In recent years, there has been a growing demand to calibrate industrial blackbodies both at short wavelengths for lower temperatures and at long wavelengths for higher temperatures. User requests cover a very wide temperature range, from −20°C to 1,500°C in the infrared bands used by thermal cameras or as defined by specific applications (especially the 1–3 μm, 3–5 μm, and 8–12 μm bands). Therefore, LNE (Laboratoire National de Métrologie et d'Essais) has developed a radiance comparator with a mirror-based optical system, an associated set of interference filter wheels, a modular holder for several infrared detectors, and a lock-in amplifier. This setup is designed to be very versatile in terms of wavelength and temperature. Targeted performances have a thermal resolution better than 0.05°C, and a known and controlled size-of-source effect (SSE). A silicon detector and a visible-to-near infrared integrating sphere were used to assess the stray light inside the housing, and supplementary baffles and stops were used to reduce it to an acceptable level. The investigation included measurement of the SSE for this comparator layout. Then, the performance in the 3–5 μm and 8–12 μm bands, using, respectively, indium antimonide (InSb) and mercury cadmium telluride (MCT) detectors, was evaluated using a water heat-pipe blackbody. This paper describes the modeling and the technical solutions implemented to optimize the optical system. Preliminary results are presented for the short-term stability, the thermal resolution between −20°C and 960°C, and also the SSE up to 60 mm in these bands.