Joel B. Fowler is an electrical engineer whose work at NIST included the design and characterization of a water-bath blackbody source for radiometry. His 1995 paper describes a cavity whose temperature and effective emissivity could be controlled and measured accurately.[ref]
The apparatus placed a shaped copper cavity in a stirred, temperature-controlled water bath. The cavity geometry increases the number of internal specular reflections, raising its effective emissivity toward one, while the bath reduces temperature variation. The paper reports operation from 278 K to 353 K and calculated emissivity 0.9997, increased to 0.99997 with a restricted aperture. Geometry, thermal control and measurement uncertainty accounting therefore work together to create a useful measurement calibration source.[ref]

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