The performance of thermal detectors is derived for devices incorporating materials with non-uniform spectral absorption. A detector designed to have low absorption in the primary thermal emission band at a given temperature will have a background-limited radiation noise well below that of a blackbody absorber, which is the condition typically assessed for ultimate thermal detector performance. Specific examples of mid-wave infrared (λ ∼ 3-5μm) devices are described using lead selenide as a primary absorber with optical cavity layers that maximize coupling. An analysis of all significant noise sources is presented for two example room-temperature devices designed to have detectivities up to 4.37×1010 cm Hz1/2 W -1, which is a factor 3.1 greater than the traditional blackbody limit. An alternative method of fabricating spectrally selective devices by patterning a plasmonic structure in silver is also considered.