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Irradiance measurements of Re-C, TiC-C and ZrC-C fixed point blackbodies
K. Anhalt, P. Sperfeld, J. Hartmann
Physikalisch- Technische Bundesanstalt Braunschweig und Berlin, Germany
M. Sakharov, B. Khlevnoy, S. Ogarev, V. Sapritsky
All-Russian Research Institute for Optical and Physical Measurements (VNIIOFI), Moscow, Russia
Abstract. Specially designed Re-C, TiC-C and ZrC-C
fixed-point cells for irradiance measurements have been
produced by VNIIOFI. In several investigations at
VNIIOFI and PTB the usability of the design for irradiance
measurements has been demonstrated. Current
developments of furnace uniformity demonstrate further
improvements to the systems.
Introduction
High-temperature fixed-points using Metal carbon (MC)
and metal carbide carbon (MC-C) systems provide
innovative reference standards for radiometry and
thermometry [ 1 ]. While temperature metrology would
benefit from any additional fixed-point higher than the
freezing temperature of gold (1337.33 K) [2], the needs for
radiometry are primarily highest temperature to provide
sufficient and stable radiance and irradiance even in the UV
spectral range with a considerable large cavity aperture [3].
Fixed-point design
VNIIOFI introduced a special design of MC and MC-C
fixed-point cells for the use in the VNIIOFI/VEGA
BB3200/BB3500 furnaces [4]. These furnaces are in use
worldwide in research institutes and NMIs. The cells have a
length of approx. 95 mm and a diameter of 30 mm, as can
be seen in Fig. 1.
Figure 1: Schematics of the large aperture eutectic fixed-point
cells manufactured by the VNIIOFI
Due to their special design a diameter of the radiating cavity
of 10 mm is realized, which is more than a factor of three
larger than the 3 mm diameter usually obtained with the
smaller cells [3]. This large radiating area offers the
possibility to perform irradiance measurements using
absolutely calibrated filter radiometers. If the spectral
irradiance is measured in absolute units under a
well-defined geometry and the emissivity of the cavity is
known, it is possible to determine the thermodynamic
transition temperature of the eutectic material via Planck's
law of thermal radiation. With such a calibration a
fixed-point cell can serve as an absolute standard for
radiance and irradiance in radiometry and photometry.
Experimental setup
The large cavity diameter allows a precision aperture to be
positioned in front of the cavity outside the furnace,
defining the radiating area. A schematic of the experimental
set-up is given in Fig. 2.
HTBB with eutectic cell and
straylight baffles
Figure 2: Scheme of the experimental set-up used for
measuring the thermodynamic temperature of the cells
In the experiments at VNIIOFI and PTB the precision
aperture in front of the HTBB has a diameter of 3 mm,
while the apertures of the filter radiometers are approx.
5 mm in diameter, the distance between the two apertures
was about 1110 mm.
Detectors
precision
aperture
Two different types of filter radiometers were used: Three
narrow-band interference filter radiometers with centre
wavelengths around 676 nm, 800 nm, and 900 nm, and one
broad-band glass filter radiometer, having a centre
wavelength around 550 nm [5]. The spectral irradiance
responsivity of the filter radiometers has been calibrated
traceable to the cryogenic radiometer of the PTB [6].
Experiments at VNIIOFI and PTB
Filter
radiometer
The HTBB furnace was stabilized at a temperature approx.
25 K below the melting temperature of the eutectic material.
The melting process was initiated by a step-like current
increase of approx. 40 A after which the furnace was
stabilized at a temperature of about 25 K above the melting
temperature. For initiating the freezing of the eutectic alloy
the temperature in the furnace was suddenly reduced and
stabilized at a temperature of about 25 K below the freezing
temperature.
From the measured temperature profiles the point of
inflection during the melt was taken as the transition
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 291