At the DLR Solar Furnace, calorimeters or circular foil radiometers are used for calibrating video camera based flux mapping systems. For many years, we have been investigating the accuracy of these devices and have undertaken several efforts to check their performance. Furthermore, we developed new instruments and compared these devices to the existent ones. The most recent measurements were collected within the frame of an international campaign, called Intercomp 2000, located at the DLR Solar Furnace in Cologne. This was the opportunity to test our own flux gauges, the 10000 suns Kendall radiometer, the 5000 suns SunCatch calorimeter, 2 HyCal circular foil radiometers for 3000 suns and 2 more HyCal radiometers for 11000 suns. About 100 measurements were taken in the range from 0,5 $MW/m2$ up to 5 $MW/m2.$ The Kendall radiometer and the SunCatch calorimeter agreed well within the 2% margin. The circular foil HyCal radiometers showed significantly larger deviations. This leads to the conclusion that we have to change the manufacturer-given calibration constants of the HyCal radiometers by using the data collected with either the SunCatch or the Kendall flux gauge.

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