Luminaire covers – be it so-called diffusers, cover plates or optical lenses – permit extended maintenance intervals since they provide protection from dust and, as the case may be, humidity. Furthermore, they provide mechanical protection for the light sources as well as additional photometric characteristics: Optical lenses combined with LED light sources allow for highly precise light distribution with simultaneously low scattering losses. However, prismatic covers (see figure b) also achieve high degrees of efficiency with excellent glare reduction when combined with specular optics. Opal luminaire covers (see figure a) achieve uniformly illuminated lighting bodies which are experiencing a certain renaissance as "classics" for aesthetic reasons. However, they feature relatively low degrees of transmission, often below 50%. This is why luminaire covers with scattering effects are usually made of translucent material today (see figure c). Due to the elevated degree of transmission and reduced backscatter, this achieves high degrees of transmission and thus significantly improved profitability while still featuring a very homogeneous appearance.
Photometric covers are considerably gaining significance over reflectors as fluorescent lamps are replaced by LED light sources.
In terms of materials, polymethyl methacrylate (PMMA) or polycarbonate (PC) are used predominantly. Improper strain and the influence of damaging substances can decrease the durability of polymethyl methacrylate or polycarbonate. Such substances include e.g. plasticisers released by non-heat-resistant cable insulation when exposed to heat which can cause tearing and decrease stability. For this reason, only heat-resistant inner wiring and through-wiring should be used in luminaires with plastic covers. Compatibility with other chemicals can be gathered from table 5.20 in chapter 5.14 "Safety from chemicals and other influencing factors".
Polymethyl methacrylate (PMMA) is a photometric starting material proven over years of use. It is a thermoplastic highly convenient for processing which can be formed using a great variety of methods such as deep-drawing, extrusion or die-casting. For all these methods it is available either with transparent, translucent or opal condition. PMMA features excellent transmission over the entire visible spectrum. Special PMMA versions additionally provide improved impact resistance. In such versions, it is a natural fit e.g. for luminaire covers in outdoor applications where elevated stability in the face of mechanical strain is required. PMMA is virtually permanently non-yellowing. This is why yellowing and embrittlement do not set in even after years of operation in areas exposed to solar radiation.
Much like PMMA, polycarbonates (PC) are thermoplastics highly convenient for processing. Through variations in chemical composition, their optical properties such as refractive index and other physical parameters can be diversified extensively. When compared to PMMA , polycarbonate features a slightly diminished degree of transmission. It is used predominantly in transparent versions, but also in opal versions; it is not available in translucent texture, however. The material is mostly processed using die-casting to achieve precise optical lens systems. Furthermore, it is characterised by excellent impact resistance. Due to this, polycarbonate is also suitable for lamp protection tubes or luminaire covers exposed to elevated mechanical strain in indoor applications or the risk of vandalism in outdoor applications. In long-term use with intense UV exposure e.g. direct sun irradiation, polycarbonate can become yellow. This yellowing involves a decrease in impact resistance.
Silicate glass is only used in special cases due to its weight and inferior impact resistance (excepting toughened glass). Such cases include e.g. decorative covers for downlights which are often made of so-called float glass, which is fabricated using special methods, or covers with particular optical effects such as fresnel glasses.
Films, z.B. made e.g. of polycarbonate, are sometimes used in luminaires. They serve as scattering materials for luminaires with large surfaces or light ceilings. Films featuring a micro-structure surface facilitate particular luminous intensity distributions in lighting technology.
The photometric characteristics of a luminaire cover are essentially determined – besides the starting material’s distribution characteristics – by its shape.
Classic prismatic covers achieve a preferred direction of the luminous flux onto the working plane and reduce luminance for the purpose of glare reduction in the viewing angle range through their inner and outer prisms’ refraction and total reflection of light. They are either fabricated in one piece using die-casting, or through blowing or extrusion. Out of all these methods, die-casting achieves the highest degree of precision and thus facilitates a very precise direction of light (see figure a).
Conical fine prisms (CDP – conical de-glaring prism) also facilitate a precise direction of light. They are embossed on one side of a cover. With this method, there are also wide-ranging variations in quality. Using high-quality PMMA and special embossing methods, finished photometric disks can serve as covers for high-efficiency direct-distribution luminaires while simultaneously limiting luminance for distribution angles > 60° to a VDU-compliant 1,500 cd/m² (see figure b).
Finely structured light guiding plate systems (e.g. Binary Light Guide System (BLGS)) are optical systems composed of stacked photometric plastic plates with embossed structures. Within the plates, total reflection leads to a dispersion of light while the surface structure leads to an emission of light via refraction and scattering. In the late 1990s and the 2000s, the first developments in this direction were released in combination with the T5 fluorescent lamp featuring a reduced cross section. The light of modern LED light sources can be coupled in much more efficiently in comparison, which means that this technology today offers way more technical and creative possibilities. Through targeted structuring of the surface embossments, the light emission can create a highly precise direct-indirect luminous intensity distribution. For example, the indirect portion can be emitted especially widely for small distances from ceilings, while the direct portion is emitted VDU-compliantly according to EN 12464-1 for elevated light quality (see figure c).
Covers made of photometric plastics with integrated lens systems are especially suitable for use in combination with LED light sources. Their highly efficient direction of light is achieved using refraction and total reflection. By designing systems of this type, exact luminous flux distributions can be devised for application-specific optimisation. The resulting optics are often used for medium to elevated mounting heights, e.g. in supermarkets, workshops and manufacturing as well as storage halls (see figure d).