Reflectors
Reflectors are probably the most important elements in the construction of luminaires for controlling light. Reflectors with mirrored surfaces are mainly used. Diffusely reflective surfaces - usually white or with a matt finish are also used.
Reflectors - general
Material
Anodized aluminium or chrome-plated or aluminium-coated plastic are generally used for reflectors. Plastic reflectors are reasonably low-priced, but can only take a limited thermal load and are therefore not so robust as aluminium reflectors, whose highly resistant anodized coating provides mechanical protection and can be subjected to high temperatures.
Reflector surfaces: specular
Matt
Textured
Facetted
Surface
The surfaces of the reflectors can have a specular or matt finish. The matt finish produces greater and more uniform reflector luminance. If the reflected light beam is to be slightly diffuse, be it to attain softer light or to balance out irregularities in the light distribution, the reflector surfaces may have a facetted or textured finish. Metal reflectors may receive a dichroic coating, which can control light luminous colour or the UV or IR component.
Reflectance of reflectors:
mirror-finish
Specular
Satin matt
Reflectance
Reflectors can be divided into different reflectance groups: mirror-finish, specular and satin matt..
Mirror-finish reflectors with good material quality are free of interference. The high reflectance and the highest specular quality make the luminaire appear as a "dark hole" in the ceiling. Reflections of items such as bright room furnishings are possible in the reflector. A further characteristic is high luminance contrasts in the reflector.
The lower specular quality of specular reflectors reduces the disadvantages associated with highly specular reflectors.
Satin-matt reflectors are also interference free if the anodising thickness is sufficient. The high reflectance and the low specular quality lead to low contrast within the reflector. This means that disturbing reflections fron room furnishings are prevented and it also produces a calm room ambiance. Diffuse surface reflection can cause luminances of >200cd/m2 in the area beyond the cut-off angle. There is usually no disturbance on VDU screens.
Geometry
Light distribution is determined to a large extent by the form of the reflector. Almost all reflector shapes can be attributed to the parabola, the circle or the ellipse.
Parabolic reflectors
Reflector contours for parallel beam / parabola
Converging beam / ellipse
Diverging beam / hyperbola
Converging-diverging beam
Reflector contour
The most widely used reflectors are parabolic reflectors. They allow light to be controlled in a variety of ways, e.g. narrow-beam, wide-beam or asymmetrical distribution, and provide for specific glare limitation characteristics. If the reflector contour is constructed by rotating a parabola or parabolic segment around its own axis, the result is a reflector with narrow-beam light distribution. In the case of linear light sources a similar effect is produced when rectangular reflectors with a parabolic cross section are used.
Focal point
In the case of parabolic reflectors, the light emitted by a light source located at the focal point of the parabola is radiated parallel to the parabolic axis.
If there is a short distance between a parabolic reflector's focal point and its apex, the reflector will act as a shield to direct rays.
If this distance is large, then the direct rays will not be shielded. However, these can be shielded using a spherical reflector.
Wide-beam light distribution
If the reflector contour is constructed by rotating a parabolic segment around an axis, which is at an angle to the parabolic axis, the result is a reflector with wide-beam to batwing light distribution characteristics. Beam angles and cut-off angles can therefore basically be defined as required, which allows luminaires to be constructed to meet a wide range of light distribution and glare limitation requirements.
Linear light sources
Parabolic reflectors can also be applied with linear or flat light sources, e.g. PAR lamps or fluorescent lamps, although these lamps are not located at the focal point of the parabola. In these cases, the aim is not so much to produce parallel directional light but optimum glare limitation. In this type of construction, the focal point of the parabola lies at the nadir of the opposite parabolic segments, with the result that no light from the light source located above the reflector can be emitted above the given cut-off angle. Such constructions are not only possible in luminaires, but can also be applied to daylight control systems; parabolic louvres, e.g. in skylights, direct the sunlight so that glare cannot arise above the cut-off angle.
Darklight reflectors
In the case of the conventional parabolic reflectors clearly defined light radiation - and effective glare limitation - is only possible for exact, point light sources. When using larger radiating sources, e.g. compact fluorescent lamps, glare will occur above the cut-off angle; glare is visible in the reflector, although the lamp itself is shielded. By using reflectors with a variable parabolic focal point (so-called darklight reflectors) this effect can be avoided; brightness will then only occur in the reflector of larger radiating sources below the cut-off angle, i.e. when the light source is visible.
Spherical reflectors
In the case of spherical reflectors the light emitted by a lamp located at the focal point of the sphere is reflected to this focal point. Spherical reflectors are used predominantly as an aid in conjunction with parabolic reflectors or lens systems. They direct the luminous flux forwards onto the parabolic reflector, so that it also functions in controlling the light, or to utilize the light radiated backwards by means of retro reflection back towards the lamp.
Involute reflectors
With involute reflectors the light that is emitted by the lamp is not reflected back to the light source, as is the case with spherical reflectors, but reflected past the lamp. Involute reflectors are mainly used with discharge lamps to avoid the lamps over-heating due to the retro-reflected light, which would result in a decrease in performance.
Elliptical reflectors
Double-focus downlight
Double-focus wallwashers
Spotlight
In the case of elliptical reflectors the light radiated by a lamp located at the first focal point of the ellipse is reflected to the second focal point. The second focal point of the ellipse can be used as an imaginary, secondary light source.
Elliptical reflectors are used in recessed ceiling washlights to produce a light effect from the ceiling downwards. Elliptical reflectors are also ideal when the smallest possible ceiling opening is required for downlights. The second focal point will be an imaginary light source positioned at ceiling level; it is, however, also possible to control the light distribution and glare limitation by using an additional parabolic reflector.
Double reflector systems
Double reflector systems consist of a primary and secondary reflector. The primary reflector aligns the light in a parallel or narrowly focused beam and directs it to the secondary reflector. The actual light distribution is created by the secondary reflector. The direct view of upon the high luminance of the lamp is prevented with double reflector systems, resulting in improved visual comfort. The precise alignment of the reflectors determines the efficiency of the system.
