Absorption-Based, Passive Range Imaging from Hyperspectral Thermal Measurements

TL;DR

This paper presents a novel passive hyperspectral infrared method for estimating object range and properties without active illumination, effectively handling scenes with minimal temperature contrast and atmospheric effects.

Contribution

It introduces a joint estimation technique for range and object properties from passive thermal data, explicitly modeling atmospheric absorption and reflection effects.

Findings

Successfully estimated ranges from 15m to 150m in natural scenes.

Regularization and spectral band availability are crucial for accurate estimates.

Method performs well when reflected downwelling is negligible.

Abstract

Passive hyperspectral longwave infrared measurements are remarkably informative about the surroundings. Remote object material and temperature determine the spectrum of thermal radiance, and range, air temperature, and gas concentrations determine how this spectrum is modified by propagation to the sensor. We introduce a passive range imaging method based on computationally separating these phenomena. Previous methods assume hot and highly emitting objects; ranging is more challenging when objects' temperatures do not deviate greatly from air temperature. Our method jointly estimates range and intrinsic object properties, with explicit consideration of air emission, though reflected light is assumed negligible. Inversion being underdetermined is mitigated by using a parametric model of atmospheric absorption and regularizing for smooth emissivity estimates. To assess where our estimate is likely accurate, we introduce a technique to detect which scene pixels are significantly influenced by reflected downwelling. Monte Carlo simulations demonstrate the importance of regularization, temperature differentials, and availability of many spectral bands. We apply our method to longwave infrared (8--13 $\mu$m) hyperspectral image data acquired from natural scenes with no active illumination. Range features from 15m to 150m are recovered, with good qualitative match to lidar data for pixels classified as having negligible reflected downwelling.