Calibration of the industrial pyrometers in the temperature range between 1 000 °C and 3 000 °C requires a blackbody cavity source which temperature is measured using a reference pyrometer. The use of such blackbody cavity source is imposed by the characteristics of the pyrometers to be calibrated, because of the large field-of-view (usually of the order of 10 mm to 30 mm), which is not compatible with the dimensions of the fixed points used to calibrate reference pyrometers. The reference pyrometer and the pyrometer under the calibration can also have very different spectral characteristics. The filter’s transmission bandwidth of the reference pyrometer filter is generally of the order of 20 nm, but it can reach several hundred nanometers for the pyrometer under calibration. An unbiased comparison of the two instruments can be only obtained with a transfer blackbody cavity with emissivity equal to 1. The different spectral and spatial characteristics of the compared pyrometers require a determination of the effective emissivity of the cavity and a determination of the radial profile of surfaces intercepted by the field-of-view of pyrometers. The high temperature furnace Thermogauge HT-9500 LNE-Cnam has been characterized for use as a transfer cavity. For this, the longitudinal and radial gradients in the cavity have been measured by various methods. The effective emissivity of the cavity was calculated. An estimate of the corrections and uncertainties associated with non-unit emissivity are presented in this article.