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

Venkatesulu, Erica; Shaw, Joseph A.

doi:10.1364/ao.454801

Cited by 6 publications

(5 citation statements)

References 24 publications

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“…2, according to the process described in previous work. 9 The monochrome camera was used with a near infrared (NIR) filter for one experiment. The spectral transmission of this filter was measured with an integrating sphere and a spectrometer.…”

Section: Instruments and Methodologymentioning

confidence: 99%

Contrast enhancement through an air-water interface with polarization imaging

Venkatesulu,

Buglione,

Field

et al. 2023

Polarization Science and Remote Sensing XI

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It is well known that underwater objects become more readily visible when viewed through a vertical polarizer that suppresses horizontally polarized reflections from the air-water interface. However, quantitative measurements of the contrast enhancement achieved with a polarizer do not seem to have been reported in the literature. To measure the polarization-enabled contrast enhancement, we placed white and black tiles next to each other, immersed in water, then measured the optical contrast between them as a function of viewing angle (relative to the surface normal) with a polarization camera that simultaneously recorded images with linear polarization oriented 0°, 90°, and 45°from horizontal. Images were recorded with an RGB polarization camera through approximately 45 cm of water at Bozeman Pond and with a monochrome polarization camera through approximately 5 cm of water at Bozeman Beach. Images also were recorded with the monochrome camera and a filter to isolate the near infrared band of approximately 750 to 1000 nm. Indoor laboratory measurements also were recorded to verify the role of the color of the reflecting background. All experiments used carefully calibrated division-of-focal-plane polarization cameras. The observed contrast decreased with viewing angle, but less so for the vertically polarized images. The contrast enhancement, represented by the ratio of vertically polarized to unpolarized contrast, increased with viewing angle, even past the Brewster angle (approx. 53°). The contrast enhancement only began decreasing for viewing angles larger than 70°. In outdoor experiments with a mostly clear sky, the highest contrast enhancement was in the blue spectral band. The contrast was essentially the same for red, green, and blue bands with a white background. In all measurements, the black tile exhibited much larger degree of linear polarization, which is an example of the Umov effect. In this paper we describe the experiments, show and explain polarization images, and show and explain plots of contrast and contrast enhancement as a function of viewing angle.

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Section: Instruments and Methodologymentioning

confidence: 99%

Contrast enhancement through an air-water interface with polarization imaging

Venkatesulu,

Buglione,

Field

et al. 2023

Polarization Science and Remote Sensing XI

Self Cite

View full text Add to dashboard Cite

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“…The gain, or "ISO" setting as defined in the Raspberry Pi documentation, was held at a value of 1 for all measurements; however, the exposure time, or "Shutter Speed", setting of the cameras was governed by the output power of the monochromator. Since the output power of the monochromator varied with wavelength, 45 the exposure time required adjustment across the wide spectral range to maintain ample signal. This variation in the exposure time of the images was accounted for by relying on the linear relationship that has been shown to exist 46 between the exposure time and signal such that a pixel multiplier could be applied to each image based on the exposure time.…”

Section: Relative Spectral Response Characterizationmentioning

confidence: 99%

Developing a low-cost multispectral imager for detecting algal blooms in rivers

Hamp¹,

Logan²,

Shaw³

2023

SPIE Future Sensing Technologies 2023

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A low-cost multispectral imager is described for routine monitoring of Cladophora nuisance algae and bluegreen algae in narrow rivers that are not spatially resolved by satellites. The goal is to identify algal blooms and estimate the chlorophyll a (chl a) and phycocyanin content from a network of low-cost imagers that can be mounted on bridges, trees, or other convenient objects at key river locations. The preliminary design uses Raspberry Pi cameras and computers with bandpass filters at 568, 671, 700, and 825 nm, based on data gathered with an airborne hyperspectral imager on the Upper Clark Fork River in southwestern Montana USA. This paper summarizes the initial design, calibration measurements, and preliminary reflectance data.

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“…Polarimetric corrections were performed with each imaging system on the camera. The relative spectral response of the detector was also measured using a grating monochromator, 13 and the detector had a response from 400 to 1000 nm.…”

Section: Imaging System Characterizationsmentioning

confidence: 99%

“…We previously found that the horizontally polarized subpixels on the detector have polarizers with a higher transmission than the vertically polarized subpixels, which causes images to have a bias towards horizontal polarization. 13 For the 300-mm lens system, the system matrix developed in the polarimetric calibration corrects both sources of error. For the telescope, we expected the first type of error to be very small, since an f/10 Schmidt-Cassegrain telescope design is symmetric about the axis with weakly curved surfaces.…”

Section: Imaging System Characterizationsmentioning

confidence: 99%

Disk-resolved and disk-integrated polarization state of moonlight as a function of lunar phase

Venkatesulu,

Dabby,

Shaw

2023

Polarization Science and Remote Sensing XI

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To use moonlight as a source for on-orbit calibration of satellite instruments and for nighttime passive remote sensing, considerable effort has been made to develop and improve radiometric models of the Moon. However, to enable calibration of polarization-sensitive instruments and nighttime polarimetric remote sensing, the polarization state of moonlight must be known as well. While several observations of moonlight polarization have been published, there is no known database of disk-integrated polarization measurements or disk-resolved polarization images as a function of phase angle. We used a monochrome division-of-focal-plane polarization imager with a telescope (focal length of 2 m) and a telephoto lens (focal length of 300 mm) to record images of the degree and angle of linear polarization for lunar phases ranging from full Moon to about 15% full. We calculated the disk-integrated polarization state from the images. We present a plot of disk-integrated polarization state as a function of phase angle, which agrees well with similar plots published previously for small regions of the Moon. The disk-integrated degree of linear polarization (DoLP) reaches a maximum of approximately 8% at a phase angle near 100° and a minimum near 0% at a phase angle of 0°. We also present S0, DoLP, and angle of polarization (AoP) images and use them to explain the lunar locations where polarized light primarily originates. To our knowledge, the presented DoLP images are higher resolution than previously published DoLP images, and AoP images presented here have not yet been published.

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

Cited by 6 publications

References 24 publications

Contrast enhancement through an air-water interface with polarization imaging

Contrast enhancement through an air-water interface with polarization imaging

Developing a low-cost multispectral imager for detecting algal blooms in rivers

Disk-resolved and disk-integrated polarization state of moonlight as a function of lunar phase

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