The number of photovoltaic (PV) system installations around the world is rapidly expanding, driven by a combination of factors including steadily falling costs, tax and rebate incentives, and interest in carbon reduction. Optimizing the total energy production of PV systems is dependent on minimizing losses and avoiding conditions which can adversely affect performance and lifetime. Module shading is one such critical element. Shading can arise from inter-row effects as well as from vegetation, buildings, and precipitation. In order to make objective measurements of shading, we have designed a system employing spot metering and sensor arrays. In order to bring array monitoring costs to affordable levels, we have employed low-cost photodiodes that are deployed in the mobile and computer market. Our shading sensor is organized in a linear array. This paper describes the development and characterization of the sensor array and provides examples of its use to evaluate the effects of inter-row shading that could be experienced by a typical PV module.