The electrical output of photovoltaic devices is dependent on the spectral content of the source illumination and its intensity. To make accurate measurements of the performance of photovoltaic devices under a variety of light sources, it is necessary to account for the error in the short-circuit current that occurs if the relative spectral response of the primary reference cell is not identical to the spectral response of the cell to be calibrated. A similar error occurs if the spectral irradiance distribution of the test light source is not identical to the desired reference spectral irradiance distribution. These errors are accounted for by the spectral mismatch parameter M (Test Method E 973), a quantitative measure of the error in the short-circuit current measurement. It is the intent of this test method to provide a recognized procedure for calibrating, characterizing, and reporting the calibration data for secondary photovoltaic reference cells.

A secondary reference cell is calibrated to the same reference spectral irradiance distribution as the primary reference cell used during the calibration. Primary reference cells can be calibrated by use of Test Method E 1125 or Test Method E 1039.

Note 1-No standards for calibration of reference cells to the extraterrestrial spectral irradiance distribution presently exist.

A secondary reference cell should be recalibrated yearly, or every six months if the cell is in continuous use outdoors.

Recommended physical characteristics of reference cells are provided in Specification E 1040.

Because silicon solar cells made on p-type substrates are susceptible to a loss of I_sc upon initial exposure to light, it is required that newly manufactured reference cells be light soaked at an irradiance level greater than 850 W/m² for 2 h prior to initial charcterization in Section 7.