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arXiv · 2607.05003

In-Band Scattering and Absorption of Infrared Blocking Foam Filters for Millimeter-wave Cameras

Alex Thomas·Bugao Zou·Shreya Sutariya·Yuhan Wang·Gabriele Coppi·Samuel Day-Weiss·Nicholas Galitzki·Kathleen Harrington·Erin Healy·Claire Lessler·Aashrita Mangu·Jeffrey McMahon·Michael D. Niemack·Edward J. Wollack
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Abstract

Expanded closed-cell polymer foams are widely used as thermal infrared (IR) blocking filters in millimeter-wave cameras, particularly for Cosmic Microwave Background observations. Precise knowledge of their millimeter-wave properties is essential for optimizing sensitivity. We present broadband (150 GHz - 2 THz) transmittance spectroscopy of Styroace-II and several Zotefoam filters, fitting their spectra with a radiative transfer model incorporating dielectric absorption and Rayleigh, Mie, and higher-order scattering. For a typical 5~cm thick filter stack at 280~GHz, Styroace-II exhibits ${\sim}10\%$ scattering with absorption estimated as ${\lesssim}5\%$ by effective-medium theory, while Zotefoam HD30 offers superior performance at ${\sim}3\%$ scattering and absorption likewise bounded to ${{\lesssim}0.3\%}$. Each model component is constrained at the ${\sim}0.1\%$ transmittance level for millimeter wavelengths. We observe batch-to-batch scattering variability of up to 2 percentage points in foams with multiple tested batches. Less commonly used Zotefoam formulations (LD15 and LD24) can further reduce in-band scattering to ${<}1\%$ while maintaining negligible in-band absorption and likely comparable IR blocking due to shared polyethylene absorption features and similar cell sizes. Based on this work, a filter constructed from the best measured LD24 batch has replaced the Styroace-II filter in a Simons Observatory 220/280 GHz Small Aperture Telescope.

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Alex Thomas, Bugao Zou, Shreya Sutariya, Yuhan Wang, Gabriele Coppi, Samuel Day-Weiss, Nicholas Galitzki, Kathleen Harrington, Erin Healy, Claire Lessler, Aashrita Mangu, Jeffrey McMahon, Michael D. Niemack, Edward J. Wollack. 2026-07-07. In-Band Scattering and Absorption of Infrared Blocking Foam Filters for Millimeter-wave Cameras. https://arxiv.org/abs/2607.05003

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