Electric Luminaire Components

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Sources & Ballasts

Since the beginning of electric lighting (Y. Edison incandescent lamp, 1879) a large number of light sources have been developed with wide variety in power, color, efficiency, size and mounting type. The main categories of lamp types based on their operation principle are:

  • Incandescence
    • Incandescent
    • Halogen
  • Discharge
    • High intensity discharge
      • Mercury
      • Sodium
      • Metal Halide
    • Standard intensity discharge
      • Fluorescent
        • Compact, linear
        • Cold cathode
      • Neon
  • Electroluminescence

A special category of lamps is the electrodeless lamp in which all the power needed for lamp operation is transferred through the use of electromagnetic field.

Selection criteria for lamps are :

  • Efficacy usually referred to system efficacy (lamp+ballast) in lm/W
  • Wattage
  • Lamp life which can be distinguished in economic life (when efficacy drops below a certain level) or useful life (when light output falls below a predefined value). Lamp Lumen Depreciation is a crucial parameter describing the degradation of lamp output due to various reasons such as exhaustion of coatings, blackening of lamp envelope etc.
  • Color. Two parameters used to evaluate the spectral output distribution emitted by lamps. Correlated Color Temperature (CCT) measured in Kelvin degrees (e.g. warm color 30000K) and Color Rendering Index (CRI) with maximum value of 100 (incandescent lamps). Comparison of various lamps CRIs is performed when these have the same CCT.
  • Bases. Is the part that holds lamp in its position in the interior of a luminaire. Is a critical parameter when an upgrade of a lighting system is designed and are categorized using the International Electrotechnical Commission (e.g. E27, GU 5.3)
  • Size which in turn might affect luminaire size and reflector efficiency.

Lamps are designated using standard abbreviations which give information about their wattage, CCT, CRI and size (according to the International Lamp Coding System).

Except standard incandescent lamps, halogen low voltage lamps need a transformer, LEDs a driver and discharge lamps a ballast to operate properly.

A LED intensity strongly depends on current thus a voltage source and a resistor can dim led light output but not linearly. The preferred method to dim the LED is with a constant current device.

Magnetic ballasts (upper) versus electronic with daylight sensor

A discharge lamp needs a device not only to start but to regulate the current passing through the circuit. The last generation is the electronic ballast which in comparison to the old magnetic ones operate the lamp to much higher frequency (20-40 kHz) eliminating flickering and improving lamp-ballast efficacy by 15-20%. They have less weight, are quieter and have the ability to dim lamp’s light output either centrally connected to a bus system or individually. Proper choice of a ballast (rapid or instant start) for a given lamp is necessary to improve efficiency and to optimize operation.


Section Key Resources
  • IESNA Lighting Handbook, 2000
  • National Lighting Product Information Program , Specifier Report , “Electronic Ballasts”, Vol. 8, No 1, 2000
Links

Lead Author(s): Aris Tsangrassoulis

Reflectors

Fig.1  Spectral reflectance of various materials (Courtesy of BartenBach LichtLabor)
Fig.2  Types of luminaire reflector materials (Courtesy of BartenBach LichtLabor). Multi-faceted/segmented or hammered material is used when more diffuse distribution is needed or correction to photometric irregularities should be performed.
Fig.3  Cut-off angle
Fig.4  Raytracing using Photopia (http://www.ltioptics.com/Photopia/overview.html)

Reflectors are fundamental parts of a luminaire capable to control lamp's light flux efficiently, achieving the desired photometric properties. Selection of the proper material (specular, diffuse) affect light distribution and luminaire efficiency. The most common materials are aluminum (chrome in some cases) or aluminum coated plastics. The latter should be used according to lamp temperature characteristics. Anodized aluminum can have >85% reflectance (in some cases up to 95%) offering neutral color rendition while at the same time can be easily processed and recycled.

Reflectors are available in a number of shapes with specular/semi-specular or diffuse finish. The latter is used to create wider distributions with more uniform luminaire luminances (see Figure 1).

The use of specular reflector material (see Figure 2) can offer more control to emitted light. By minimizing the number of reflections, efficiency is increased as well and that is achieved when reflector design is based on lamp physical/photometric and thermal properties. Lamps with large dimensions need a larger reflector to control light rays efficiently. If specific (i.e. narrow) intensity distribution is needed, a part of lamp’s light flux should be cut off to avoid spilled light.

The most common used shape is the parabola although circular, elliptic or hyperbolic segments can be used as well. The position of the lamp in relation to a parabolic reflector defines the intensity distribution (narrow or wide) and the cut-off angle as presented in Figure 3.

Baffles and louvers can be used to reduce glare. If diffuse material is used the cut-off area receives some light while in the case of parabolic shape specular louvers this area appears dark, which makes them suitable for spaces with VDTs. A special type of reflectors are the involute ones which do not allow 1st reflection rays to return to the lamp thus reducing overheating stress.

Today optical design is performed by the use of computer modeling permitting performance evaluations of various reflector shapes (see Figure 4).

Section Key Resources

Links

Lead Author(s): Aris Tsangrassoulis

Shielding/Diffusion Components

These components (baffles, reflective/diffuse louvers, prismatic panels, lenses, diffusers) are used in an effort to modify intensity distribution. Adoption of these components has as design targets:

  • To reduce glare (decreasing luminaire luminance)
  • To avoid direct view of the lamp
  • To modify cut-off angles
  • To reduce UV emission
  • To provide safety protection
  • To redirect light rays due to deflection (prismatic systems, light guides)
  • To modify spotlight optical properties (lenses)
Hybrid luminaire with light guide components (http://new-learn.info/helioptics/outline.html)

Materials used for refractors and diffusers are mainly acrylic, polycarbonate and glass. Suitability of each material is decided on a number of criteria such as discoloration, abrasion and impact resistance, visible transmittance and clarity. Important parameter for estimating discoloration is the ASTM yellowness index. Acrylic even after some period of use, maintain a high value of transmittance in relation to polycarbonate but cracks more easily. Polycarbonate is more sensitive to discoloration/haze. Glass is heavier than plastics with excellent optical properties, anti-static, chemical resistant and more expensive as well.

One important component is the light guides (such as PRISMEX) which are capable to transfer light flux across the material sheet using edge-positioned lamps. Printed dot matrix on the surface can produce even illumination.

Fig.1  Parabolic louver. Its upper part which is used as lamp bearer absorbs light flux reducing luminaire efficiency.

Diffusers can be prismatic or opal with their main function is to hide the lamp and if possible to redistribute light flux on their surface. To achieve this a certain distance is needed between the lamp and the diffuser. There are of white appearance and the main concern during design is to reduce light absorption. Diffusers cause more uniform luminance distribution in contrast to shielding components (baffles, perforated metal , specular parabolic louvers(see Figure 1)). Shielding is used to limit unwanted light rays or to block direct view with the light source. These components can have an impact on luminaire efficiency. Parabolic louvers reduce direct glare without compromising efficiency but they can cause an abrupt luminance transition form the dark cut-off to viewing area.

Section Key Resources
  • IESNA Lighting Handbook
  • Advanced Lighting Handbook, 2003 New Building Institute Inc, (www.newbuildings.org)
Links

Lead Author(s): Aris Tsangrassoulis

Housings

All parts of luminaire (lamps, ballasts, wiring, reflectors/diffusers, sensors) are contained inside an especially designed fixture which is usually made of aluminum alloys or surface treated (powder coated) steel or synthetic. Key design targets is construction simplicity, resistance to corrosion , excellent heat dissipation, pleasing appearance, long life expectancy for all materials and easy maintenance.

Aluminum housings can affected by increased acidity although its effect is rather limited due to the protection of the surface by aluminum oxide. Galvanized steel is more sensitive especially in humid environments where synthetic housings are preferred (e.g. pools)(1). Minor components such as rubber gaskets (sensitive to oil products), metallic brackets , heat resistive cables and sockets should withstand corrosion as well, since they can affect housing's service life.

Heat dissipation is a major issue with housing capable to remove heat from lamp sockets and interior and dissipate this heat to the environment. Temperature inside the housing can have a dramatic influence on lamp's and ballast's life expectancy and luminaire efficiency. LEDs are affected as well (2) by poor thermal management since it can cause an increase in their junction temperature deteriorating optical performance and service life. All luminaires should comply with a set of standards (e.g.IEC 60598) concerning their safety requirements.

Section Key Resources
  • Chemical resistance of luminaire components, Application Guide, Philips Lighting
  • T. Dong & N. Narendran, “Understanding heat transfer mechanisms in recessed LED luminaires”, 9th International Conference on Solid State Lighting , August 3-5, 2009, San Diego, Proc. Of SPIE 7422
Links
  • No links specific to this section have been listed.

Lead Author(s): Aris Tsangrassoulis

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