The absolute direct irradiance calibration facility of PMOD/WRC aims to provide traceablility of direct solar irradiance measurements performed with spectroradiometers and filter radiometers, and to allow aerosol optical depth retrievals to be conducted.
The ATLAS system (A Pulsed Tunable Laser System for the Characterisation of Spectrometers) allows the accuracy of array spectroradiometers to be improved, which are widely used for satellite validation of various atmospheric products. The stray-light and linearity of spectroradiometers is measured, and algorithms and post-correction functions can then be applied.
Surface-based optical radiation measurements are a key requirement for the validation of remote-sensing products from satellites. State-of-the-art systems consist of array spectroradiometers to measure solar radiation in the UV-VIS-NIR range in order to determine a large range of parameters, from surface radiation to atmospheric composition products such as the total concentration of trace gases as well as their profiles. In order to fulfil their role as reference systems for the validation of satellite products, traceable surface-based measurements to the international system of units are required, including a comprehensive uncertainty budget based on an extensive characterisation of the measuring system. One key aspect in this chain is the radiometric characterisation of the optical system, including its stray-light characteristics and non-linearities in order to produce correction functions for these parameters.
Figures 1 and 2 show the ATLAS tunable laser system in the PMOD/WRC optical laboratory. The in-range stray-light correction of array spectroradiometers are based on line-spread function (LSF) measurements across the whole wavelength range of the spectrometer.
The ATLAS tunable laser system consists of:
The light source current is regulated by a current control unit, which consists of an Agilent 34420A voltmeter, a 100 mΩ shunt from Burster (1282-0.1, SN 187275), and a 1000 W switched power supply from Xantrex. A current of 8.1 A is used to power the 1000 W lamp, with a feedback precision of 0.25 mA. The temperature of the shunt is monitored, as well as the current and the lamp voltage.
The shunt is calibrated yearly relative to our reference shunt and is traceable to SI units using the procedure described in PMOD/WRC quality management procedures. The calibration laboratory is temperature stabilised to 22°C.
The ATLAS tunable laser system can be used to determine the:
Figure 1. 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.
Figure 2. View of a different ATLAS set-up.