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A normal broadband irradiance (Heat Flux) calibration facility
M. Ballico and E. Atkinson
National Measurement Institute of Australia, Lindfield Australia
Abstract. The existing heat flux scale in Australia is
based on a set of thermopile and Gardon type sensors,
calibrated against a room temperature ESR using an 800 o C
blackbody filtered by crown glass. NMIA has recently set
up a new calibration system based on heat-pipe
blackbodies with temperatures measured on the ITS-90,
calibrated apertures and careful control of the ambient
infra-red radiances. As measurements are made in air,
corrections based on LOWTRAN data are made to the data,
and the accuracy of this correction has been assessed using
inert purging of part of the optical path. The facility is used
to allow confirmation of the spectral flatness of heat-flux
sensors into the thermal IR.
Introduction
Heat flux sensors are often calibrated under total
hemispherical irradiance conditions by inserting the sensor
fully into the mouth of a large blackbody source at a
known temperature and using Planck’s law to calculate the
irradiance [1]. This technique places significant thermal
load on the detector, requiring water cooling, but avoids
problems of atmospheric absorption, since the gas in the
blackbody is at essentially the same temperature as the
radiation field.
The present standard used in Australia [2] calibrates heat
flux sensors using an 800 o C blackbody filtered by crown
glass (and reported as such). Whilst giving reproducible
calibration results, these sensors are usually used to
measure surface irradiance due to thermal sources as low
600 o C. In such cases, a substantial fraction of the radiation
is above the 1-2 µm radiation source against which they
were calibrated, and spectral blackness of detector is
essential for the calibration to be meaningful.
Accordingly, NMIA has developed a facility to measure
the total normal irradiance responsivity to a range of
blackbody source spectra.
Experimental apparatus
The facility is based on the blackbody sources and
pyrometers from the NMIA radiation thermometry
calibration facility [3]. For 400-1100 o C Cs and Na
heatpipes are used whilst for 1000-2900 o C, a Thermogage
blackbody is used. Temperature gradients have been
measured and emissivities both measured and
calculated [3,4,5], and are included as uncertainties.
Temperature measurements are made using NMIA’s
850 nm and 650 nm pyrometers (MTSP and HTSP)
calibrated on the ITS-90 give uncertainties from 0.2 o C
over 400 o C-1100 o C rising to 0.5 o C at 1700 o C.
The measurement is essentially trivial: the blackbody
temperature is measured, and an aperture of known area in
front of the blackbody provides a calculable irradiance in
the measurement plane. However, at low source
temperatures the irradiance from the surroundings is
comparable to that from the aperture. Retro-reflection from
the thermopile and its surrounds, ambient reflection and
thermal emission from the plane containing the aperture
must be controlled.
The aperture is mounted in a 300 mm square ribbed
anodized panel (Figure 1). Its IR reflectance, is measured
[4] at below 0.5%. The aperture itself is covered by a light
trap consisting of anodized concentric sharpened annuli,
with a reflectance estimated below 0.2%. Over 100 W flux
from the blackbody illuminates the aperture assembly, but
its temperature rise (and consequent re-radiation to the
thermopile) is kept below 0.1 o C by water cooling. Either
side of the aperture is clear of an F/1 viewing cone to
minimize the risk of stray reflections.
The pyrometers, aperture assembly and thermopile are on a
translating stage in front of a range of blackbody sources,
and a typical measurement sequence consists of measuring
the blackbody temperature, then measuring the thermopile
signal with the aperture assembly opposite either the
blackbody or a light trap at ambient temperature.
Atmospheric absorption corrections
The commercial computer package LOWTRAN has been
used to compute the transmission through a range of
atmospheres and paths (Figure 2) It is interesting to note
that the integrated absorption predicted by LOWTRAN
increases nearly as the square root of the distance (naively,
one may expect something like Beer’s law). This results
from the strong overlapping of numerous absorption lines.
One consequence of this is that the absorption initially
decreases very quickly (with distance) however, after
600 mm the rate falls to 5%/m for an 800 o C source. At
higher source temperatures, less of the spectrum falls in
the major CO 2 and H 2 O bands and the total absorption is
lower.
The total path length between the aperture of the
blackbody and sensor is about 600 mm however the air
column in the mouth of the blackbody is not at ambient
temperature. Air near the blackbody temperature will
radiate equal to its absorption, and may be considered part
of the blackbody, but air at temperatures intermediate
between the blackbody and ambient will have lower
density and hence shorter effective optical thickness, but
will also have thermally broadened absorption bands. We
have not yet fully studied this effect, and as the insulation
in the front of the blackbody is 100 mm in length, it is
simply included as a 70 mm uncertainty in the effective
atmospheric path. This results in an uncertainty in
transmission varying from 0.3% at 600 o C to 0.2% at
1500 o C.
Experimental measurements of atmospheric
absorption
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 273