Evaluation of Hemispherical Total Emissivity for Thermal Radiation Calorimetry

An attempt to derive the hemispherical total emissivity from the normal emis- sion spectrum is proposed for Vycor and fused silica glasses. The normal emis- sion spectrum from a clear surface has been measured at steady state in the temperature range from 400 to 750 K. The sample is heated on one metal- backed face by thermal radiation from a heater. Temperatures inside the sample were monitored by thermocouples at two points near the surfaces. Evaluation of the hemispherical total emissivity from the normal emission spectrum is deter- mined by means of Kramers Kro nig analysis and virtual mode equations. Assuming a linear temperature distribution within the sample, the thermal con- ductivities of silicate glasses were obtained at elevated temperatures. The results are comparable with those obtained by previous investigators. The effect of radiation heat transfer in a sample is also discussed.