Here we will refer to the temporal instability definitions from the IEC standard, as it is the most comprehensive of the three standards with regard to stability and takes both short term and long term instability (STI, LTI) into consideration. For all standards, the general method of temporal instability measurement is to measure the irradiance of the simulator beam over a specified time period and to then calculate the instability using the following equation:
The STI and LTI are defined as:
For measuring the temporal instability of steady state or continuous solar simulators, the simulator irradiance is measured via an IV measurement system which consists of a suitable test cell connected to a source meter unit (SMU) which is in turn controlled via computer software. For the STI the short circuit current (Isc) of the cell is measured repeatedly over the required time period (usually seconds, as per the ASTM standard) and the STI calculated via the equation above. For IV applications, as described in the IEC standard, the LTI refers to the length of time required to conduct an IV sweep. With modern metal halide sources this can be as short as 1-2 seconds.
It is important to note that the IEC standard states:
"For the evaluation of STI, the I-V data acquisition system may be considered an integral part of the solar simulator. If a solar simulator does not include the data acquisition system, then the simulator manufacturer shall specify the corresponding data sampling time as related to the reported STI classification."
Thus it is acceptable to employ filtering, averaging and NPLC integration etc. via the SMU in order to improve the stability of less stable sources such as xenon arc lamps. As noted above, sources such as metal halide arc lamps require only NPLC control in order to achieve stability well within class A standards, allowing for much faster rates of measurement.