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Illuminating the Darkness: Photometric Analysis Under Diffuse Lighting

Presentation #101.07 in the session Moon & Earth I (Oral Presentation)

Published onOct 23, 2023
Illuminating the Darkness: Photometric Analysis Under Diffuse Lighting

Introduction: The ShadowCam instrument acquires high resolution (~2 m/pix) and high signal-to-noise ratio images of shadowed regions dimly illuminated by light reflected from the surrounding terrain. These capabilities enable mapping inside permanently shadowed regions (PSRs) near the lunar poles to investigate albedo patterns, examine morphology, and look for any changes to the surface over the lifetime of the KPLO mission.

Under typical (primary) lighting, the Sun is the dominant light source and can be modeled as a point source. With this simplification, each spot on the surface has a single incidence angle and a corresponding emission and phase angle. However, these assumptions and simplifications no longer hold under secondary illumination conditions, especially for cases where ShadowCam is imaging the interior of a shaded crater illuminated by a diffuse crescent of light along the crater rim. In these cases, a single point on the surface is illuminated from multiple incidence angles. Furthermore, these photometric angles vary substantially across the scene as we move from the wall of the crater that is being illuminated to the crater floor and the opposite wall.

Simulations: We can calculate a series of view factor maps using digital terrain models of the shaded area and the surrounding region scattering light. These view factors enable the estimation of the contribution of light from each part of the diffuse source and depend only on the local topography, so they must be combined with a primary illumination model to determine which areas can scatter light at the specified time.

We can derive the contribution of light reflected from each section of the illuminated crater wall to a point within the cavity. In doing so, we model the local photometric angles for each of these rays. We refer to these as secondary phase, local secondary emission, and local secondary incidence to distinguish them from the primary illumination conditions of the scene.

Discussion: Modeling the photometric angles is critical to interpreting differences in the reflectance values across the scene, examining the morphology, and looking for changes across the surface. Multiple observations of a local scene acquired throughout the mission with different combinations of median secondary phase angles may be required to build confidence in interpretations of reflectance variations differences and local morphology. In addition, combining the slewed images with nadir observations enables investigations of the slope of the median phase angle curve (via a phase ratio), which provide a means to constrain additional surface properties.

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