T he C ryogenic S olar A bsolute R adiometer (CSAR)
and
the
M onitor
to
M easure
the I ntegrated
T ransmittance (MITRA)
of
W indows
André Fehlmann, W. Finsterle, and W. Schmutz, PMOD/WRC, Switzerland
R. Winkler, N. Fox, and E. Usadi, NPL, United Kingdom
P. Blattner, METAS, Switzerland
On the 1st April 2010 WMO officially signed the BIPM Mutual Recognition Agreement.
Thereby all signatories recognise the World Radiometric Reference (WRR) as the reference for solar radiative flux measurements. WMO in turn agreed on ‘... the importance of
SI traceable measurements to monitor climate change.’
Although the WRR is currently not directly traceable to the primary SI radiant power standards, comparisons using transfer instruments demonstrated that the two scales agree
to better than 0.03%, however with a large uncertainty of 0.14 %. This is still within the
WRR‘s originally stated uncertainty of 0.3 % (k= 1).
PMOD/WRC as WMO’s designated institute for irradiance measurements, and the two
national metrology institutes National Physical Laboratory (NPL) and Federal Office of Metrology (METAS) make a collective effort to introduce a possible new directly SI traceable
solar irradiance standard. We are adopting cryogenic technology to build a Cryogenic Solar Absolute Radiometer (CSAR).
Vacuum can
CSAR will be fully characterised and directly compared to NPL’s primary standard radiometer. At the end of September 2010 it will participate in the 11th International Pyrheliometer
Comparison (IPC) in Davos where it has to demonstrate its performance compared to the
WRR. To achieve the anticipated absolute accuracy of 0.01 %, we need to measure the
integrated transmittance of the entrance window. Our monitor MITRA is designed to fulfil
this task.
Once CSAR has proven its stability in two IPCs it may be considered a potential substitute
for the WRR. Thereby we could guarantee the continuity and SI traceability of the ground
based solar irradiance standard. Furthermore, this CSAR instrument serves as an engineering model for the proposed TRUTHS space mission. The ultimate goal of this satellite
is to bring a primary standard for radiant power and total solar irradiance into space. This
would give us the opportunity to recalibrate instruments in orbit and hence increase the
accuracy of earth observation data.
Reference blocks
RhFe temperature sensors
20 mm diameter TSI cavity
To measure temperatures, we use RhFe
sensors from LakeShore.
To avoid heat being dissipated in the wires
feeding the heaters, we are using superconducting materials. We are testing NiTi
(Tc ~ 9 K) which is only suitable for a working temperature of 4 K and MgB2 (Tc ~ 39 K)
which is still superconductive at 15 K.
CSAR
We are testing two different cavity diameters (15 and 20 mm). The cylindrical part of
the cavities are painted with a diffuse Nextel
black paint. The back plates are coated with
either Nextel black paint or NEC Toshiba NiP
black. First reflectivity measurements have
been made at a wavelength of 647 nm. Further measurements at different wavelengths
are planned.
absorptance
15 mm cavity; Nextel back plate
99.983 %
20 mm cavity; Nextel back plate
99.982 %
15 mm cavity; NiP back plate
99.996 %
Mechanical Gifford McMahon cryo-cooler
from Sumitomo
• 1st stage15 W @ 40 K
• 2nd stage 0.4 W @ 4 K
10 cm
Terrestrial solar Spectrum
(MODTRAN)
Quartz (Suprasil)
Saphir
Diamond
Since the cryocooler for space applications
has less cooling power, we introduced additional optional heat links to simulate its
15 K and 120 K stages.
Both instruments have diamond
turned, vulcano shaped precision
apertures made from aluminium.
This geometry prevents light from
from being reflected via the window
into the cavity.
MITRA
housing
We used a ultrasonic welding technique to
realise an ideal thermal contact between the
MITRA cavities made from silver and the labyrinth thermal resistor made from aluminium.
This design gives us a thermal relaxation constant of the cavity of 14 s and a temperature
rise of 1 K with 20 mW power absorbed in the
cavity.
MITRA
The window transmittance needs monitoring
because a fraction of the solar spectrum lies
above the cut off wavelength of the window
material. And due to changes in the atmosphere this fraction may change.
Initial measurements will be made with flat
Quartz (Suprasil 3001) windows which have a
diameter of 120 mm. CSAR and MITRA each
have a similar window. The equivalence of the
two windows is going to be characterised by
measuring the spectral transmittance over
both disks.
cut off
fraction of TSI above cut off
4 µm
0.36 %
8 µm
0.10 %
15 µm
0.0006 %
5 cm
References:
MITRA cavity, thermal
resistor and heat sink
MITRA finite element
simulation
Dual cavity design allows differential measurement of the solar irradiance. Thus
values of the integrated transmittance of windows may be determined.
Finsterle W., et al., “Third comparison of the World Radiometric Reference and
the SI radiometric scale”, 2008, Metrologia, 45(4), 377-381
Fox N.P, et al., “Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS)”, 2003, Adv. Space Res., 32, 2253-2261