretical approaches to understanding the performance of lidar are considered (the time-dependent radiative transfer equation, Monte Carlo simulations, and the quasi-single-scattering assumption). Basic lidar instrument design considerations are presented, and examples of lidar studies from surface vessels, aircraft, and satellites are given.

Lidar measurements of the diffuse attenuation coefficient in Yellowstone Lake.

Airborne lidar study of lake ecosystems is still a relatively unexplored field. In this paper we present measurements of the diffuse attenuation coefficient of downwelling irradiance (Kd) obtained using a 532 nm airborne lidar in flights during 2004 and 2016 over Yellowstone Lake, Yellowstone National Park, Wyoming, USA. We compare the lidar measurements with MODIS Kd data, discuss the impact that local weather and river inflows/outflows may have had on the data, compare to previous models of the diffuse attenuation coefficient, and examine several published relationships converting Kd to Secchi disk depth.

Digital all-sky polarization imaging of the total solar eclipse on 21 August 2017 in Rexburg, Idaho, USA.

All-sky polarization images were measured from sunrise to sunset and during a cloud-free totality on 21 August 2017 in Rexburg, Idaho using two digital three-camera all-sky polarimeters and a time-sequential liquid-crystal-based all-sky polarimeter. Twenty-five polarimetric images were recorded during totality, revealing a highly dynamic evolution of the distribution of skylight polarization, with the degree of linear polarization becoming nearly zenith-symmetric by the end of totality. The surrounding environment was characterized with an infrared cloud imager that confirmed the complete absence of clouds during totality, an AERONET solar radiometer that measured aerosol properties, a portable weather station, and a hand-held spectrometer with satellite images that measured surface reflectance at and near the observation site. These observations confirm that previously observed totality patterns are general and not unique to those specific eclipses. The high temporal image resolution revealed a transition of a neutral point from the zenith in totality to the normal Babinet point just above the Sun after third contact, providing the first indication that the transition between totality and normal daytime polarization patterns occurs over of a time period of approximately 13 s.

Detection of polarization neutral points in observations of the combined corona and sky during the 21 August 2017 total solar eclipse.

We report the results of polarimetric observations of the total solar eclipse of 21 August 2017 from Rexburg, Idaho (USA). We use three synchronized DSLR cameras with polarization filters oriented at 0°, 60°, and 120° to provide high-dynamic-range RGB polarization images of the corona and surrounding sky. We measure tangential coronal polarization and vertical sky polarization, both as expected. These observations provide detailed detections of polarization neutral points above and below the eclipsed Sun where the coronal polarization is canceled by the sky polarization. We name these special polarization neutral points after Minnaert and Van de Hulst.

All-sky polarization imaging of cloud thermodynamic phase.

Knowing the cloud thermodynamic phase (if a cloud is composed of ice crystals or liquid droplets) is crucial for many cloud remote sensing measurements. Further, this knowledge can help in simulating and interpreting cloud radiation measurements to better understand the role of clouds in climate, weather, and optical propagation. Knobelspiesse et al. [Atmos. Meas. Tech.8, 1537 (2015)10.5194/amt-8-1537-2015] showed that, for simulated zenith observations, the algebraic sign of the S1 Stokes parameter (related to the difference between perpendicular and parallel linear polarization in the scattering plane) can be used to detect cloud thermodynamic phase when observed with a ground-based passive polarimeter. In this paper, we describe the use of our all-sky imaging polarimeter to experimentally test this proposed method of detecting cloud thermodynamic phase in the entire sky dome. The zenith cloud phase was validated with a dual-polarization lidar instrument.

Reflective all-sky thermal infrared cloud imager.

A reflective all-sky imaging system has been built using a long-wave infrared microbolometer camera and a reflective metal sphere. This compact system was developed for measuring spatial and temporal patterns of clouds and their optical depth in support of applications including Earth-space optical communications. The camera is mounted to the side of the reflective sphere to leave the zenith sky unobstructed. The resulting geometric distortion is removed through an angular map derived from a combination of checkerboard-target imaging, geometric ray tracing, and sun-location-based alignment. A tape of high-emissivity material on the side of the reflector acts as a reference that is used to estimate and remove thermal emission from the metal sphere. Once a bias that is under continuing study was removed, sky radiance measurements from the all-sky imager in the 8-14 µm wavelength range agreed to within 0.91 W/(m2 sr) of measurements from a previously calibrated, lens-based infrared cloud imager over its 110° field of view.

Extended visual range during solar eclipses.

During the solar eclipse on 21 August 2017, observed from Rexburg, Idaho, the eastern horizon for a short time period close to totality showed the contours of the Grand Teton mountain range from distances between about 80 and 90 km. In most of the partial phase before and after the eclipse, the visual range was below 80 km and the mountains could not be seen by the naked eye because of wildfire smoke in the air. We present photographic evidence of extension of the visual range during the eclipse and explain it in terms of a simple model for the daytime visual range. In this model, contrast between a black object and the sky increases whenever the ratio of nearby to distant air light reduces. As a consequence, we predict asymmetries with regard to totality and also that similar phenomena may be observed if part of the line of sight close to the observer is in deep shade of a thick cloud cover.

The VIS-SWIR spectrum of skylight polarization.

The skylight degree of linear polarization (DoLP) was previously shown to vary primarily with aerosol optical depth and underlying surface reflectance for visible-to-near-infrared (VNIR) wavelengths. This paper extends the study of skylight polarization to 2.5 µm in the short-wave infrared (SWIR). A successive-orders-of-scattering radiative transfer code was used to model skylight polarization with measured inputs that included aerosol properties retrieved from a ground-based solar radiometer (extrapolated into the SWIR) and spectral surface reflectance from a handheld spectrometer. The modeled DoLP depended heavily on the aerosol size distribution at SWIR wavelengths and on the aerosol optical depth at VNIR wavelengths. Once the aerosol optical depth became greater than the Rayleigh optical depth, the predicted polarization deviated significantly from Rayleigh scattering theory. The SWIR polarization spectrum generally decreased at wavelengths beyond 1 µm at a rate dependent on the aerosol size distribution. The surface reflectance affected the polarization in the same manner throughout the visible (VIS)-SWIR spectrum, with higher reflectance decreasing the skylight polarization. Validation measurements of SWIR skylight polarization in a 1.5-1.8 µm band are also shown. These measurements were made on clean and smoky days using a SWIR imaging polarimeter. In both simulations and measurements, the SWIR skylight polarization was greater in the smoky atmosphere than in the clean atmosphere.

Airborne lidar detection and mapping of invasive lake trout in Yellowstone Lake.

The use of airborne lidar to survey fisheries has not yet been extensively applied in freshwater environments. In this study, we investigated the applicability of this technology to identify invasive lake trout (Salvelinus namaycush) in Yellowstone Lake, Yellowstone National Park, USA. Results of experimental trials conducted in 2004 and in 2015-16 provided lidar data that identified groups of fish coherent with current knowledge and models of lake trout spawning sites, and one identified site was later confirmed to have lake trout.

Inflight observation of Bottlinger's rings.

On the morning of 5 November 2013, a bright subsun was consistently visible during a flight from Bozeman, Montana, to Salt Lake City, Utah. Just after passing over the Wasatch Mountains and beginning to descend into the Salt Lake Valley, the subsun expanded to a rare display of Bottlinger's rings-an elliptical halo surrounding the subsun. The rings remained visible for 1 to 2 min. This paper shows photographs of the sequence, along with meteorological data from a nearby radiosonde. The display occurred in virga below clouds at an air temperature in the approximate range from -8°C to -12°C, in air saturated with respect to ice, at an altitude of approximately 2600-3600 m above mean sea level.

Blue sun glints on water viewed through a polarizer.

Sun glints are formed by specular reflections of the sun from capillary waves formed by wind blowing over water. These glints are normally colorless for a high sun or take on the color of the light source, such as orange-red during sunset or sunrise. However, when the glints are highly polarized by reflection near the Brewster angle, i.e., with relatively high sun they can change from colorless to a blue appearance caused by blue light leakage through a polarizing filter oriented orthogonal to the plane of polarization of the reflected light. Measurements are shown of crossed-polarizer transmission spectra exhibiting blue and near infrared light leakage for photographic polarizing filters and polarized sunglasses. A variety of photographs is shown to confirm blue light leakage as the source of the blue glint color.

Atmospheric optics in the near infrared.

Digital near-infrared photography opens up new observation possibilities and applications for atmospheric optics. We discuss necessary conditions and requirements for observing a variety of atmospheric optical phenomena in the infrared spectral range and report for the first time near-infrared photographs of 22° ring halos and inferior mirages. Our emphasis is on optical phenomena observable in the troposphere, excluding the large body of work addressing near-infrared airglow and aurora.

Light and color in the open air-introduction to the feature issue.

This feature issue reports recent progress in scientific understanding of optical phenomena in the natural world, visible to the naked eye. The issue contains papers largely arising from presentations given at the 12th International Conference on Light and Color in Nature, held at the University of Granada from 31 May to 3 June 2016.

Colors of thermal pools at Yellowstone National Park.

The brilliant visible colors of various hot springs and pools in Yellowstone National Park are explained with a combination of scattering from the water and from microbial mats that coat the bottoms of these thermal features. A simple 1D radiative transfer model was used to simulate the colors recorded in visible photographs and the spectrum of light making up these colors. The model includes attenuation in water by absorption and molecular scattering as well as reflection characteristics of the microbial mats and surface reflection of the water. Pool geometries are simulated as simple rough cones scaled to have depths and widths that match published data. Thermal images are also used to record the spatial distribution of water skin temperature. The measurements and simulations confirm that colors observed from shallow-water features arise primarily from the spectral properties of the microbial mat, which is related to the water temperature, while colors observed from deeper water arise primarily from the wavelength-dependent absorption and scattering in the water.

Infrared Moon imaging for remote sensing of atmospheric smoke layers.

Simultaneous visible and long-wave infrared (IR) images of the Moon were used with a simple energy-balance model to study the spatial pattern of lunar surface temperatures. The thermal images were obtained with a radiometrically calibrated, compact, low-cost, commercial IR camera mounted on a small telescope. Differences between the predicted and measured maximum Moon temperatures were used to determine the infrared optical depth (OD), which represents the path-integrated extinction of an elevated layer of wildfire smoke in the atmosphere. The OD values retrieved from the IR Moon images were combined with simultaneous OD measurements from a ground-based, zenith-pointing lidar operating at a wavelength of 532 nm to determine an IR-to-visible OD ratio of 0.50±0.18 for moderately aged wildfire smoke aerosol.

Visible and invisible mirages: comparing inferior mirages in the visible and thermal infrared.

Visible (VIS)-light and thermal infrared (IR) inferior mirages in the 8-14 µm waveband have been observed simultaneously for the takeoff and landing of various airplanes at distances of several kilometers. Similarities as well as differences between the VIS and IR mirages are discussed.

Light and color in the open air: Introduction to the feature issue.

This is a feature issue devoted to optical phenomena in nature. Many of the papers published in this feature issue are based on presentations given at the "Light & Color in Nature" conference held in August 2013 at the University of Alaska-Fairbanks.

Wavelength dependence of the degree of polarization in cloud-free skies: simulations of real environments.

The visible and NIR maximum degree of polarization (DoP) of cloud-free skylight depends on many factors, including wavelength, sun zenith angle, surface reflectance, and aerosol properties. For clear-sky environments, radiative transfer models accurately estimate the sky DoP when each of these properties is well constrained. (The model used here was recently compared with full-sky polarization measurements with excellent agreement.) Using coincident Hyperion satellite observations and AERONET retrievals to provide model inputs, we simulate the maximum sky DoP for a variety of locations. Results show large variations in the wavelength dependence of sky polarization across different Earth environments. Therefore, accurate modeling of the sky DoP depends largely upon proper representation of the surface and aerosols in the model. Simple models which do not incorporate accurate aerosol and surface information have limited utility for simulating cloud-free sky DoP.