A Novel Instrument Design for Studying Photometric Effects in Reflectance and Emission Data in Visible and Mid-IR Wavelengths

Background and Motivation

Decades of space missions have advanced our understanding of planetary surfaces through remote sensing and in-situ spectroscopy [1, 2, 3]. These efforts produce high-quality data but suffer from observational limitations and often requiring complex models and lab validation [4, 5, 6]. A major source of uncertainty is the effect of viewing geometry on spectral measurements, causing modifications in spectral features such as shape, depth, and slope, among others [7, 8, 9, 10]. This can cause various issues with misidentification of surface materials [11, 12]. Phase angle variations influence surface brightness and albedo, complicating surface temperature measurements from instruments like Diviner on the Lunar Reconnaissance Orbiter [10]. These complications have also been seen on Mars as phase reddening and variable brightness temperatures [13, 14]. Here, we present our Spectrogoniometer designed to measure effects of varying phase angles on spectral measurements and the ”first-light” data.

The instrument presented here supports both reflectance and emission measurements. In reflectance mode, the viewing geometry is defined by the phase angle (ϕ, Figure 1), which depends on the incidence angle (i, Figure 1), emergence angle (α, Figure 1), and their respective azimuthal directions (θ, Figure 1) [4]. In emission, it is characterized by the emission angle (e, Figure 1) [4]. Laboratory studies of viewing angle effects are limited due to challenges in instrumentation and automation. Most lab measure ments use fixed geometries (e.g., 30◦ phase angle with no azimuthal offset), while mission data has often relied heavily on modeled data for interpretation [15, 16]. Efforts to characterize these effects have focused primarily on the visible (VIS) and short-wave
IR (SWIR) ranges, with growing work in longer IR wavelengths (e.g. Figure 1) [17, 18]. Additionally, most existing setups only utilize reflectance, lacking emission capabilities [e.g. 18].

To support spacecraft data, lab measurements must replicate similar phase angles. Rovers and landers, unlike orbiters, frequently view surfaces at high emergence/emission angles. These large phase angles complicate comparison to nadir-viewing instruments and hinder data interpretation by affecting the angular distribution of scattered light, which depends on grain size, shape, and internal structure [5, 6]. Absorbing surface layers and small geometric changes at high angles further amplify scattering complexities and cause significant variations in the phase curve, making accurate analysis more difficult [6].

Due to the lack of well-characterized spectral measurements across varying phase angles, high-phase-angle data is often interpreted through complex models that rely heavily on estimations and assumptions. While a dedicated phase angle measurement campaign won’t eliminate these complexities, it can provide valuable constraints and improve model accuracy, enhancing space craft data interpretation.

With the growing volume of spacecraft observations and measurements, including past, current, and future missions, the need to collect lab data that matches observation geometries is increasingly important. Figure 2 ([19]) illustrates limited laboratory measurements across a small phase angle range, but it highlights an example of phase angle characterization in a laboratory setting. Even in the case of a few phase angles measured, there are spectral features that are changing or in some cases disappearing from the spectra.

Spectrogoniometers capable of measuring variable phase angles do exist [e.g. 17, 18], but [18] is restricted to VIS and SWIR ranges. Though helpful, current instruments have limited capacity to support a broad range of mission science objectives. Specifically, phase angle measurements in the thermal IR (TIR) remain largely unsupported in a laboratory setting. The instrument described in [17] was originally designed to cover both VIS and IR, but [20] notes the
IR capability is no longer functional. Improved characterization of phase angle effects across a wider spectral range will enhance the support and accuracy of modeling efforts used in the interpretation of both remote sensing and in-situ data.

Methodology and Instrument Description

The spectrogoniometer used in this work (Figure 3) is a multi-wavelength spectrogoniometer with a measurable spectral range of .285 μm - 18 μm with a spectral resolution of approximately 1cm−1. The instrument utilizes a dual arm design. One arm, the receiving arm (Rx Arm) "A", uses a series of relay optics "B" to collect light reflected or emitted from the sample to the detector. The transmit arm (Tx Arm) "C", holds the light source "D" used in reflectance measurements. The motion of each arm is depicted in Figure 4. Both arms have an approximate elevation angle range of ±70◦. The Tx arm has an azimuth range of 230◦. The sample tray "F" contains four fixed positions and rotates to bring each position under the arms. It contains the sample and three calibration targets. This design allows for efficient measurements and immediate calibration as needed for both reflectance and emission modes.

Data collection and calibration was performed using an automated system developed for the instrument. The Rx arm moves incrementally over its whole range then the Tx arm increments and the Rx arm repeats its range. After the Tx arm completes its elevation range, it moves in azimuth and the process is repeated until a measurement is performed at each combination of arm elevations and azimuth. In total, 14 incidence angles, 23 incidence azimuths, 14 emergence angles, and 28 emission angles are characterized per material. Each sample measured produces 4508 reflectance spectra and 28 emission spectra at various phase angle combinations. Data are processed using a pipeline developed for the instrument. For each material analyzed, a hyperspectral reflectance spectrum, a hyperspectral emission spectrum, and visible images are produced. Those data products will be used to identify
modifications (among other things) in spectra as phase angles change. Well characterized materials (e.g. olivine, pyroxenes, clays, etc.) were selected for the initial characterization of this instrument. Later, other materials commonly found in spectral databases (e.g. ASU spectral library [21]) will be analyzed to provide a number of phase angle characterizations for these already well characterized materials. The data produced by this instrument will be analyzed and produce science products which will aid in interpretations of spacecraft-based measurements (e.g. identify commonly altered spectral features, identify phase reddening in materials, surface brightness temperatures, etc.).