Petri Kärhä

Method for characterization of filter radiometers

We have developed a new method for characterizing the irradiance responsivity of filter radiometers. The method is based on a spatially uniform, known irradiance, generated by combining several identical laser beams. The measurement setup and the experimental demonstration at one wavelength are presented. The diffraction correction related to the generated irradiance is studied experimentally. The uncertainty analysis of the method indicates a relative standard uncertainty of 1 x 10(-3). The results with the new method are compared with the characterization measurements based on our present spectral-irradiance scale. The results have a relative deviation of 1 x 10(-3), which is well within the combined standard uncertainty of the comparison.

Monte Carlo analysis of uncertainty of total atmospheric ozone derived from measured spectra

We present a Monte Carlo based model to study effects that possible correlations in spectral irradiance data may have on the derived total ozone column values. Correlations may produce systematic errors in the spectral irradiance which behave differently from uncorrelated data. The effects are demonstrated by analyzing the data of one day’s measurements.

Facility for determining action spectra of UV photodegradation

We have developed an ultraviolet (UV) ageing facility consisting of a spectrograph which can be used to expose material samples with spectrally dispersed UV radiation at the wavelength range 273 – 423u2005nm, and an automated, high-resolution laser-based setup for measuring the induced color changes in the transmittances of the exposed samples. The samples are scanned using a suitable laser wavelength to obtain the wavelength dependence on the color change. The locations on the samples are converted to exposure wavelengths with the known dispersion of the spectrograph. We demonstrate the facility by exposing and measuring a set of atactic polystyrene sheets. By studying samples aged for different time periods we derive a complex action spectrum for the photoyellowing that is nonlinear with respect to the radiant exposure. We further derive linear activation spectra for the photoyellowing demonstrating how they depend on the selected threshold level.