I Principle Let E(λ) the spectral irradiance of a source at wavelength λ at a specific distance between the source and the instrument entrance pupil. In such condition, the instrument generates a signal S(λ). The response of the instrument is R(λ) = E(λ)/S(λ) R(λ) is the instrument spectral irradiance response in W/cm2/nm per count s-1 for UV and visible channels, and in W/cm2/nm per mV for the IR channel. Consequently, when a source as the sun is measured providing a signal of value S(λ), the solar irradiance is obtained as E(λ) = R(λ) S(λ) This assumes the linearity of the detection or S(λ) being corrected for the non linearity. To perform the absolute calibration, a black body radiator is chosen because as a primary standard source. Furthermore, its irradiance cover most of the domain of interest for SOLSPEC, with exception for wavelength below 200 nm due to the limits of Air-UV. Consequently, a specific bright source in UV is needed. For that purpose, we have chosen a D2 lamp calibrated by the Physikalisch-Technische Bundesanstalt (PTB) to extend the domain of calibration down to about 165 nm. II The PTB Black Body II-1 General Descritpion At the Physikalisch-Technische Bundesanstalt (PTB) BB3200 series black-bodies radiators manufactured by the All-Russian Institute for Optophysical Measurements (VNIIOFI), Moscow are used to realise and maintain fundamental radiometric units by utilizing Planck’ s law. One of these black-bodies, the BB3200pg represents the primary standard for the realization of the spectral irradiance scale [Sperfeld et. al. 1995], [Sperfeld et. al. 1998/01]. BB3200pg is part of the spectral irradiance calibration equipment (SPICE) at the PTB and is also intended to be used by collaborating institutes as a standard radiation source with calculable spectral irradiance [Gröbner et. al. 2005] .

I Principle

Let E(λ) the spectral irradiance of a source at wavelength λ at a specific distance between the source and the instrument entrance pupil. In such condition, the instrument generates a signal S(λ).

The response of the instrument is

R(λ) = E(λ)/S(λ)

R(λ) is the instrument spectral irradiance response in W/cm2/nm per count s-1 for UV and visible channels, and in W/cm2/nm per mV for the IR channel.

Consequently, when a source as the sun is measured providing a signal of value S(λ), the solar irradiance is obtained as

E(λ) = R(λ) S(λ)

This assumes the linearity of the detection or S(λ) being corrected for the non linearity.

To perform the absolute calibration, a black body radiator is chosen because as a primary standard source. Furthermore, its irradiance cover most of the domain of interest for SOLSPEC, with exception for wavelength below 200 nm due to the limits of Air-UV. Consequently, a specific bright source in UV is needed. For that purpose, we have chosen a D2 lamp calibrated by the Physikalisch-Technische Bundesanstalt (PTB) to extend the domain of calibration down to about 165 nm.

II The PTB Black Body

II-1 General Descritpion

At the Physikalisch-Technische Bundesanstalt (PTB) BB3200 series black-bodies radiators manufactured by the All-Russian Institute for Optophysical Measurements (VNIIOFI), Moscow are used to realise and maintain fundamental radiometric units by utilizing Planck’ s law. One of these black-bodies, the BB3200pg represents the primary standard for the realization of the spectral irradiance scale [Sperfeld et. al. 1995], [Sperfeld et. al. 1998/01].

BB3200pg is part of the spectral irradiance calibration equipment (SPICE) at the PTB and is also intended to be used by collaborating institutes as a standard radiation source with calculable spectral irradiance [Gröbner et. al. 2005] .

Home

II-3 Use of Deuterium Lamps

As the temperature of the BB3200 is less than the temperature of the Sun (5870 K), the radiation emitted by the black body is very weak below 200 nm. This domain is important as it is photochemically active for the Earth's atmosphere. Then, in this spectral range we need to rely on deuterium lamps. We use a set of lamps calibrated at PTB. Calibration measurements with both BB3200 and D2 lamps are extended in wavelengths in order to allow overlapping. Consequently, it is possible to link the deuterium lamps calibration (below 200 nm) with the black body calibration (above 200 nm).

See Calibration Photos

II-2 Temperature Measurement

The black-body can be used as a primary radiation standard by utilizing Planck’s radiation law. There the radiation temperature is the main parameter that has to be determined very accurately. At the PTB the temperature is determined radiometrically using filter radiometers [Sperfeld et. al. 1995]. The broadband spectral irradiance responsivity of these radiometers has been absolutely calibrated against a cryogenic radiometer [Friedrich et. al. 1995],[Werner et. al. 2000]. The temperature of the blackbody is determined by numerically solving the integral of the black-body irradiance EBB given by Planck’s law weighted with the spectral responsivity sFR of the filter radiometer

This equation can be transposed to the black-body temperature TBB using a fifth degree polynomial [Sperfeld et. al. 1995]. For a measured photocurrent iFR of the filter radiometer in consideration of the geometric parameters of the measurement, the black-body temperature can be instantly determined during its operation. Therefore the compact-sized filter radiometers was mechanically moved into the optical beam between SOLSPEC and the BB3200pg and the corresponding photocurrent was measured. This procedure lasts less than one minute and allows to measure the temperature whenever no spectral measurement was taken with the spectroradiometer. During the spectral measurements, when no temperature measurement could be performed, a rear detector system focused on the back of the black-body cavity bottom could monitor the temperature drift respectively its stability.

The main part of the blackbody is the cavity that consists of a stack of pyrolitic-graphite (pyrographite) rings that are pressed together and directly resistance-heated by electrical current. The cavity has a length of 200 mm with an inner diameter of 37 mm. The cavity bottom is made of graphite and has concentrical grooves to enhance it’s emissivity. Several baffles at the cavity opening are used to improve the temperature uniformity inside the cavity and to prevent stray light. The maximum output aperture diameter is 22 mm. The effective emissivity of this cavity is calculated to be better than 0.9995. The cavity together with several heat insulation shields is mounted in a water cooled housing. A quartz window at the back of the housing allows the radiation emitted from the rear of the cavity bottom to be monitored. The lifetime of one radiator operated at the maximum Temperature of 3200 K is about 100 h. When operating the black-body at the PTB at temperatures around 3000 K, the lifetime could be considerably extended. The construction and design of BB3200pg are described in detail in [Sapritsky et. Al. 1997]. The black-body has been extensively characterized by several National Metrological Institutes [Sperfeld et. al. 1998/02].

II-3 Performance of BB3200PG during the SOLSPEC Calibration

Geometry

An invar high-precision aperture with 11,9 mm diameter was placed in front of the black-body cavity opening to limit the field of view for the instruments in front of the black-body. Therefore the aperture opening area of 111.38 mm² forms the reference plane for the black-body opening. The field of view for the filter radiometers placed in 420 mm distance to the aperture was limited to a 24 mm diameter area of the cavity bottom. For the SOLSPEC placed at appr. 1380 mm distance, a maximum diameter of 16 mm of the uniform irradiated bottom could be seen. This corresponds with the field of view of the instrument towards the sun.

Stability

The black-body radiator is operated in constant current mode. After a relaxation time of appr. 1 hour, this leads to a stable temperature regime around 3050 K with temperature drifts of less than 0.5 K/h. In the figure below, the temperature stability of a typical operating day is shown. After the current has been slightly ramped up to 545 A (reached at 150 min. operating time), the temperature stabilized to 3045 K. During more than five hours of measurement the temperature drifted less than 0,1 K/h. The temperature could be measured before and after each spectral measurement with SOLSPEC. A rear detector system monitored the temperature changes during the whole day and could therefore verify the high temperature stability during the whole measurement day. Additionally a multi-channel filterradiometer was placed in front of the black-body below the entrance port of SOLSPEC. The six channels of this radiometer could also verify the temperature stability with a resolution of 0.05 K.

Irradiance uniformity

The irradiance uniformity under this configuration was measured by scanning the irradiated field using the filter radiometers at 420 mm distance. It turns out that over an area of 16 mm diameter the irradiance varies less than ±0,25 %. This result correspondents with a temperature variation of less than 0.15 K. It can be extrapolated to the measuring distance of SOLSPEC, where this uniformly irradiated area expands to a diameter of 70 mm.