Thermal (2.0 to 5.6 um) measurements of the normal emissivity EN of several diathermanous materials having slightly different refractive indices were made at 15.2 °C, 4.9 °C, and -5.6 °C. Calculations of the total hemispherical emissivity EH were made from EN and plotted against the optical depth PyaxX. A comparison of these data with a model proposed by R. Gardon [J. um. Ceram. Soc. 39(8), 278 (1956)] indicates that at near-ambient temperatures they agree very closely. This comparison presumes that the narrow range of refractive indices about n = 1.5 associated with these specimens would not preclude them from being treated as having a value of 1.5. It has been observed that EN > EH by 5% for both weakly and strongly absorbing materials. This is attributable to phase differences in the multiply reflected internal radiation attempting to exit the specimen throughout r steradians. Other radiation properties of the materials, i.e., diffuse transmittance TD, absorption coefficient Thx, and absorption index k, were calculated.