International

3.10
temporal instability of irradiance 
(2) 
where the maximum and minimum irradiance are those measured within the relevant time 
interval. 
Note 1 to entry: Temporal instability is defined by two relevant time intervals: 
a) Short term instability (STI) 
This relates to the data sampling time of a data set (irradiance, current, voltage) during an I-V measurement. 
This value of temporal instability may be different between data sets on the I-V curve. In that case the short 
term instability is determined by the worst case. 
For batch testing of cells or modules with no irradiance monitoring during I-V measurement the STI is 
irrelevant and LTI related to the time period between irradiance determinations shall be used for classification. 
b) Long term instability (LTI) 
This is related to the time period of interest. Three cases can be distinguished: 
– For a three channel I-V measurement (irradiance, current, voltage) with a pulsed or steady-state solar 
simulator, the LTI value is the time for acquiring the I-V characteristic. 
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IEC 60904-9:2020 © IEC 2020 
– 11 – 
– For a two channel I-V measurement (voltage, current) with a pulsed or steady-state solar simulator, 
irradiance shall be measured before and after I-V measurement. The LTI value shall be calculated from 
these two irradiance values. The LTI value may depend on the I-V data acquisition time and the stability of 
the light source. The maximum averaging interval for irradiance shall correspond to the time interval 
between I-V data points. 
– For irradiation exposure LTI shall be verified for the specifications (time period, data recording interval for 
irradiance) given by the supplier. If such information is not available LTI shall be related to at least 100 
irradiance values, with a minimum of one data point per hour, taken at equal intervals over the exposure 
period. 
Note 2 to entry: Irradiance corrections, such as those of IEC 60891, are often used to minimize the effects of 
irradiance fluctuations on PV module output power measurements. The uncertainty related to irradiance correction 
will depend on the difference “measured irradiance –target irradiance” and the precision of relevant I-V correction 
parameters of the PV device. For other applications, the use of such corrections should be considered together 
with requirements for solar simulator temporal instability.