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A neutral density (ND) filter made of an LWIR-transmissive, heat-resistant material is interposed between a longwave infrared (LWIR) imager and an entity that is highly emanative of both light and heat. According to exemplary inventive practice, the ND filter is a thermoplastic (e.g., polystyrene) sheet characterized by a thickness in the 1-2 mm range and a thermal conductivity≦0.13 W/m-K. Important parameters of the ND filter include LWIR transmittance (which depends on material and thickness) and thermal conductivity (which depends on material). The quality of the image taken of the entity is affected by the respective degrees of LWIR attenuation by, and heating up of, the ND filter. Accordingly, the material and the thickness of the ND filter are selected to optimize the image, in particular so as to avoid saturation and/or whiting-out of the image due to insufficient LWIR attenuation and/or excessive ND filter temperature.