The Sodium Line Emission Pyrometer (SLEP) concept has been successfully utilized to perform nonintrusive temperature measurements in a coal combustor. Measurements were performed during many hours of operation, and no degradation in either the optical access or the signal were observed. Temperatures between 1700 to 2000 K were measured with accuracy of -10 K (often -1 K). An analysis indicates that errors due to the presence of particles or walls do not exist for the parameter range at which coal conversion facilities operate. The SLEP apparatus is at a laboratory development stage and it consists of the following components: an aerodynamic window optical access port, a fiber optic cable to transfer the collected emission, a high-resolution echelle spectrometer, a linear detector array and an optical multichannel analyzer. The theoretical background, design details, and laboratory and field measurements test results are discussed in this report. The instrument has a potential of serving as a key diagnostic for the fossil energy industry. Recommendations for future work are discussed. 9 refs., 32 figs., 15 tabs.