Astronomical instruments operating in the infrared-millimeter region often require internal sources for detector monitoring, instrument calibration, and health checking. We describe the design, modeling, and experimental evaluation of thermal emitters with a fast speed of response and low-power dissipation, suitable for the far-infrared-submillimeter region. The development of an internal illuminator for the Herschel Spectral and Photometric Imaging Receiver (SPIRE) satellite instrument is used as an example of the optimization of the design to meet particular requirements. A prototype illuminator design for SPIRE has been developed that, for a power dissipation of 1.5 mW, provides an equivalent blackbody temperature of 45 K with a 90% settling time of 220 ms.