High photometric accuracy is essential for precise measurements of optical properties. The effect of source radiation power on the photometric accuracy is a mechanism of growing importance, since modern FTIR spectrometers possess strong sources for high signal-to-noise ratio. A related problem is radiation incident on the detector from a high-temperature sample. This paper presents an analysis of the photometric error in the reflectance measurements for samples at high temperatures. The maximum temperature that can be investigated as a function of the required photometric accuracy is determined for several dielectric crystals and film- substrate composites.