Devices that control the distribution of the light emitted by a lamp or lamps and that include all the items necessary for fixing and protecting the lamps (and sometimes the gear, too) and for connecting them to the electricity-supply circuit .
The optical characteristics of a luminaire determine the shape of its light beam, or light-output distribution. The light distribution of a luminaire defines how the luminous flux radiated by the lamp or lamps is distributed in the various directions within the space around it. Different lighting applications require different light distributions and thus different luminaires.
The desired light distribution of a luminaire is obtained through the application of one or more of the physical phenomena: reflection, refraction, and diffuse transmission. Many luminaires also make use of shielding in one form or another, principally to obtain the required degree of glare control and to limit light pollution. The shielding function may be performed by refractors or diffusers or by mirror reflectors, by white-painted surfaces, or, where very stringent glare control is required, by black surfaces.
Many conventional luminaires are provided with a reflector (sometimes in conjunction with another light-control element) in order to create the appropriate light distribution. The reflecting material that is used for reflectors can be specular, spread, or diffuse.
Specular reflectors (also called high-gloss mirror reflectors) are used when a precise form of light distribution is required, as in floodlights, spotlights, and road-lighting luminaires. The reflector creates multiple images of the light source. The most widely used material is sheet aluminum, which has the strength needed to produce a stable reflector. Reflectance values are around 0.70. Alternatively, commercial-grade aluminum can be clad with a thin layer of super-purity aluminum or silver. With aluminum, reflectance values of up to 0.80 can be obtained, while with silver a reflectance of more than 0.90 is possible. Finally, there is vacuum metalizing, in which a specular layer of aluminum is deposited on a suitably smooth substrate (metal, glass, or plastics). The resulting reflectance, which is somewhere between 0.80 and 0.90, is dependent on both the substrate material and the quality of the metalizing process.
At the other extreme from specular reflection is diffuse reflection, which is also called matt reflection. Here light incident on the reflector is scattered in all directions, so there is no mirror image of the light source. Matt reflectors cannot provide sharp beam control, but are employed where diffuse or non-focused light distributions are required. Matt-finished metals and white glossy paints on metal or glossy-white plastics provide near-diffuse reflection. The small specular component due to the gloss is of no practical optical significance; the gloss merely serves to facilitate cleaning. Reflectance values can be in the range 0.85–0.90. Ceramic materials or finishes have completely diffuse reflection characteristics with extremely high reflectances of up to 0.98.
When using a simple plane (or straight-sided) reflector, the light emitted by the light source is reflected according to the material of the reflecting surface, viz., specularly or diffusely.
Plane reflectors are often used to screen off the direct light from the light source. Accurate beam shaping is scarcely possible with plane reflectors, but by changing the symmetry of the reflectors, the direction in which the bulk of the light is emitted can be changed.
The best optical performance is obtained when using a curved reflector. Depending on the curvature, many different types of beams can be created. A curved reflector may be cylindrical, parabolic, elliptical, hyperbolic, or some other contour to suit a particular application. The circular and parabolically shaped reflectors are the ones most commonly used.
The most important optical property of a parabolic reflector is that a point source of light placed in its focus will produce a parallel beam of reflected rays with the greatest intensity in its center. If the light source is not at the focus but in front or behind it, the reflected rays are no longer parallel. Thus, by choosing the position of the light source relative to the focus point, the desired beam shape (narrow to wide) can be created. Since a lamp is never a real point source, deviations from the theoretical beam shape for a point light source as sketched above will always occur. The smaller the light source relative to the size of the reflector, the more accurately can the beam be shaped.
Smooth-curved reflectors have to be produced to a high degree of accuracy, because even small deviations from their intended shape will produce undesirable discontinuities in their light distribution (striations). This will not occur with a faceted reflector. A faceted reflector consists of a number of adjacent, plane or curved, facets that together approximate a curve that is an approximation of a parabolic curve. The width of beam produced by the faceted reflector is somewhat greater than that of a smooth-curved reflector.
Translucent diffusers enlarge the apparent size of the light source. They scatter the light of the lamp in all directions without defining its light distribution. They serve mainly to reduce the brightness of the luminaire and thus the glare created by it. Diffusers are made of opal glass or translucent plastic, commonly acrylic or polycarbonate. The material should be such that it scatters the light while producing the minimum amount of absorption.
Screening the Lamp from Direct View
Screening is employed to hide the bright lamp or lamps from direct view. The higher the brightness (luminance) of the lamp, the stricter the requirements for the shielding.
Shielding devices are often combined with the function of defining the light distribution, in which case highly reflective material is used for the louvre.
Basic photometric data that can be calculated from the light distribution are the beam spread and the luminaire light-output ratio. For all types of luminaires and for all types of application, these data provide an insight into the photometric quality of the luminaire.
The mechanical function of the luminaire housing is threefold: it accommodates the various component parts of the luminaire, such as the optical system and the various components of the electrical system; protects these against external influences; and provides the means of mounting the luminaire in the installation.
Sheet steel is generally chosen for the manufacture of tubular fluorescent luminaire housings for use indoors. The pre-painted sheet steel from the roll is white with diffuse reflection properties. Thus, after having been shaped in the luminaire factory into the desired luminaire form, no finishing-off operations are required.
Stainless steel is widely used for many of the small luminaire components, such as clips, hinges, mounting brackets, nuts, and bolts, that have to remain corrosion free.
Plastics are used for complete luminaire housings, for transparent or translucent luminaire covers, and for many smaller component parts. All-plastic houses can of course only be employed for light sources that have a relatively low operating temperature.
Plastic covers are of methacrylate or polycarbonate. Methacrylate maintains its high light transmission properties over a long period, but its impact resistance is relatively low. The impact resistance of polycarbonates is very high and thus offers a high degree of protection against vandalism. It can be chemically treated to protect it from yellowing under the influence of ultraviolet radiation.
Normal glass, where no special demands are placed on heat resistance.
Hard glass, where heat resistance, chemical stability, and resistance to shock are important. Should hard glass break, it will disintegrate into small pieces.
Luminaires made completely out of glass are extremely heavy and nowadays are seldom employed.
Ceramic material is used in compact housings that are exposed to very high temperatures such as very compact halogen lamp luminaires.
All luminaires should have housings of sufficient rigidity to withstand normal handling, installation, and use. With indoor-lighting luminaires for fluorescent lamps, stiffness and rigidity of construction is particularly important, since these lamps are relatively large and awkward to handle. Perhaps the most critical part of a luminaire as far as strength is concerned are the mounting brackets. The strength required here is covered by a safety factor: the mounting bracket (s) must be able to support at least five times the weight of the luminaire itself. With road-lighting and outdoor floodlighting luminaires, the mounting brackets must also be strong enough to withstand the highest conceivable wind loading for the location. Here a good aerodynamic shape for the luminaire (characterized by its so-called shape factor) can be advantageous, as it also serves to reduce the strength required for the lighting mast.
Under some circumstances, the impact resistance of the luminaire itself is also important, particularly where protection against vandalism is called for.
Resistance to Pollution and Humidity
The first numeral classifies the degree of protection against the ingress of solid foreign bodies (ranging from fingers and tools to fine dust) and protection against access to hazardous parts.
The second numeral classifies the degree of protection against the ingress of moisture.
The higher the IP values, the better the protection and thus the lower the dirt accumulation and the lumen depreciation.
Ease of Installation and Maintenance
Many luminaires are of such a shape, size, and weight as to make mounting them a difficult and time-consuming operation. Mounting, but also relamping and cleaning, must usually be carried out high above ground level. So the ergonomic design of the luminaire should be such as to make these operations as easy and as safe as possible to perform. For example, covers should be hinged so that the electrician has his hands free to work on the lamp and gear. A good, ergonomically designed luminaire is one that can be mounted in stages: first the empty housing or a simple mounting plate, which is light and easily handled, then the remaining parts.
The electrical function of a luminaire is to provide the correct voltage and current for the proper functioning of the lamp in such a way as to ensure the electrical safety of the luminaire.
The electrical wiring in a luminaire must be such as to ensure electrical safety. This necessitates great care in the choice of wire used and its installation. There are a great many different types of wire available, in both single-core (solid) and multi-core (stranded) versions, all with various cross-sectional areas and clad with various thicknesses and qualities of insulation. The cross-sectional area (thickness) of the wire must be matched to the strength of the current flowing through it. The insulation of the wire used must be resistant to the high air temperature in the luminaire and the temperatures of the luminaire materials with which it is in direct contact. This is true not only under normal conditions of operation, but also in the presence of a fault condition.
The method used to connect a luminaire to the power supply must be both quick and safe. The practice generally adopted is to incorporate a connection block in the housing, although prewired luminaires in which the electrical connection to the mains is automatically made when the unit is placed in position are also available.
Electrical Safety Classification
Class 0: Applicable to ordinary luminaires only. These are luminaires having functional insulation, but not double or reinforced insulation throughout and with no provision for earthing.
Class I: Luminaires in this class, besides being electrically insulated, are also provided with an earthing point connecting all those exposed metal parts that could become live in the presence of a fault condition.
Class II: This class embraces luminaires that are so designed and constructed that exposed metal parts cannot become live. This can be achieved by means of either reinforced or double insulation. They have no provision for earthing.
Class III: Luminaires in this class are those designed for connection to extra-low-voltage circuits (referred to as Safety Extra-Low Voltage, SELV). They have no provision for earthing.
A considerable amount of the electrical energy supplied to the lamp is converted into heat. The ballast adds to this heating effect within the luminaire. With very high-powered lamps, the ballast should be placed outside the luminaire in a special ballast box.
The volume of the luminaire. The greater the volume, the lower will be the temperature rise inside the luminaire.
The ease with which the heat generated within the luminaire can be conducted through it to the surrounding air. One way of promoting airflow through the housing is to make use of heat-conducting materials in its construction. Most metals are good in this respect, while plastics, on the other hand, are thermal insulators and cannot therefore be employed as housing materials where high-power lamps are involved.
The cooling effect of the surrounding air. Good heat dissipation calls for large surface areas to be in contact with the surrounding air. Luminaires for high-power lamps, such as high-bay luminaires, floodlights, and some LED luminaires that are very sensitive to high temperatures, are therefore provided with cooling fins (Fig. 11). Some types of industrial luminaires are provided with air vents in the top of the housing to allow the warm air to escape.
Protection Against Flammability
The flammability of a luminaire operating under fault conditions is an issue with luminaires made of plastics. But the combustion behavior of such luminaires is not just material dependent; it also depends on the shape and thickness of the luminaire housing.
No less important than the functional characteristics of a luminaire is what is termed its aesthetical appeal, that is to say its appearance and styling. In interiors, all non-recessed luminaires are clearly visible, and so whether switched on or not, their design should be in harmony with that of the interior. In outdoor lighting, it is usually only the daytime appearance of the luminaires, when these are clearly visible, that is important: particularly in built-up areas, their design can make a positive contribution to the attractiveness of the locality.