Crystalline materials are indispensable to modern life. The production of crystals usually entails high temperatures, making radiation the dominant mode of heat transfer. Knowing the emissivity of the various surfaces in a crystal growth furnace is essential for numerical simulations which are widely used to better understand the complex, multiphysical nature of crystal growth. Moreover, emissivity data are needed for optical temperature measurement which is crucial to process control. The standard practice in both cases, namely extracting the closest available emissivity data from the literature, can lead to significant errors. The present paper reports on the development of a setup for in-situ measurement of emissivity in crystal growth furnaces. The setup is based on a radiometric technique using pyrometers as radiation sensors. Radiation shields with amplification are used to block background radiation and enable accurate measurement of low emissivities, e.g. for metallic surfaces. The accuracy of the measurements is assessed theoretically and by comparison to literature data for reference samples. Selected results are presented from ex-situ and in-situ measurements.