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NIST Reference Cryogenic Radiometer
- Primary Optical Watt Radiometer (POWR) -

Photo of the POWR laboratory

The POWR laboratory
 The Optical Technology Division of NIST realizes and maintains the unit of optical power (watt). A new NIST reference cryogenic radiometer, Primary Optical Watt Radiometer (POWR), has been constructed and replaced the previous reference radiometer (HACR) in 2005. POWR realizes and maintains the optical power unit (watt), which serves as the basis for all radiometric and photometric units and scales realized at NIST. The POWR, including the detector module, has been completely designed by NIST. The new radiometer is designed to have the versatility to grow with NIST's needs, to embrace new technologies, and be able to provide optical power measurements with uncertainties of 0.01 % (k=1), which has been verified by a recent intercomparison with two other cryogenic radiometer facilities at NIST.

Compared with HACR, POWR is designed to have a greater dynamic range, faster response time, lower noise, and improved modular construction. Furthermore, it 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. The POWR facility is currently operational at the standard laser wavelengths of 488 nm, 514.5 nm, and 632.82 nm.

Improvements of the POWR facility over its predecessor HACR facility

  • Greater dynamic range of primary standard power levels (0.01 % at 1 mW to 1 mW)
  • Periodic shuttering with active cavity, feed-forward, and phase-sensitive detection.
  • Optional 2 Kelvin operational temperature mode: gives greater sensitivity compared with the standard 4.2 K operational temperature.
  • Continuous spectral coverage possible at the primary standard (using tunable lasers from the adjacent SIRCUS and IR-SIRCUS facilities).
  • Extended IR coverage and UV coverage using the primary standard.
  • Possibility for windowless transfer from the primary standard, decreasing transfer uncertainties.
  • Horizontal optical path eliminates the need for turning mirror, and provides much more space for larger transfer detectors to be calibrated.
  • Beyond radiant power, irradiance measurements using the POWR facility are conceivable.
  • Captures future, unknown primary standard requirements through modular design.
  • 2-weeks between liquid helium transfers, even when operating at 2 K temperature.
Drawing of the cryostat using liquid nitrogen at 77 K, liquid helium at 2 K. Receiving cavity is mounted at the bottom of the cryostat.

Figure 1. Construction of the cryostat.		 Photograph of the detector module - the external view.

Figure 2a. Detector module


Photograph of the detector cavity.

Figure 2b. Detector cavity.

Internal intercomparison

Optical power measurements on POWR were compared with two other absolute cryogenic radiometers at NIST:
  1. LOCR (Laser-Optimized Cryogenic Radiometer) located in NIST Boulder,
  2. a portable ACR located in the NIST SIRCUS facility.
The spectral power responsivity of two trap detectors measured at 488 nm, 514 nm, and 633 nm with the two cryogenic radiometers agreed to within ±0.02 %, comparable to the uncertainties of these radiometers. 		Photograph of the detector module mounted on the bottom of cryostat.

Figure 3. Detector module mounted on the cryostat (bottom view)


Photograph of Joe Rice and Jeanne Houston aligning the optics of the radiometer.

Figure 4. Joe Rice and Jeanne Houston aligning the optics.

Return to Novel Detectors and Transfer Standards
For technical information or questions, call:
Joe Rice
Phone: (301) 975-2133
Email: joe.rice@nist.gov 		      Jeanne Houston
Phone: (301) 975-2327
Email: jeanne.houston@nist.gov

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Online: September 1997   -   Last updated: July 2006