Shock temperature measurements of various ionic solids and metals have recently been only somewhat corroborated by static experiments using laser-heated diamond anvil cells. In each technique, optical pyrometry is employed; however, the shock temperature measurements of the several groups usually use from four to twelve wavelengths between 400 and 1050 nm, while the diamond cell experiments involve optical multichannel analyzer data over a smaller spectral range. We propose that the argon pressure medium often used in laser-heated diamond anvil cell measurements exhibits a spectral transmittance filtering effect that affects the apparent temperature measured. This is due to the large thermal gradients and the concomitant refractive index variations. We use an optical transfer matrix approach to calculate the effect on the measured spectral radiance and temperature, and find that measured temperature may be inaccurate due to this interference effect by as much as 10&percent;. ©2000 American Institute of Physics.