Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo


Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_137



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 [1].

Luminaire Characteristics

The principal characteristics of luminaires can be listed under the following headings:
  • Optical

  • Mechanical

  • Electrical

  • Thermal

  • Aesthetics

Optical Characteristics

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

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.

Spread Reflectors

With spread reflectors (sometimes also called half-matt reflectors), there is no sharp mirror image of the light source. They are employed where a moderate degree of optical control is required, with the emphasis on producing a beam with smooth transitions. Such reflectors also help to smooth out discontinuities in the light distribution caused by inaccuracies in the shape of the reflector. Spread reflection is produced by brushing or etching or by hammering very small dimples and bumps into a specular surface (Fig. 1).
Luminaires, Fig. 1

A spread finish as produced by brushing (left) or by hammering (right) a specular surface

Diffuse Reflectors

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.

Reflector Forms

There are three basic reflector forms: plane, curved, and faceted (Fig. 2).
Luminaires, Fig. 2

Basic reflector forms

Plane Reflectors

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.

Curved Reflectors

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.

Faceted Reflectors

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.


Refractors are used to create the desired luminaire light distribution by passing the light from the source through a refractor (Fig. 3). The angle through which the light is bent is dependent on both the shape of the refractive material and its refractive index (Snell’s law).
Luminaires, Fig. 3

Bending of light by refraction from the incident angle to the refraction angle

$$ \frac{ \sin \left({\alpha}_i\right)}{ \sin \left({\alpha}_r\right)}=\frac{n_{air}}{n_{material}} $$
Refracting devices are either lenses or prisms. The type of refractor most commonly employed in indoor lighting is the lens found in tubular fluorescent-lamp luminaires intended for general lighting. It consists of a horizontal plastic panel which is mounted just below the lamp. The panel is flat on the top and has a special pyramidal (prism) or lens structure on the underside, which directs the light in certain directions and reduces the brightness under specific angles. Where in the past prismatic controllers were used, we see today more advanced lens-type refractors that give more accurate possibilities to shape the light distribution. These types of refractors are also used to produce LED luminaires for general indoor lighting and for road lighting (Fig. 4). Refracting glass bowls were in the past sometimes used for high-pressure mercury and sodium road-lighting luminaires. They have become obsolete because they are heavy, but more so because lighting control in the upward direction, and therefore control of light pollution, is not easily attainable.
Luminaires, Fig. 4

Micro-lens type of refractor for a LED luminaire for general indoor lighting


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.

The luminaire reflector housing itself, or a built-in baffle, can provide the screening function (Fig. 5). When the sole purpose of the louvre is to shield the lamp from view, diffuse-reflecting material is used, such as a white-plastic louvre or, in the case of floodlights, matt-black metal rings (Fig. 6).
Luminaires, Fig. 5

Lamp shielding by the reflector itself (left) and by an internal baffle (right)

Luminaires, Fig. 6

Simple louvre (left) to shield the lamp in a fluorescent-lamp luminaire and (right) a floodlight louvre

Shielding devices are often combined with the function of defining the light distribution, in which case highly reflective material is used for the louvre.

Color Filters

In certain lighting applications, in particular display lighting and decorative floodlighting, color is sometimes used to help achieve the desired aesthetic effect. In the past, color filters attached to luminaires containing white light sources were extensively used for this purpose. Both absorption and dichroic (interference) filters were used, although absorption filters in particular (Fig. 7) lower the efficacy of the total lighting system. Typical transmittance values are as follows: for blue absorption filters 5 %, for green absorption filters 15 %, and for red absorption filters 20 %. The consequence of the light absorption is that these filters become warm, which with high-power floodlights may damage the filter. The solution where such floodlights are employed is to use dichroic filters, which are a more expensive alternative. Today, colored LED light sources are normally employed where colored lighting is required. Here the color comes directly from the lamp itself, so the efficacy of the lighting system is much higher.
Luminaires, Fig. 7

Absorption-type color filters

Light-Distribution Characteristics

The light distribution of a luminaire defines how the luminous flux radiated by the luminaire is distributed in the various directions within the space around it. It is the result of the optical devices used in the luminaire as described above. This is also called luminous intensity distribution, since it is specified in terms of luminous intensities in all the directions in which the luminaire radiates its light (Fig. 8). The luminous intensity diagram is in fact the light fingerprint of a luminaire, in digital form (I-Table), and is the basis of all lighting calculations.
Luminaires, Fig. 8

Light distribution of a luminaire given by its luminous intensity diagram. The arrows represent the luminous intensities in the directions specified. Here the light distribution is given for all planes, although it is usually only given for one (e.g., the blue curve) or two, mutually perpendicular, planes

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.

Mechanical Characteristics

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

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

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.

Aluminum Alloys

Aluminum alloys, in which other elements have been added to the pure aluminum to improve their mechanical, physical, and chemical protective properties, are used to manufacture cast, extruded, and sheet-metal luminaires. Cast aluminum refers to the process in which molten aluminum alloy is poured (cast) in a mold. Extrusion is the process in which softened aluminum alloy is pressed through the openings of a die. Cast and extruded aluminum alloys are much used in housings for floodlighting and road- and tunnel-lighting luminaires because they can be employed in humid and damp atmospheres without having to add protective finishes. Sheet aluminum is chiefly employed in luminaires for reflectors (Fig. 9). The reflectors are anodized to improve their reflection properties and to protect them from becoming matt.
Luminaires, Fig. 9

Mirror reflector of sheet aluminum protected by a plastic film, which must be removed before use


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.


Although glass is heavy, glass covers are used where these have to be positioned close to a light source having a high operating temperature. This is the case, for example, with HID flat-cover road-lighting luminaires and with most floodlighting luminaires. Two sorts of glass are used:
  • 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 atmosphere can contain many potentially corrosive gases which, in the presence of moisture vapor, will form highly corrosive compounds. In all areas where this danger exists (notably in outdoor applications, indoor swimming pools, and certain industrial premises), luminaires made from corrosion-resistant materials or having protective finishes should be used. In such areas, the luminaire should protect the optical and electrical components it houses. It should, of course, be fully enclosed. The degree of protection provided by the luminaire is classified according to the International Protection code (IP code) as described in an international IEC standard [2]. The IP code consists of two numerals: IP • •.
  • 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.

Electrical Characteristics

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.

Lamp Holders

The most usual types of holder are the Edison screw, the bayonet, and the pin (Fig. 10). Most Edison screw and bayonet holders are made of plastics or porcelain, with metal parts for carrying the current. Porcelain is resistant to high temperatures and has a high voltage-breakdown resistance, which is important considering the high ignition voltage of HID lamps. The pin lamp holders for tubular fluorescent and compact fluorescent lamps are nearly always made of plastic. The metal contacts are spring loaded to ensure a constant contact pressure.
Luminaires, Fig. 10

A variety of lamp holders made of both plastics and porcelain

Electrical Wiring

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.

Mains Connection

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

The electrical safety classification drawn up by the IEC embraces four luminaire classes [2]:
  • 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.

Thermal Characteristics

Temperature Control

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.

For a given lamp/ballast combination, the working temperature reached by the luminaire is dependent upon three factors:
  • 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.

Luminaires, Fig. 11

Metal halide high-bay luminaire (left) and recessed LED downlight with cooling fins

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.


Luminaires always have to comply with the appropriate safety rules. UL (Underwriters Laboratories) is the US mark for demonstrating compliance with all US safety standards, and ENEC (European Norms Electrical Certification) is the similar European mark (Fig. 12).
Luminaires, Fig. 12

UL and ENEC certification marks. The number in the ENEC mark indicates the country of the institute that has given the European approval



  1. 1.
    CIE Publication: S 017/E: 2011, International lighting vocabulary. International Lighting Commission, CIE, Vienna (2011)Google Scholar
  2. 2.
    IEC International Standard, 60598-1: ed 7.0, Luminaires – part 1: General requirements and tests. (2008)Google Scholar

Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.NuenenThe Netherlands