Overview
Three laboratories are available to characterise radiometers, each run by the Sections: Solar Radiometry (SRS), the UV Radiometry (WCC-UV), and Atmospheric Turbidity (WORCC).
1) Solar Radiometry Section: Optics Laboratory
1.1) Characterisation of Reflectivity
The set-up consists of a chopped laser beam, a lens, an integrating sphere with an Si photodiode detector and a monitor diode (see Figure 1). The integrating sphere collects the reflected light from the samples which is then measured by the photodiode and a lock-in amplifier. The integrating sphere can be moved in two directions perpendicular to the laser beam, allowing each point of the test object to be individually sampled. Figure 2 on the right shows measurements of a cavity radiometer which can be conducted at three different wavelengths (375, 532 and 633 nm).
Figure 1. Laboratory setup for reflectivity measurements.
Figure 2. Measurements of a cavity radiometer.
1.2) Heliostat
A Heliostat is a system of two flat mirrors that guides sunlight into a laboratory to serve as a source for irradiance experiments or measurements (see Figure 3). A tracking system follows the sun, enabling use for long periods of the day.
Although sunlight might not always be available, the Heliostat has the great advantage of delivering a solar beam into a laboratory with a well-defined beam geometry and spectrum. Laboratory experiments can then be conducted under controlled environmental conditions.
Figure 3. The Heliostat showing one of the two mirrors.
2) WCC-UV and WORCC Sections: Optics Laboratory
An optics lab serves the WCC-UV and WORCC sections, and allows UV, spectral and filter radiometers to be characterised. Solar UV reference spectroradiometers, and a spectral and an angular response facility are also available. Two further setups are available for the absolute calibration of irradiance standards as well as the irradiance calibration of global and direct UV and UV-VIS spectroradiometers.
2.1) Spectroradiometers for Solar UV Irradiance Measurements
The instrument of choice for the measurement of absolute spectral solar radiation is a well characterised spectroradiometer. At PMOD/WRC, the reference instrument is the transportable reference spectroradiometer QASUME. The instrument consists of a Bentham double monochromator DM-150. Irradiance is sampled with a diffusing head built by CMS Schreder where a 6 m fiber cable connects the input head to the entrance of the monochromator.
This reference instrument was validated in the QASUME (Quality Assurance of Solar Spectral Ultraviolet Irradiance Measurements carried out in Europe) European project. The aim of the project was the development of a new approach.
Figure 4. The spectroradiometers QASUME-I on the left and QASUME-II on the right.
In 2016, QASUME-II was constructed as a copy of QASUME with improved input optics and a hybrid solid-state detector system. Both QASUME spectroradiometers can measure global and direct solar irradiance in the 250 – 500 nm wavelength range (Figure 4).
The third spectroradiometer of the WCC-UV is the double monochromator Brewer #163. This instrument is mainly designed to measure the total ozone column. It is also capable of recording spectral direct solar UV irradiance and global solar irradiance in the 285 – 365 nm wavelength range. The instrument was equipped with a diffusing entrance optics. The Brewer #163 was modified during the 2008 – 2016 period to measure absolute and polarised solar sky radiance. It is designed to continuously measure the total ozone column and aerosol optical depth in the UV range.
2.2) Laboratory equipment
2.2.1) The Acton SP2500 monochromator
The SP2500 is a double monochromator with 500 mm focal length. It consitsts of 2 input and 3 exits ports. The wavelength can be selected within the range 200 nm to 1200 nm with a precision of 0.1 nm using two set of grating with 2400 lines/mm or 12000 lines/mm. The slit width can be adjusted to the application for input slit 1 one exit slit 3. The variable setup facilitates two different measurements:
A) Spectral Response Function Measurements
Input slit no. 1 of the Acton SP2500 double monochromator (Figure 5) is illuminated using a 1000 W Xenon lamp for the measurement of the relative spectral response of UV filter radiometers. The test device is illuminated by quasi monochromatic light through the exit port 3.
The wavelength scale of the monochromatic source was determined by measurements of selected spectral emission lines of a mercury lamp. In addition, we characterise the source in its usual operating state using the QASUME reference spectroradiometer as reference detector and using a reference Silicon Trap detector.
Figure 5. The Acton SP2500 double monochromator.
B) Irradiance Standard Calibration Setup
The setups for the irradiance calibration are shown in the pictures below (Figures 6 – 8). The basis of the irradiance scale consists of a set of 1000 W FEL lamps traceable to the primary irradiance standard of the Physikalisch-Technische Bundesanstalt (PTB), Germany. This irradiance reference has become the de-facto standard for spectral UV measurements in Europe. DXW-type 1000 W lamps (vertical calibration, left picture) or any other transfer standards can be calibrated relative to the WCC-UV irradiance reference using the QASUME spectroradiometer (horizontal calibration, middle picture). An example is shown in the picture on the right using the transportable calibration unit from CMS Schreder loaded with a 250 W lamp.
Figure 6. Horizontal calibration.
Figure 7. Low-power calibration system.
Figure 8. Vertical calibration.
The light emitted from irradiance standards is calibrated relative to at a well defined orientation and distance from the lamp. The irradiance calibration table facilitates the measurements of the spectral emission of this lamps with two lamp holder. The irradiance can be either recorded by the reference spectroradiometer QASUME or is fiber coupled to input port of the Acton SP2500. The wavelength range from 200 nm to 500 nm is recorded in the later case by an Hamamatsu Photocounter on exit port 2 and the visible light (500-1200nm) is measured by an InGaAs-Detector at exit port 1 (first stage of the double monochromator). The wavelength scale of the system was determined by measurements of selected spectral emission lines of a mercury lamp.
2.2.2) The Angular Response Function Setup
The angular response function (ARF) of a radiometer is measured on a 3 m long optical bench (Figure 9). A 1000 W Xenon lamp mounted at one end of the optical bench serves as a radiation source.
The detector is mounted on a goniometer at the other end of the optical bench. The resolution of the rotation stage is 29642 steps per degree, or 0.12 arcsecond.
Figure 9. The set-up to measure the Angular Response Function (ARF) can be seen in the foreground. A UV radiometer is being illuminated (on the left) by a light source (on the right).
2.2.3) Lasers
Optics Lab 2 also has a HeCd Laser (325 nm) , a HeNe Laser (632.8 nm) and the ATLAS tunable laser system. ATLAS (“A Pulsed Tunable Laser System for the Characterisation of Spectrometers”; Figure 10) can be used to improve the accuracy of array spectroradiometers that are widely used for satellite validation of various atmospheric products.
ATLAS is used to measure the stray-light and linearity of spectroradiometers, and to develop algorithms and post-correction functions to deal with these aspects. Further details can be found on the ATLAS webpage.
Figure 10. View of the ATLAS tunable laser system with the optical conditioning beam unit consisting of a trap diode, a micro-lens-array system and a neutral density filter-wheel. An optical fibre then transmits radiation further into the radiometer which is to be characterised.