## 1 Introduction  
Today, concentrator photovoltaic (CPV) systems are used to increase the effectiveness of photovoltaic (PV) systems using reflective material, lenses, or mirrors to concentrate sunlight on highly efficient solar cells [1]. CPV systems convert solar energy to electricity efficiently by concentrating and focusing incident solar radiation on high-efficiency multi-junction (MJ) solar cells. A typical CPV system consists of a solar concentrator, MJ solar cells, and a sun tracking system. In a CPV system, solar radiation is concentrated and focused on MJ solar cells through the solar concentrator. Since the system requires direct sunlight, the concentrator and solar cells require the use of a sun tracking system to follow the sun's trajectory and optimize the incident solar radiation [2]. The solar concentrator could be a Fresnel lens [3], a parabolic concentrator [4], a compound parabolic concentrator (CPC) [5], a parabolic trough concentrator [6], and others [7; 8; 9]. An ideal CPV system is expected to distribute the concentrated sunlight uniformly to the solar cell. However, the use of concentrators causes non-uniform illumination [2]. The non-uniformity of the incident illumination is found in all CPV systems and has been a well-known problem in the CPV systems [10]. For CPV systems, the illumination non-uniformity causes effects in two basic categories of electrical and thermal impacts that are described in detail in [2; 10; 11; 12]. Several approaches have been proposed and developed to improve the illumination non-uniformity in CPV systems. These approaches can be classified into two categories: concentrator-design-based and secondary optical element (SOE)-based approaches. Concentrator-design-based approaches modify and focus on designs of the solar concentrator to solve the illumination non-uniformity problem. Non-imaging Fresnel lenses are designed with the objective of concentrating light rather than forming an image. The main goal of the design of a non-imaging Fresnel lens is to maximize the energy and flux uniformity of solar radiation concentrated by the lens [13; 14]. [PERSON] et al. [15] proposed a two-dimensional single optical element system for integrating both the concentrating and spectral splitting actions. Solid transparent dispersive prisms are designed to deflect and split a polychromatic collimated light beam from a given direction onto the same area of a receiving target. The resulting concentrated and spectrally divided beam is simply obtained by the superimposition of each prism contribution. The solar cell intercepts the beam exiting from the prism ensemble. Later, these authors proposed the design of a three-dimensional point-focus spectral splitting solar concentrator system [16] to minimize the optical loss. SOE-based approaches, called two-stage systems, utilize an SOE, such as a lens, a reflector, or a CPC, located before the solar cell to improve the irradiation uniformity. [PERSON] et al. [17] presented a two-stage CPV system with a Fresnel lens as the primary optical element (POE) and a dielectric totally internally reflecting non-imaging concentrator as the SOE. The two-stage design provided both a higher concentration and a more uniform flux distribution on the PV cell. [PERSON] et al. [18] investigated a design of a symmetrical two-stage flat reflected concentrator (STFC), which used reflectors to provide uniform sunlight distribution over the solar cell. The design consisted of two symmetrical off-axis concentrators and inclined flat reflectors. The incident solar radiation was concentrated by the primary off-axis concentrators and diffused by the secondary planes. A Fresnel-Kohler technology was developed [19; 20; 21]. The Fresnel-Kohler technology was based on Kohler integration. The principles of the general design procedure were described in [22].