In polarimetric instruments, it is often necessary to characterize the polarimetric dependence at various polarization states. Frequently, this is done by placing a polarizer between the instrument and light source. Certain polarizer materials (e.g., wire grids as opposed to polymer-based materials) tend to reflect a significant amount of light, which can cause second-order reflections in the region between the two polarizers. We characterize the reflections using Jones calculus and discuss their significance for polarization instruments.
On 21 August 2017 we measured skylight polarization during a total solar eclipse in Rexburg, Idaho, using two all-sky polarimetric imagers. The all-sky polarization images were recorded using three simultaneously operating digital singlelens-reflex (DSLR) cameras with good low-light sensitivity. Each camera was equipped with a 180° field-of-view fisheye lens to view the entire sky and each lens contained a fixed linear polarizer orientated at 0° , 60° , and 120° , respectively, to recover the first three Stokes parameters. Skylight polarization was measured from sunrise to sunset in the cameras’ blue, green, and red channels. Before and after totality, the maximum sky polarization occurred in its usual pattern with a band of maximum polarization positioned 90° from the sun. However, during totality skylight polarization became nominally symmetric about the zenith. This was observed clearly in the blue and green channels and less obviously in the red channel, which had a greatly diminished signal. At and near the observation site, we also operated an infrared cloud imager, a hand-held spectrometer to measure surface reflectance, and an AERONET solar radiometer to characterize the atmospheric aerosols. This ancillary data set provided a complete characterization of the conditions of the surrounding atmosphere and underlying surfaces.
Knowing the thermodynamic phase of a cloud–whether it is composed of spherical water droplets or polyhedral ice crystals–is critical in remote sensing applications and in climate studies. We recently showed that we can determine cloud phase with visible-wavelength sky polarimetry, and in this presentation we extend that method to shortwave infrared wavelength bands near 1.6 microns. We describe the instrument, a passive, three-channel polarimeter with spectral bands at 1550 nm, 1640 nm, and 1700 nm with approximate width of 40 nm and how we are using it in experiments to discriminate between liquid-water and ice clouds. This portable polarimeter measures scattered sunlight using polarizers orientated at 0° , 45‡ , and 90° with respect to the solar vertical scattering plane. It has a 4.9° field-of-view and a motorized, computer-controlled pan-and-tilt mount that controls the positioning of the polarimeter so that it can measure any point in the sky.
Getting students interested in science, specifically in optics and photonics, is a worthwhile challenge. We developed and implemented an outreach campaign that sought to engage high school students in the science of polarized light. We traveled to Montana high schools and presented on the physics of light, the ways that it becomes polarized, how polarization is useful, and how to take pictures with linear polarizers to see polarization. Students took pictures that showed polarization in either a natural setting or a contrived scene. We visited 13 high schools, and presented live to approximately 450 students.
At any given time, clouds cover approximately 60% of the Earth’s surface and they strongly influence weather and climate; however, they are one of the largest sources of uncertainty in climate models and predictions of atmospheric effects on remote sensing measurements. Knowing the cloud thermodynamic phase – whether a cloud is composed of ice crystals or liquid particles – is critical in these applications. Knobelspiesse et al. (Atmos. Meas. Tech., 8, 1537–1554, 2015) showed theoretically that the sign of the S1 Stokes parameter can be used to detect cloud thermodynamic phase when observed with a ground-based passive polarimeter and demonstrated this principle with a zenith-viewing polarimeter. In this theory, a positive S<sub>1</sub> value indicates a liquid cloud, while a negative S<sub>1</sub> value indicates an ice cloud. In this paper, we report the use of our all-sky polarimeter, operating at 450 nm (10 nm band) to detect ice, liquid, and multi-layered clouds. The cloud thermodynamic phase was independently verified with a dual-polarization lidar pointed at the zenith.
The spatial distributions of flying insects are not well understood since most sampling methods - Malaise traps, sticky traps, vacuum traps, light traps - are not suited to documenting movements or changing distributions of various insects on short time scales. These methods also capture and kill the insects. To noninvasively monitor the spatial distributions of flying insects, we developed and implemented a scanning lidar system that measured wing-beat-modulated scattered laser light. The oscillating signal from wing-beat returns allowed for reliable separation of lidar returns for insects and stationary objects. Transmitting and receiving optics were mounted to a telescope that was attached to a scanning mount. As it scanned, the lidar collected and analyzed the light scattered from insect wings of various species. Mount position and pulse time-of-flight determined spatial location and spectral analysis of the backscattered light provided clues to insect identity. During one day of a four-day field campaign at Grand Teton National Park in June of 2016, 76 very likely insects and 662 somewhat likely insects were detected, with a maximum range to the insect of 87.6 m for very likely insects