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Spectral Responsivity Scale between 1 µm and 19 µm at the NIST
G. P. Eppeldauer, J. Zeng, and L. M. Hanssen
National Institute of Standards and Technology
Gaithersburg, Maryland 20899, USA
Abstract. Most of the present spectral responsivity
measurements at the National Institute of Standards and
Technology (NIST) are based on the responsivity scale
realized on the recently developed facility for Spectral
Irradiance and Radiance Responsivity Calibrations using
Uniform Sources (SIRCUS). The SIRCUS facility and the
NIST reference responsivity scale are being extended. In
this work, the scale extension from the silicon to the
infrared (IR) wavelength range, using both NIST designed
pyrometers and commercial pyroelectric detectors, is
discussed.
Pyroelectric radiometers
New radiometers, based on LiTaO 3 pyroelectric
detectors, have been developed at the National Institute of
Standards and Technology (NIST) to realize and maintain
the extended spectral power responsivity scale. 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
was improved spatial non-uniformity of responsivity at
long wavelengths. The dome has a 4 mm diameter opening
in the optical axis. Figure 1 shows the picture of the
pyroelectric radiometer.
Fig. 1. Pyroelectric radiometer (top cover is removed)
The current measuring preamplifier is attached with a
connector to the bottom of the detector (front) unit. Two
identical radiometers were made to utilize the advantage of
group policy.
Two commercial pyroelectric detectors have also been
added to the group of the pyroelectric radiometer standards.
These two detectors are temperature monitored and black
paint coatings are applied on the detectors to convert
optical power change into temperature change.
Calibration facilities
The facility for Spectral Irradiance and Radiance
Responsivity Calibrations using Uniform Sources
(SIRCUS) made it possible to improve the uncertainty of
monochromator-based spectral radiant power responsivity
measurements and to extend the calibrations from power
mode to irradiance [1] and radiance measurement modes.
The high beam power and stability of the tunable IR lasers
of the SIRCUS facility makes it possible to calibrate a
wide range of IR detectors and radiometers for spectral
power, irradiance, and radiance responsivity.
In addition to the SIRCUS facility, the Ambient Infrared
Detector Characterization Facility was used to measure
frequency- and temperature-dependent responsivity,
signal-gain stability, and noise performance. The IR-Laser
Scatter and Detector Characterization Facility was used to
measure the spatial non-uniformity of responsivity,
linearity, angular responsivity, absolute responsivity,
stability, and repeatability. The Fourier Transform IR
Spectrophotometry Facility was used to measure the
spectral reflectance of the coated detectors to determine
the relative spectral responsivity.
Spectral responsivity calibrations
The relative spectral responsivity of all four pyroelectric
detectors has been determined from spectral reflectance
measurements. The spectral responsivity of the commercial
pyroelectric detectors is significantly structured because of
the black paint coating on the detectors. The absolute
responsivity tie points for the four radiometers have been
derived from trap and single element Si and Ge
photodiodes traceable to the NIST reference responsivity
scale [2]. More tie points have been derived from an
earlier developed LiNbO 3 pyroelectric radiometer standard
[3] and also from a single element LiTaO 3 pyroelectric
transfer detector (PD2) calibrated against the primary
standard cryogenic radiometer at 10.6 µm [4]. From these
measurements, the spectral power responsivity in the 1 µm
to 19 µm range was determined. Figure 2 shows the
spectral power responsivity curves of the two commercial
pyroelectric detectors. The relative expanded uncertainty
of the spectral power responsivity calibrations of the
pyroelectric working standard radiometers is 2.7 % within
the 1 µm to 19 µm wavelength range.
Radiometer characterizations
All four pyroelectric devices have been fully characterized
to operate them under application conditions so that the
measurement uncertainty in use will not be significantly
higher than when calibrated. The characterizations help the
users to determine the optimum spot size of the incident
beam, chopping frequency, and temperature dependent
responsivity corrections for different applications.
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 335