I. Introduction
The accumulation of dust and other contaminants on photovoltaic (PV) modules, known as soiling, is one of the principal energy loss factors in PV systems. Soiling is quantified by the soiling ratio (SR), the ratio of actual PV power output to expected output under clean conditions [1]. Early work on direct measurement of soiling ratio compared the output of soiled modules to regularly cleaned modules [2], using short-circuit current as a proxy for module power. This concept has also been extended to comparing short-circuit current of soiled and clean mini-modules or reference cells. In recent years soiling sensors employing all-optical detection of accumulated dust have been developed [3]–[5]. These approaches work well when soiling accumulations are uniformly distributed on module surfaces. However, soiling accumulations are often non-uniform, with concentrated bands of soiling at module edges, especially bottom edges. For non-uniform soiling, more accurate measurements of soiling ratio are achieved by measuring module power and calculating the ratio of measured soiled power to expected power [6]–[8] for clean conditions. Expected power, the denominator of SR, can be determined either by using clean modules identical to the soiled modules [7] (“module-module”) or by using periodically cleaned reference cells [8] (“module-cell”).