## 3 Sources of Uncertainty
### Uncertainty Budget Associated with the Spectral Responsivity  
The spectral responsivity of multi-junction devices is performed at ESTI according to standard IEC60904-8-1(2017), with suitable bias light to saturate the junction(s) not under measurement and narrow band light at wavelengths where the junction under measurement responds. Both the device under test and the reference device are over-illuminated, including not only active area, but also cover and package. The narrow band light is obtained from a broadband xenon light source filtered by interferential bandpass filters with bandwidth from 10-20 nm full-width-half-maximum (FWHM), covering the wavelength range of 300 to 1900 nm. The quasi-monochromatic light is then chopped at a fixed frequency, and the short-circuit current signal is measured by lock-in amplifiers, which enable extraction of the signal due to the quasi-monochromatic light from the static component due to the bias lights [9]. The narrow band light simultaneously illuminates the device under test and the reference device; the spectral response of the device under test is calculated from the ratio of the two device currents. The following principal components contribute to the uncertainty of the spectral responsivity:  
* Electrical uncertainty (lock-in amplifiers, current-voltage converters, experimental noise)
* Temperature uncertainty (measurement conditions at 25.0 \(\pm\) 1.0 \({}^{\circ}\)C and thermometers calibrations)
* Optical uncertainty (the spatial non-uniformity of quasi-monochromatic light on the measurement plane, alignment of device under test and reference device, area of device under test and reference)
* Reference cell uncertainty (calibration and drift)  
The electrical standard uncertainty due only to the lock-in amplifiers and the current-voltage converters is \(\pm\)0.56%, excluding the electrical noise contribution, which is measured during each acquisition by the standard deviation of the signal ratios as each wavelength point is measured a number of times. This value varies with wavelength and device. The temperature of the device and of the reference cell is controlled during the measurement by two separate base plates, controlled by a water circuit and by a Peltier element respectively. The temperature standard uncertainty contribution, considering both items, amounts to \(\pm\)0.06% and is the smallest of the four uncertainty components. The two devices are kept as physically as close as possible, but nevertheless non-uniformity of the incident quasi-monochromatic light is unavoidable, due to the non-uniformity of the Xenon light source itself and of the filters; each filter generates a different uniformity pattern on the test area. Considering the worst case, the standard non-uniformity uncertainty is \(\pm\)1.16% for small area reference cells (typically 400 mm\({}^{2}\)). Given that the two devices are placed on two different plates, a tilt alignment error within 2\({}^{\circ}\) is assumed, corresponding to a standard uncertainty of \(\pm\)0.03%. The last component of the optical uncertainty budget is due to the area of the devices, which enters in the calculation of the ratio of the short-circuit current ratio, and which has to be normalized to the unit area. The area is measured by a calibrated meter equipped with a magnification camera, traceable to SI Units with a standard uncertainty of \(\pm\)0.07%. The area contribution is excluded in the present exercise because it does not influence the SMR (see Equation (5)). The final values of the spectral responsivity (SR) uncertainty are therefore due to some components which are independent of wavelength and others that are wavelength-dependent (the calibration uncertainty of the reference cell and the electrical noise component).