An InfraRed Channeled Spectro-Polarimeter (IRCSP) was demonstrated in the near space environment as a piggyback out of NASA Columbia Scientific Ballooning Facility. The compact IRCSP is sensitive to linearly polarized long-wave infrared (LWIR) light between 7-12 microns and targets cloud micro-physical properties. Post landing the instrument was retrieved with no damage to the optical payload and collected over 150 minutes of flight data at altitudes above 30 km. The results collected both demonstrate the operation of uncooled microbolometers in the low pressure environment and are the first know high-altitude observations of a polarized signal from cloud tops in the LWIR. During deployment, the IRCSP reported brightness temperatures between 250-285K with uncertainty of < 1:5K. In addition, statistically significant polarization modulation with degrees of linear polarization (DoLP) between 1 – 20% and preferential angle of linear polarization (AoLP) trends were detected. These results support the hypothesis that the LWIR polarimetry has the potential to add new sensitivity to existing remote sensing platforms.
In remote sensing, radiometric measurements taken in the mid-wave infrared and beyond (λ > 3μm) are commonly reported in units of Kelvin by utilizing Planck's radiation law to relate measured radiance and target brightness temperature (Tb). Thus, it is desirable to match this formalism in thermal polarimetry and report the unnormalized Stokes parameters in units of K instead of radiance (Wm-2sr-1). This approach also allows common performance metrics in long-wave infrared (LWIR) imaging such as Noise Equivalent Differential Temperature (NEDT) to be modified and extended to metrics of polarimetric accuracy and precision. However, since the relationship between measured radiance and Tb are non-linear, the conversion of I, Q, and U in units of radiance to Tb, Tb,Q, and Tb,U in K is ambiguous. As a solution a metric of performance for thermal linear Stokes polarimetry, the Stokes resolved differential temperature (SRDT), is introduced.
Dichroic filters are used by instrument designers to split a field of view into different optical paths for simultaneous measurement of different spectral bands. Quantifying the polarization aberrations of a dichroic is relevant for predicting the incident polarization states downstream, which could affect the performance of diffraction limited systems. One important application is the fore-optics of exoplanet imaging coronagraphs. In this work, the polarization properties of the Edmund #69-205 650 nm roll-off dichroic are measured using a rotating retarder Mueller matrix imaging polarimeter. The polarization properties of this commercial dichroic are compared at normal and 45° angle of incidence. The normal incidence measurements verify the instrument calibration since no polarization aberrations were observed. Transmission measurements at 680 nm and 45° yield a 2.9 rad magnitude of retardance and 0.95 diattenuation. Effectively, at 630 nm the dichroic is a λ/4 waveplate with a horizontal fast-axis.
A compact long-wave infrared (LWIR) channeled spectro-polarimeter (IRCSP) has been developed for integration into the NASA Earth Science Technology Office (ESTO) funded submm-wave and LWIR polarimeters project to measure the microphysical properties of cloud ice. Once deployed, the IRCSP will produce the first linear Stokes measurements ( S0 , S1 , S2 ) of upper-tropospheric cirrus clouds from 8.5 to 12.5 μm. For the first time, a compact, light-weight, and uncooled LWIR polarimeter with off-the-shelf thermal optical components is demonstrated. We report narrowband calibration measurements which quantify metrics of polarimetric system performance. The response of the system to linearly polarized light is shown to agree with both a Mueller matrix model and modulation function for narrowband calibration measurements with an R2 > 0.98 from 8 to 11 μm. The polarimetric efficiency is >0.8 from 8 to 11 μm for narrowband measurements indicating satisfactory performance of the polarization optics. Beyond 11 μm, the agreement is significantly reduced as thermal noise compounds with reduced detector response. Ultimately, the observed system performance is limited by the spectral response of the detector past 11 μm in addition to the thermal noise inherent for the measurements at room temperature.
The trade-off between spectral resolution and instrument throughput is analyzed for a compact, uncooled, longwave infrared (LWIR) channeled spectropolarimeter (IRCSP). The IRCSP was developed as a part of the Sub-mm Wave and InfraRed Polarimeters (SWIRP) project out of NASA's Goddard Spaceflight Center. The IRCSP scientific objective targets measurements of AOLP and DOLP with 1-µm spectral resolution from 8.5 - 12.5µm in a single snapshot. The geometry of the field stop determines the field of view (FOV) of the IRCSP. This work relates the spectral resolution, instrument throughput, and polarimetric accuracy of a spectro-polarimeter to the FOV. The accuracy of linear Stokes retrievals for low temperature thermal targets are predicted for varying FOV and measurement noise conditions. This work presents a method to quantify the achievable accuracy in AOLP and DOLP as a function of field stop dimensions and signal-to-noise ratio (SNR). While smaller field stops are shown to improve accuracy as the spectral resolution is increased, low SNR is the dominant source of error for the IRCSP prototype. For the IRCSP, a SNR of at least 80 is required to produce DOLP measurements with < 5% error for targets with DOLP < 0.2.
A compact long wave infrared (LWIR) channeled spectro-polarimeter (IRCSP) has been developed for integration
into the ESTO-funded Submm-Wave and LWIR Polarimeters (SWIRP) project to measure the microphysical
properties of cloud ice. The IRCSP rotates incident linearly polarized light using the combination of a quarter
waveplate with a fast axis at 45◦ and a thick birefringent crystal; the output polarization state’s orientation is
then a function of wavelength. To modulate and then measure the rotated light, a subsequent wiregrid linear
polarizer tilted at 20◦ generates two output paths with opposite polarities in reflection and transmission to enable
joint radiometric and polarimetric measurement and correct for atmospheric attenuation. The two symmetric
optical paths following the linear polarizer each consist of a diffraction grating and uncooled microbolometer to
simultaneously measure the resulting intensity fringes. Angle and degree of linear polarization (AOLP, DOLP) are
retrieved across 8.5-12.5 µm with 1 µm resolution using Fourier decomposition of the modulated spectrum. The IRCSP will not only measure H-V variance but will produce the first full linear Stokes measurements
(I, Q, and U) of upper-tropospheric cirrus clouds in the LWIR. Following thermal and polarimetric calibration,
the polarimeter is expected to achieve 0.5% DOLP accuracy over 90% of the spectral band.
A miniaturized long-wave InfraRed (LWIR) spectro-polarimeter is being developed as a prototype for the Compact Submm-Wave and LWIR Polarimeters (SWIRP) project. The polarimeter in development is a compact (20x20x40 cm) conical-scan instrument to measure the polarimetric radiation from ice cloud scattering at mm- submm (220 and 680 GHz) and IR (8.6, 11, and 12 m) bands. The LWIR polarimeter will provide a series of polarization measurements across the 8.5 - 12.5 micron band, measuring the full set of linear Stokes parameters (I, Q, U) as a function of wavelength. The spectro-polarimeter uses a combination of birefringent crystals, a Wollaston prism, a diffraction grating, and an uncooled microbolometer array to measure both the degree and angle of linear polarization across the spectral bandwidth by modulating the polarization flux in wavelength with a high order retarder.
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