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Primary optical watt radiometer (POWR)


The Sensor Science Division of NIST realizes and maintains the unit of optical power (watt) with the NIST reference cryogenic radiometer, Primary Optical Watt Radiometer (POWR). It serves as the basis for all radiometric and photometric units and scales realized at NIST, providing optical power measurements with uncertainties of 0.01 % (k=1).


The POWR, including the detector module, was completely designed by NIST to have the versatility to grow with NIST's needs and embrace new technologies. It provides optical power measurements with uncertainties of 0.01 % (k=1), which have been verified by an intercomparison with two other cryogenic radiometer facilities at NIST.

POWR is installed adjacent to the facility for Spectral Irradiance and Radiance using Uniform Sources (SIRCUS), providing ready access to a variety of lasers. These lasers allow a broad range of wavelength and power levels to be selected for scale transfer to portable detectors, further reducing the uncertainties in the measurement chain. Additionally, the modularity of the critical detector section permits new POWR detector modules to be designed and built which are better optimized for specific transfer wavelengths and power levels.

Major Accomplishments

POWR impacts total solar irradiance measurements.  For more information, see the PowerPoint presentation Comparison of the total solar irradiance radiometer facility cyrogenic radiometer against the NIST Primary Optical Watt Radiometer.

Created September 18, 2009, Updated October 16, 2023