Setup, alignment and instrument details
The following setup was used for testing the cosine response and transmission of the diffuser.
In this setup the lamp distance is as large as possible. The reason for this is that the lamp should resemble a point light source. If the lamp is placed at a close distance this would lead to large errors especially at bigger zenith angles. The diam eter of the windings inside the lamp are about 5 mm. A high accuracy power supply is used to keep the lamp current constant (Optronics OL 83A). The voltage across the lamp filament is monitored directly. The lamp current is monitored by measuring the vo ltage drop across an accurate wire-wound 0.01 ohm resistor. The radiometric stability of the lamp is checked with a PIN-diode detector. The Photomultiplier output is measured with a Fluke 45 multimeter with PC control through an RS-232 connection.
To align this setup the diode (see figure) is replaced by a HeNe laser and the lamp is replaced by a crosshair. The diffuser is mounted on top of the rotator and its surface is placed against the rotation axis. The laser spot is aimed at the centre of the diffuser. Once the diffuser is aligned, the laser is replaced with the diode. The diode is then aligned by looking at the diode surface through the crosshair. Finally the crosshair is replaced with the calibration lamp. Accuracy of the alignment is esti mated at about 0.1 degree.
Details of the UV3 system
The diffuser is coupled into a fibre which is connected to a double monochromator with a photomultiplier detector. The gratings are selected for a high efficiency in the UVB wavelength range. The complete system is called the "UV3 spectrometer".
The fibre core consists out of a few thousand single, randomised fibres and is surrounded by a flexible sheathing. One fibre’s end surface is rectangular shaped to match the entrance slit of the monochromator. The other end is shaped as a circle to match the diffuser. Both ends are made of stainless steel for mounting purposes.
|Single fibre diameter||100 micron|
|Core diameter||5.5 mm|
|Sheathing diameter||12.7 mm|
|No. Of fibres||2800 ± 200|
Double monochromator details
|Manufacturer||Bentham (model TDM300F)|
|Grating type||plane holographic|
|Slit widths||1.0, 1.5, 1.0 mm|
|Slit height||20 mm|
The photomultiplier is selected for low dark current. This is to improve the accuracy of the detection system especially in the wavelength range between 280 and 310nm where photon rates are very low.
|Manufacturer||EMI (model 9804QA)|
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