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Standard Detector Developments
Transfer standard radiometers are being developed to propagate the low uncertainty (~0.02 %, k = 2) radiant power scale of primary standard cryogenic radiometers (POWR and L1) to other NIST reference calibration facilities (SIRCUS, SCF, FASCAL-2, Blackbody temperature and emissivity, etc.) and to improve the System International (SI) units, the candela and kelvin. Usually, the transfer standard radiometers can be operated in both power and irradiance measurement modes and can maintain the reference spectral responsivity scales for spectral power, irradiance, and radiance responsivity calibrations in the 200 nm to 2.5 µm wavelength range. Wavelength extending transfer standard radiometers are also being developed to extend the spectral responsivity scales up to 19 µm. Working standard radiometers and photometers are being developed that are calibrated against the transfer standard radiometers, to realize, maintain, and disseminate the different radiometric and photometric scales, and can be used for routine calibrations. Both the transfer and working standard radiometers are radiometrically and electronically characterized before responsivity calibrations. Silicon tunnel-trap detector
Silicon tunnel-trap detector with input aperture.

Introduction

 The goal of this detector application and radiometer-photometer development project is to decrease the large measurement-uncertainty gap between the NIST primary standard radiometer and field measurements. To achieve this goal, a large variety of transfer and working standard radiometers are being developed. The radiometers and photometers are optically and electronically characterized and verified that they can be calibrated with the lowest possible measurement uncertainty in different radiometric and photometric measurement modes depending on the diverse requirements in different scale realizations, scale propagations, and field applications.

Transfer standard radiometers
  • Reflectance-type silicon trap-detectors.
    • UV silicon trap-detectors have been developed and built based on UV damage resistant IRD UVG-100 silicon photodiodes, selected for high and equal shunt resistance. The three trap detectors are equipped with 5 mm diameter precision apertures and can maintain the reference spectral power and irradiance responsivity scales between 200 nm and 300 nm.

    • Silicon trap-detectors have been developed and built using either Hamamatsu S6337 or S1337 photodiodes. The four trap detectors are equipped with 5 mm diameter precision apertures and used as transfer standards for the IR-SIRCUS and IR-Laser Scatter and Detector Characterization facilities.

    • The UV-100 silicon trap-detector was built using three new-generation UDT UV-100 silicon detectors to compare it with the earlier fabricated UDT QED-200 silicon trap detectors (also built with UV-100 inversion layer photodiodes). The trap detector is equipped with a 5 mm diameter precision aperture.
Figure 1
Figure 1. Computer design of a reflectance-type silicon trap detector.
 Figure 2
Figure 2. Picture of the NIST developed reflectance-type silicon trap-detectors.
  • Silicon tunnel-trap detectors

    A group of six silicon tunnel-trap detectors have been developed. Each of them is based on two medium size (10 mm by 10 mm) Hamamatsu 1337 and four large size (18 mm by 18 mm) Hamamatsu 6337 silicon photodiodes that are connected in parallel. The tunnel-trap detectors are equipped with 5 mm diameter precision apertures at the front and can maintain the lowest uncertainty (reference) spectral power and irradiance responsivity scales between 300 nm and 950 nm.
Figure 3
Figure 3. Beam propagation in a 6-element silicon tunnel trap detector.
 Figure 4
Figure 4. Angular responsivity of a silicon tunnel-trap detector in irradiance mode.
Figure 5
Figure 5. Computer designed cut-out-view of a silicon tunnel-trap detector.
Figure 6

Figure 6. Noise-equivalent-power of a silicon tunnel-trap detector/photocurrent meter at different signal-gain selections in both dc and ac signal measurement modes.
  • Filter trap-radiometers

    Tunnel-trap detector based filter radiometers have been designed and built for use in remote sensing applications. The two radiometers were designed for both radiance and irradiance measurements. The filter wheel was temperature stabilized and equipped with up to 5 filters. A Gershun tube arrangement with two precision apertures was attached to the front of the radiometer. With the Gershun tube installed, the instrument operates in radiance mode; with it removed, in irradiance mode. The filter trap radiometers have been characterized for optical and electrical performance, and have been calibrated for responsivity using both narrow-band, tunable-laser-illuminated and broad-band lamp-illuminated integrating sphere sources.

Figure 7

Figure 7. Filter trap radiometer with a radiance measuring Gershun tube at the front.
 Figure 8
Figure 8. Filter trap radiometer with removed front panel.

Figure 9

Figure 9. Baffle arrangement inside the Gershun radiance tube and FOV and out-of-FOV rays of an extended source measurement.
  • LiNbO3 pyroelectric radiometer

    This radiometer was developed to extend the NIST reference spectral responsivity scale from the visible range to the ultraviolet (UV) and infrared (IR). The transmission of the gold-black coated LiNbO3 pyroelectric material is negligibly small, therefore the absorptance, equal to (1-reflectance), is proportional to the responsivity of the detector. The spectral total reflectance of the coating was measured with integrating spheres and spectrophotometers to determine the relative spectral responsivity from the UV to the IR. The relative spectral responsivity was converted into absolute spectral power and irradiance responsivities by measuring the total power in a 442 nm stabilized laser beam. The reference device for absolute calibration was a Si trap-detector calibrated against the primary standard cryogenic radiometer. The spectral power and irradiance responsivity scales of the pyroelectric radiometer have been realized between 250 nm and 2.5 µm with a relative standard uncertainty of less than 0.34 % (coverage factor k = 1).
Figure 10
Y = Mo = M1*x + ... M8*x8 + M9*x9
M0 0.998
M1 5.6222e-06
M2 -4.4275e-09
M3 29764e-139
R 0.9934
Figure 10. Spectral power responsivity of the LiNbO3 pyroelectric transfer standard radiometer.
Figure 11

Figure 11. Design of the LiNbO3 pyroelectric radiometer.
  • LiTaO3 pyroelectric radiometer

    New radiometers, based on LiTaO3 pyroelectric detectors, are being developed to extend the spectral power responsivity scale up to 19 µm. The gold-black coated pyroelectric detectors are temperature controlled using a thermoelectric cooler/heater and a thermistor sensor. A gold coated reflecting dome is mounted above the tilted detector to decrease the IR reflectance loss and increase signal absorption. The result is improved spatial uniformity of responsivity at long wavelength. This transfer standard radiometer was calibrated against a reflectance type silicon trap detector. The relative responsivity was determined from spectral reflectance measurements on the FT IR Spectrophotometry Facility when the detector was equipped with the dome.
 Figure 12

Figure 12. LiTaO3 pyroelectric transfer standard radiometer.

Working standard radiometers
  • First generation radiometers

    The NIST developed first generation radiometers and photometers made the transition possible from traditional lamp-based scales to lower uncertainty detector-based scales.

    • Irradiance meters

      Figure 13

      Figure 13. Working standard silicon (b) and InGaAs (a) irradiance meters calibrated on the SCF facility and maintain the irradiance responsivity scale for the Night Vision Radiometer Calibration Facility.

    • Illuminance meter

      Figure 14

      Figure 14. Cross section of a temperature controlled photometer package used to disseminate the scale.

    • Silicon radiance meter

      Figure 15

      Figure 15. Cross section of the silicon radiance meter that holds the NIST spectral radiance responsivity scale as realized on the SIRCUS facility.
  • Extended-InGaAs radiometer The extended-InGaAs working standard radiometer is working in irradiance measurement mode. The 1 kΩ detector shunt resistance is transformed into 400 MΩ using a boot-strapped current meter. The radiometer holds the spectral irradiance responsivity scale of the IR-SIRCUS between 1 µm and 2.5 µm.

    Figure 16
    Figure 16. Cross-section of the ext-InGaAs radiometer. The shunt resistance is increased to 400 MΩ (up to 10 Hz).
Figure 17
Figure 17.
  • InSb radiometer
    The InSb power and irradiance measuring radiometers have a NEP of 0.2 pW/Hz1/2. They hold the spectral power and irradiance responsivity scale up to 5.1 µm.
Figure 18
Figure 18.		 Figure 19
Figure 19.

Figures 18 and 19. InSb power and irradiance meter. Spatial uniformity tested for power mode measurements.
  • HgCdTe (MCT) irradiance meter
    The MCT working standard radiometers are working in irradiance mode instead of power mode where the spatial non-uniformity of responsivity can change from 15 % to 90 %. The dominant uncertainty component in irradiance mode is the shown 2 % maximum error in the angular response relative to the ideal cosine response function.
Figure 20
Figure 20.		 Figure 21

Figure 21.

Figures 20 and 21. Spatial uniformity and angular response of a typical PC-MCT detector at 10.6 µm.


Figure 22

Figure 22. NIST developed working standard radiometers.
  • New generation radiometer-photometer developments
    A new generation modular radiometer-photometer system has been designed at NIST to satisfy the increased measurement and uncertainty requirements both inside and outside of NIST. In the new modular design, each radiometer has two independent temperature control loops. The first loop controls the temperature of the photodiode to low temperatures to obtain low noise and drift amplification for the radiometer output. There is a 1 to 4 stage thermoelectric cooler and a thermistor inside of the sealed can of the commercially purchased photodiodes. The bottom of the can is attached to a copper base plate that delivers the dissipated heat to the heat sink. The second loop can control the temperature of a filter holder where filters can be positioned in front of the photodiode. The holder is pulled with three nylon screws against three thermoelectric coolers that are attached to the copper base plate. In irradiance mode measurements, instead of filters, diffusers can be positioned in front of the photodiode. The diffuser will eliminate any responsivity changes in the radiometer for different input beam geometries. A thin aperture touches the front surface of the filter or diffuser. The front surface of the aperture is the reference plane of the radiometer for irradiance measurements. Regular and UV damage resistant Si, Ge, InGaAs, and extended-InGaAs single-element photodiodes can be applied in this design. The input optics can be easily changed according to the application requirements. The measuring electronics, located in the bottom cylinder of the measuring head, can measure DC or AC optical radiation within a dynamic signal range of 14 decades.
Figure 23
Figure 23. Design scheme of the measuring head of a new generation radiometer.

Figure 24

Figure 24. Circuit diagram of silicon photodiode current meters to measure radiant power over 14 decades using either manual or remote signal-gain control.
  • Common radiometer-photometer developments with Gamma Scientific
    The NIST radiometer-photometer designs were further developed and implemented in a cooperation with Gamma Scientific to produce a versatile radiometer and photometer system. The design considerations are discussed in a PPT presentation.

    Figure 25

    Figure 25. Modular design of the Gamma Scientific photometer.

References
  • Chapter 4: Transfer standard filter radiometers: applications to fundamental scales, G.P. Eppeldauer, S.W. Brown, and K.R. Lykke, Experimental Methods in the Physical Sciences 41 (Elsevier Inc., 2005), pp. 155-211.

  • Spatial and angular responsivity measurements of photoconductive HgCdTe LWIR radiometers, H. Gong, L.M. Hanssen, and G.P. Eppeldauer, Metrologia 41, 161-166 (2004).

  • Gold-black coatings for freestanding pyroelectric detectors, J. Lehman, E. Theocharous, G. Eppeldauer, and C. Pannell, G Measurement Science and Technology, 14 (Institute of Physics Publishing Ltd., UK, (2003), pp. 916-922.

  • Spectral responsivity determination of a transfer-standard pyroelectric radiometer, G. Eppeldauer, M. Racz, and L. Hanssen, Proc. SPIE Infrared Spaceborne Remote Sensing X Symposium, 4818, July 10-11, Seattle, Washington (2002), pp. 118-126.

  • NIST Technical Note 1438, Optical Radiation Measurement with Selected Detectors and Matched Electronic Circuits Between 200 nm and 20 µm, G.P. Eppeldauer, U.S. Government Printing Office, Washington, DC, 20402 (2001).

  • Spectral power and irradiance responsivity calibration of InSb working-standard radiometers, Eppeldauer G. and Racz M., Appl. Opt. 39(31), 5739-5744 (2000).

  • Realization of a spectral radiance responsivity scale with a laser-based source and Si radiance meters, G.P. Eppeldauer, S.W. Brown, T.C. Larason, M. Racz, and K.R. Lykke, Metrologia 37, 531-534 (2000).
  • Optical Characterization of Diffuser-input Standard Irradiance Meters, G. Eppeldauer, M. Racz, and T. Larason, SPIE Proc. Optika'98, 5th Congress on Modern Optics, 14-17 September, Budapest, Hungary. 3573, 220-224 (1998).

  • Near Infrared Radiometer Standards, Eppeldauer G., SPIE Proc. 2815, 42-54 (1996).
For technical information or questions, call:

George Eppeldauer
Phone: (301) 975-2338
Fax: (301) 869-5700
Email: geppeldauer@nist.gov

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Online: March 2007