## 2 Review and Evaluation of Solar Irradiance Forecasting Sensors
### All-Sky Cameras
[94] utilizes a professional all-sky imager consisting of a camera (640 \(\times\) 480 pixels) pointed at a spherical mirror (420 \(\times\) 420 pixels) that reflects the sky image, by which it estimates future cloud motion. Alternatively, Ref. [44] uses one IP camera per site (Newcastle and Canberra, Australia) to predict the time period covered and its duration. Similar is the equipment used in [43] which, by employing two 3.1 MP cameras, one per site (Merced and Folsom), combines sky images with irradiance measurements to classify three future sky conditions: clear, cloudy, and partly cloudy. In addition, Ref. [42] takes images from a 1280 \(\times\) 1280 pixels camera at 180 m from a solar panel and with different exposition times (11,88,176,264 ms) to predict the short-term power output of the solar panel. Meanwhile, between 2016 and 2017, Cheng performed various studies [103; 104; 105] always utilizing a 640 \(\times\) 480 pixels camera in conjunction with irradiance readings to forecast fast motion clouds, characteristic of a maritime climate, at the National Taiwan Central University. Finally, Ref. [42] uses images from a 2592 \(\times\) 1744 pixels all-sky imager and 99 pyranometers distributed over 10 km \(\times\) 12 km to predict different cloud scenarios. It is also very important to take into account the configuration of the cameras. Recent studies have highlighted the importance of certain relevant aspects of all-sky cameras, such as their angular calibration, and how it impacts the results obtained [106]. Additionally, other studies have demonstrated the wide range of applications of this technology [107]. In order to analyze how many all-sky cameras have been used in the above-mentioned studies, Figure 3 has been developed showing the number of all-sky cameras used in each location. In this way, it will be possible to know in which areas these cameras are most used to carry out these investigations. To expand upon the investigations cited in this section, Table A1 is included in Appendix A, which details each of the studies, the objectives set, as well as the type of data used for it, and the final results obtained, which, in turn, are measured and compared with certain models and specified error metrics. Table A1 includes only the research that innovates with a solar irradiance predictive model (excluding research that is limited to evaluating and comparing models), or those studies involving concepts that support the subject of this article, such as, for example, those concerning the distortion correction of fisheye, the amount of aerosol in the atmosphere, or cloud profiles. In order to analyze the location of the solar forecasting studies shown in Table A1, Figure 4 has been developed. This figure shows a map indicating the number of studies carried out at each location. In this way, we can know how many studies have been developed in different locations. Figure 3: Number of all-sky cameras used in the surveys at each location. This bar chart displays the number of all-sky cameras deployed across various geographic locations for survey purposes. Each bar represents a location and the number of cameras used, providing insight into the distribution and extent of survey coverage.