How are wavelengths arranged

Double monochromator radiometer

The double monochromator radiometer is an apparatus that divides the sunlight into its components very finely graded according to wavelengths and measures the irradiance of the individual wavelengths. This decomposition is necessary because the different wavelengths of UV radiation have very different effects on the skin. For example, the UVB rays with the shortest wavelengths, which can even reach the earth's surface, are around 10,000 times more effective than the longest-wave UVA rays, which can almost be seen by the eye as violet.

The measurements of UV radiation at the Institute for Medical Climatology at the University of Kiel are carried out with a spectroradiometer system that has been in operation in the Westerland branch since the summer of 1993. The entrance optics are attached to a fixed stainless steel frame on the roof of the institute. The institute building is located on the edge of the dunes on the west coast of Sylt in the immediate vicinity of the beach. The distance from the edge of the flood is around 100 m. The location enables measurements with a completely free horizon under largely clean air conditions.

Measuring system

The measuring system comprises a horizontally aligned cosine-true diffuser, which consists of a Teflon spherical cap with a shut-off ring. The total deviation from the ideal cosine characteristic is 4% (Kaase and Chen 1992). The entrance optics are connected to the entrance slit of a double grating monochromator (DM 150, Bentham Comp. Reading, England) via a quartz light waveguide 4 m in length. The two monochromators, whose focal length is 150 mm, are arranged in series for additive dispersion and high stray light suppression and separated by a slit. The holographic diffraction gratings have a line density of 2400 per mm (blaze wavelength at 240 nm). The width of the slit diaphragms is 0.65 mm, the nominal resolution resulting for the monochromator is 1 nm. The two gratings are rigidly coupled to one another via a connecting rod. To record the spectra, the grids are adjusted step by step with the aid of a spindle. The spindle is driven by a stepper motor. A photomultiplier, which is mounted directly on the exit slit of the double monochromator, acts as a radiation detector. The bialcalic cathode gives the photomultiplier increased UV sensitivity. After amplification by a programmable current amplifier, the photocurrent of the photomultiplier is fed to an analog-digital converter. The measured values ​​are stored in a computer file after calculating the respective dark current and the system sensitivity. The control of the monochromator grating, the current amplifier and the calculations are carried out by a special program on a personal computer.



calibration
The calibration is carried out on the overall system. The wavelength calibration is carried out with a mercury vapor lamp based on the known emission lines. The system sensitivity is calibrated with a 1000 W quartz halogen lamp, which is operated with a constant operating voltage and regulated constant current. A calibration file is created based on the intensity distribution known for the emitter, which is converted into the spectral irradiance (Wm-2 nm-1 ) is used.

Measuring cycle

The measurements are taken from shortly after sunrise to shortly before sunset at 6-minute intervals. After measuring the photomultiplier dark current, a spectrum from 290 to 400 nm is recorded in each case. The step size in the range from 290 to 320 nm is 0.5 nm, otherwise 5 nm. The surroundings of the Fraunhofer line Ca K, which is particularly striking in the solar spectrum, is scanned in close steps with the step 0.075 nm for a control of the wavelength accuracy.

Double monochromator radiometer

The double monochromator radiometer is an apparatus that divides the sunlight into its components very finely graded according to wavelengths and measures the irradiance of the individual wavelengths. This decomposition is necessary because the different wavelengths of UV radiation have very different effects on the skin. For example, the UVB rays with the shortest wavelengths, which can even reach the earth's surface, are around 10,000 times more effective than the longest-wave UVA rays, which can almost be seen by the eye as violet.

The measurements of the UV radiation at the Institute for Medical Climatology at the University of Kiel are carried out with a spectroradiometer system that has been in operation in the Westerland branch since the summer of 1993. The entrance optics are attached to a fixed stainless steel frame on the roof of the institute. The institute building is located on the edge of the dunes on the west coast of Sylt in the immediate vicinity of the beach. The distance from the edge of the flood is around 100 m. The location enables measurements with a completely free horizon under largely clean air conditions.

Measuring system

The measuring system comprises a horizontally aligned cosine-true diffuser, which consists of a Teflon spherical cap with a shut-off ring. The total deviation from the ideal cosine characteristic is 4% (Kaase and Chen 1992). The entrance optics are connected to the entrance slit of a double grating monochromator (DM 150, Bentham Comp. Reading, England) via a quartz light waveguide 4 m in length. The two monochromators, whose focal length is 150 mm, are arranged in series for additive dispersion and high stray light suppression and separated by a slit diaphragm. The holographic diffraction gratings have a line density of 2400 per mm (blaze wavelength at 240 nm). The width of the slit diaphragms is 0.65 mm, the nominal resolution resulting for the monochromator is 1 nm. The two gratings are rigidly coupled to one another via a connecting rod. To record the spectra, the grids are adjusted step by step with the aid of a spindle. The spindle is driven by a stepper motor. A photomultiplier, which is mounted directly on the exit slit of the double monochromator, acts as a radiation detector. The bialcalic cathode gives the photomultiplier increased UV sensitivity. After amplification by a programmable current amplifier, the photocurrent of the photomultiplier is fed to an analog-digital converter. The measured values ​​are stored in a computer file after calculating the respective dark current and the system sensitivity. The control of the monochromator grating, the current amplifier and the calculations are carried out by a special program on a personal computer.



calibration
The calibration is carried out on the overall system. The wavelength calibration is carried out with a mercury vapor lamp based on the known emission lines. The system sensitivity is calibrated with a 1000 W quartz halogen lamp, which is operated with a constant operating voltage and regulated constant current. A calibration file is created based on the intensity distribution known for the emitter, which is converted into the spectral irradiance (Wm-2 nm-1 ) is used.

Measuring cycle

The measurements are taken from shortly after sunrise to shortly before sunset at 6-minute intervals. After measuring the photomultiplier dark current, a spectrum from 290 to 400 nm is recorded in each case. The step size in the range from 290 to 320 nm is 0.5 nm, otherwise 5 nm. The surroundings of the Fraunhofer line Ca K, which is particularly striking in the solar spectrum, is scanned in close steps with the step 0.075 nm for a control of the wavelength accuracy.