## 2 Review and Evaluation of Solar Irradiance Forecasting Sensors
### All-Sky Cameras  
All-sky cameras have demonstrated high effectiveness in measuring relative sky radiance, and their potential continues to be revealed each year through new studies. One noteworthy example is a study that utilized multi-exposure techniques to showcase the capabilities of these cameras [72]. As previously discussed, solar irradiance forecasting has primarily focused on two approaches: physical models based on numerical methods for weather prediction (NWP) and direct prediction utilizing real-time satellite images for cloud cover forecasts. However, the limited resolution and uncertainty of initial conditions make NWP models invalid for cloud forecasts at specific locations and times [4; 73]. While NWP can provide information days in advance, there are biases and random errors in irradiance predictions [74]. Various studies have reported on the limitations of these two approaches [10; 75; 76]. As mentioned earlier, all-sky cameras and satellites are primarily designed to predict cloud patterns and dynamics, and their generated images serve as a valuable tool for solar forecast research. While numerous studies have explored this aspect (as seen in reference [77]), the prediction resolution in both spatial and temporal domains has been significantly improved using a more cost-effective system. This system provides a resolution of a few kilometers, which is typically used in solar power plants, and a temporal resolution of a few minutes. Since irradiance variability for short-term horizons has a significant impact on solar power plants and, in turn, power systems, immediate prediction systems based on high-resolution sky images can offer direct irradiance forecasts to optimize the performance of solar power plants or solar farms, particularly in this scenario. These systems are usually based on high-resolution digital cameras equipped with fisheye lenses (\"US6844991B2--Fisheye lens--Google Patents\", 2005) and a dome for weather protection, see left part of Figure 1, as shown in the processing conducted in [78]. Hemispherical sky images are obtained (180\({}^{\circ}\) field of view to which the distortion produced by the lens is corrected [79; 80; 81] (see Figure 2)). In many cases, this is supported by pyranometers utilized for validating prediction models and shadow-bands to avoid a direct solar arc view [82; 83; 84]. These enhance the average life of the camera sensor but can significantly reduce cloud-tracking capability close to the sun; see the central part of Figure 2, which is the result of the segmentation carried out in [85]. Instead, in cameras without these shadow-bands, as direct sunlight is not blocked, the circumsolar region is influenced by glare effects resulting from direct focus and for specific angles of incidence, and thus, a scattering may be produced in the dome; see image of the all-sky camera associated to the left part of Figure 2. Otherwise, there are advantages to employing non-weather-specific cameras, such as easy installation, low cost, and no moving parts (solar tracker), with its corresponding apparition of robotic arms in the images. A modality that also avoids the lenses is based on images obtained from a reflection of a convex mirror oriented to the sky [85; 86; 87] (see right part of Figure 2), which is also the result of the segmentation carried out in [83]. The mirror can be designed to correct lens distortion, which eliminates the need for software correction and corresponding computational savings [88]. Another option to avoid image saturation resulting from direct focus on the sun (without using a shadow disc or band) but require a more expensive infrastructure and more complicated maintenance are so-called shadow cameras. These are located on a mast sufficiently high to record a complete panoramic view of the solar power plant. The processing is now performed on images of solar irradiance maps, which are the result of shadows cast by clouds on the solar farm [78, 89, 90].