Radiation pyrometry is often used in the measurement of temperature during polymer processing operations such as film blowing. Since polymers are semitransparent in the infrared, the temperature measured by the pyrometer will correspond to some average of the internal temperature distribution. In the present study, an expression is derived for this average when a sheet with a thermal gradient across its thickness is considered. This expression is used to study the effect of the absorption characteristics of the sheet on this average when the material is a gray medium.