## 1 Introduction  
Greenhouse covers constitute one of the main climate modifiers to improve the environments for crops, adjusting such things as solar radiation, temperature and humidity [1]. Nowadays, a large part of intensive agriculture models has been developed within the so-called \"agriculture under plastic\", which has allowed a significant increase in the productive yield and quality of horticultural and ornamental crops [2], especially those located in tropical and subtropical regions and those located in regions characterized by mild climates, such as southern Europe. Nevertheless, the diversity of climates throughout the world and the specific requirements of each crop [3] has meant that the plastics industry has had to develop a multitude of different covers that meet the required needs [4]. An advantage associated with the use of high-tech plastics is the saving of water and energy used in cooling systems, therefore generating a more sustainable production model. The diversity of materials used in the composition of a plastic cover modifies its spectral properties, such as the transmittance patterns along the electromagnetic spectrum [5], which have an impact on agronomic behavior [6]. Examples include: (a) a reduction in the temperature inside a greenhouse due to the natural heating produced by solar radiation,which is caused by blocking near-infrared radiation (NIR); (b) blocking part of the ultraviolet (UV) radiation, which limits the activity of insects that are harmful to plants, as well as avoiding burning in certain ornamental crops and, finally, (c) an improvement in the \"greenhouse effect\" due to blocking the transmission of long-wave thermal radiation to the outside during the night. In addition, the composition impacts the mechanical properties of greenhouse covers, such as their durability and the useful life of the product [7]. Polyethylene is mainly used due to its low density and high molecular weight and comes in the following forms: (a) Low-density polyethylene (LDPE), whose structure presents many branches (2-8 carbon atoms) linked to the main chain, which confers low resistance to tensile stress but good impact resistance [8]. (b) Linear low-density polyethylene (LLDPE), which has short branches without interconnections with neighboring chains. This is very flexible, and it has great stretchability and chemical resistance [8]. (c) Metallocene linear low-density polyethylene (mLLDPE), which is related to metallocene catalysts and presents a homogeneous distribution of the polymeric molecular chains. This has greater transparency, elasticity, processing speed and recyclability [8]. Finally, (d) ethylene-vinyl acetate (EVA) and ethylene butyl acrylate (EBA), which are co-polymers with excellent processability, impact resistance, flexibility and a high compatibility with other polymers, additives, and fillers [8]. Additives are mixed with these polymers in order to give the plastic covers specific agronomic, radiometric and mechanical characteristics. These include: (a) nickel/benzoph-enone-type UV stabilizers that absorb UV radiation and protect from photodegradation [9]. In addition, this material confers resistance to phytosanitary products but causes a reduction in light transmission [10]; (b) HALS-type UV stabilizers (hindered amine light stabilizers), which are photoprotectors that capture the free radicals caused by photodegradation but allow greater light transmission [11]; (c) additive diffusers, which make the incident light more evenly distributed [11]; (d) thermal additives, which help to retain infrared radiation inside the greenhouse [12]; (e) anti-thermal additives, which prevent the passage of short-wave radiation, thus achieving a drop in temperature [1]; (f) photoselective additives, which select and transform certain wavelengths into others [13;14]; and (g) anti-drip additives, which prevent water condensation on the inside of the roof [11]. Related to plastic covers, the incident solar radiation may be transmitted, absorbed and/or reflected [15].