Moses Kodur, Surface Optics Corp; Samuel Dummer, Surface Optics Corp
https://doi.org/10.64861/GVUO9955
Keywords: Roughness Crossover, Diffuse Gold, Satellite Characterization, SDA, BRDF, HDR Reflectance, Specular vs Diffuse, MWIR/LWIR, Glint, EO/IR Signature Modeling, Surface Metrology, Specular Fraction, Emissivity, Hemispherical Emittance
Abstract:
In the field of Space Domain Awareness (SDA), the fidelity of electro-optical and infrared signatures is fundamentally limited by the accuracy of the underlying material property models used for glint prediction and stray-light analysis. Traditionally, many signature simulation frameworks rely on simplified assumptions where material surfaces are categorized as either purely diffuse or purely specular, often treating reflectance as a static material constant. However, experimental data from this study reveals that the distribution of reflected energy is a highly dynamic function of surface morphology that evolves significantly with wavelength. As the incident wavelength increases relative to the characteristic scale of surface roughness, a roughness crossover occurs, fundamentally altering the directional scattering behavior of the asset. This work provides an extensive experimental dataset quantifying this transition, demonstrating that directional reflectance partitioning can shift by orders of magnitude even when total hemispherical reflectance remains deceptively stable. For the SDA community, understanding this shift is critical: a surface that appears dark or diffuse in the visible may produce high-intensity specular glints in the infrared, leading to significant errors in apparent radiance and thermal balance modeling if not properly characterized._x000D_
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To provide these high-fidelity insights, this study utilizes a controlled set of gold-coated, machined substrates with nominal grit levels ranging from 120 to 1500, creating a morphology gradient that isolates the impact of surface texture on optical performance. The primary experimental highlights come from a dual-track optical characterization involving spectral Hemispherical-Directional Reflectance (HDR) and multi-wavelength Bidirectional Reflectance Distribution Function (BRDF) mapping. The HDR results demonstrate a high sensitivity of specular partitioning to subtle morphology changes, showing that for intermediate surface finishes, the specular-to-diffuse ratio can change by over ten times across the mid-wave to long-wave infrared transition. Crucially, in many of these cases, the total hemispherical reflectance remained within a few percent of its baseline value, providing empirical evidence that total reflectance is an incomplete and potentially misleading descriptor for signature modeling in the infrared. _x000D_
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The BRDF results further illuminate the non-linear nature of these transitions by providing direct observations of specular lobe evolution. The study captures the resulting peak amplitudes, angular widths, and off-specular tails for each controlled finish, revealing that while the specular peak amplitude rises rapidly in the crossover regime, the angular width of the specular lobe does not always follow a simple linear relationship with RMS roughness. Instead, the results show that nominally identical finishing protocols can produce distinct slope statistics that drive measurably different scattering distributions. For the SDA modeler, this means that even if the total energy in the specular channel is known, the spread of a glint—and thus the probability of detection by a ground-based sensor—can vary significantly based on the substrate preparation. These findings are presented as fitted parametric representations, such as microfacet-based models, enabling the data to be directly ingested into ray-tracing and signature-prediction software._x000D_
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Finally, the study connects these optical observations to operational thermal performance through elevated-temperature spectral emissivity measurements. By measuring emittance from room temperature to above 400°C, the data shows how directional reflectance partitioning influences the apparent radiance of a body. Even when two surfaces have similar hemispherical emittance—and thus similar bulk cooling rates—their directional emittance, and therefore their brightness to an infrared sensor at a specific look-angle, can differ by a factor of two or more due to morphology-driven specular effects. This work provides the SDA community with a measurement-grounded framework for predicting the signatures of complex surfaces, offering a pathway to validate and refine the next generation of space object models. The results emphasize that surface finish is not merely a manufacturing byproduct but a primary design and signature-control parameter that must be accounted for to ensure the accuracy of space-based asset detection and characterization._x000D_
Date of Conference: September 15-18, 2026
Track: Satellite Characterization