## 1 Introduction  
Concentrator photovoltaics (CPV) occupy a small niche of the global photovoltaic (PV) market. However, several power plants with capacity of more than 30 MWp have been realized, mainly in China and in the U.S., with a total installed capacity of about 360 MWp already grid-connected [1]. Even though manufacturing capacity has recently decreased worldwide, mainly due to the unprecedented rapid drop of crystalline silicon prices, CPV technology remains very promising: the highest efficiency values for concentrator multi-junction cells reached 46.0 \(\pm\) 2.2% at 508 sums and the scientific outlook is robust [2]. However, in order to be competitive with Crystalline silicon (c-Si) technology CPV has to increase system efficiency to at least 40%, and concentration ratios up to 1000\(\times\)according to [3]. Given that the price depends on the rated power, as is the case for flat-plate modules, reliable measurements of maximum power (and consequently of the efficiency) are crucial. In 2017 the international standard IEC62670-3 \"Photovoltaic concentrators (CPV)--Performance testing--Part3: Performance measurements and power rating\" [4] was published; it specifies different methods to measure the electrical characteristics of CPV devices both indoor and outdoor. In order to explore the standard's applicability, several measurements on a triple-junction CPV cell have been performed at the European Solar Test Installation (ESTI) of the European Commission's Joint Research Centre, located in Ispra, northern Italy (45\({}^{\circ}\)49\({}^{\prime}\) N 8\({}^{\circ}\)37\({}^{\prime}\) E). Some preliminary results have already been presented and published in a previous work [5]. The approach for outdoor measurements is to acquire current-voltage (IV) curves of the CPV device under clear sky conditions, together with the relevant meteorological quantities at ground level and the direct normal spectral irradiance with simultaneous acquisitions throughout the day. Subsequently filtering is applied to the acquired data in order to reduce the data set only to the points near Concentrator Standard Operating Conditions (CSOC) conditions, defined in [6], which the IV curves have been translated to. The most stringent requirement is related to the spectral match between the reference spectrum AirMass 1.5 Direct (AM1.5D) and the measured spectrum at time \(t\), which is determined by calculating the three spectral matching ratios \(\text{SMR}_{12}\), \(\text{SMR}_{13}\), \(\text{SMR}_{23}\), which must lie in the interval 1.00 \(\pm\) 0.03 in order for the measured IV curve to be valid (in this notation 1 denotes the top, 2 the middle and 3 the bottom cell). This limit is specified in the standard and has an impact on the final calculated performance of the CPV device: A larger interval would have the advantage of an increased statistical population size, with the disadvantage that points far from the target CSOC conditions would be considered in the calculation. However, this stringent acceptance limit makes sense only if the measurement uncertainty of each SMR is much less than the limit itself. In the present study the uncertainty sources of the SMR quantities are considered, using real data from the ESTI calibration chains and data acquired in-the-field, in order to estimate the uncertainties of each SMR by a Monte Carlo simulation. Each run of the algorithm consisted in 100,000 iterations, in order to generate a sufficient number of random errors which are added to the measured quantities. The resulting values are then compared to the acceptance threshold prescribed in the standard.