Workplace Illumination – Part 11: Luminaires

            Thus far, the “Workplace Illumination” series has presented primarily conceptual information (i.e. physics) and the human side (i.e. physiology) of lighting and vision.  This installment begins an exploration of the hardware (i.e. physical structures) used to bring designed lighting to fruition.  Luminaires are fundamental building blocks of lighting systems and embody a substantial portion of their design specifications.
            Despite their central role, many users of lighting systems fail to appreciate the extent to which the specification of luminaires influences the comfort, safety, and productivity of workspaces.  The functions of a luminaire, how its components serve those functions, and various types of luminaires are presented in this installment.  A basic understanding of luminaire functions is required to optimize lighting system design or even to create functional lighting.

            When considering the lighting system on which their work depends, many only see a “light,” a “light bulb,” or possibly a “fixture.”  A luminaire is all of these, if “loose” terminology is accepted, and much more.  A very basic definition describes a luminaire as a device employed to produce, control, and distribute light.  A complete definition references additional functions and components used in various combinations to achieve desired effects.
            Rather than create an inclusive, but cumbersome, single-sentence definition, this installment expands the definition of a luminaire into more-palatable presentations of components, organized by the function it performs or supports.  This begins with the most salient function, the production of light.

Light Production
            The key component responsible for producing light or, more accurately, emitting optical radiation, is the lamp.  Lamps are often called “light bulbs,” though this is a colloquialism.  A bulb is actually one component of a lamp; it is the outer glass shell that contains the fill gas required to create an environment in which the emitter operates effectively.
            In an incandescent lamp, the emitter is a filament, usually made of tungsten, heated by electrical current.  Incandescence is the emission of visible light caused by thermal excitation of a material’s atoms.  Characteristic of emitted light, such as apparent color and CCT, are dictated by the filament material and temperature.  Filament temperature is dictated by the electrical current supplied to it.
            Characteristics of observed light output can be altered by treating the bulb.  Coating or etching the interior glass surface is common; this is often done to diffuse the emitted light for a uniform light distribution.  A bulb can also be coated with material to selectively absorb wavelengths of light (i.e. filter), customizing the lamp’s spectral output.  An example of incandescent lamp construction is shown in Exhibit 1.

            In discharge lamps, a multi-step process is required to produce light.  The most common type of discharge lamp is the fluorescent “tube,” available in several sizes.  Energizing an electrode causes the emission of electrons from it.  Collisions of these electrons with atoms of mercury, the primary constituent of the tube’s fill gas, releases ultraviolet (UV) radiation.  Phosphors coating the interior of the glass tube absorb the UV radiation and emit visible light; this process is known as down-conversion.  Characteristics of the visible light emitted are formulated by the combination of phosphors used.  A typical fluorescent tube construction is shown in Exhibit 2.
            Other discharge lamps, such as high-intensity discharge (HID) metal-halide (MH) and high-pressure sodium (HPS) lamps have similar operating characteristics, but differ in key ways.  Fluorescent tubes are double-ended (i.e. cap/connector at each end) with a single-layer glass enclosure.  Compact fluorescent lamps (CFLs) often use single-ended configurations of size comparable to standard incandescent lamps.  A typical CFL construction is depicted in Exhibit 3.

            HID lamps are often single-ended with the discharge tube enclosed in an outer bulb.  Phosphors may be used to modify the output of HID lamps, but are not required to convert the radiation emitted (i.e. emission is in the visible spectrum).  Construction of a typical HPS lamp, a common type of HID lamp, is shown in Exhibit 4.
            A characteristic shared by fluorescent and HID lamps is negative resistance.  Discharge lamps require use of ballasts to limit current and maintain the electrical supply to the lamp.  The ballast is often housed in the luminaire, external to the lamp, but may also be mounted remotely if physical space limitations or temperature extremes warrant.  Some lamps, such as CFLs, are constructed with an integral ballast.  It is its integral ballast that allows a CFL to be used as a drop-in replacement for a less-efficient incandescent lamp.  Electronic ballasts are preferred to magnetic types for the performance improvements they provide.  These include increases in efficacy and lamp life, integrated circuits with advanced control capabilities, reduced weight, and the elimination of flicker (see Part 8).

            Solid-state lighting (SSL) is rapidly eclipsing all other lamp types in many applications.  Their long service life, high efficacy, “programmability,” durability, light weight, flexible form factor, low operating cost, and ever-improving installation cost provide economic and performance advantages in a variety of applications.
            SSL is an umbrella term for various types of LED or light-emitting diode.  LEDs are semiconductor diodes; a semiconductor is a material characterized by resistance between that of a conductor and an insulator that can be modified by the application of an electric field and a diode is a device that allows electrical current to flow in only one direction.  LEDs emit visible light via injection luminescence, where electrons and electron holes are recombined, ejecting photons.  An electron hole is a position in a material’s valence band where an electron could exist, but does not; it is an empty space in the valence.  The wavelength of emitted light is dictated by the diode material.
            Further exploration of the inner workings of LEDs is left to the interested reader.  The complexities of semiconductor physics is far beyond the scope of this series, but the increasing importance of SSL should make good references relatively easy to find.
            Optical radiation emitted by LEDs can be manipulated by methods similar to those used for other light sources.  Phosphors can be used to convert UV radiation to visible light and multiple LEDs can be combined, using additive color mixing (see Part 7) to achieve desired output.
            Like discharge lamps, LEDs require a control device between the electrical power supply and the lamp, called a driver.  Drivers are made in constant-current and constant-voltage configurations to match the construction of the LED package.  The driver converts AC power to DC and conditions it to maintain proper function of the LEDs.
 
            Other lamp types are also available, including combination lamps that use more than one of the types described in a single unit.  Those described above represent the vast majority of relevant installations, as well as a progression of lighting technology over the past century.
 
Light Control
            The control of light, as a function of luminaires, is subject to interpretation.  Some control factors were discussed in the previous section, as they pertain to the production of light.  For example, ballasts and drivers, as well as other components, such as igniters, control light by establishing and maintaining conditions conducive to visible light emission.  These controls are electrical in nature.
            Dimming is another electrical control capability; it may be integral in a discharge lamp ballast or LED driver.  It may also be achieved by an external device, as is the case for incandescent lamps.  The mechanism used to dim a lamp’s output varies with the type of lamp; compatibility of devices and objectives must be verified before integrating them in a lighting system.
            Controls of a chemical nature are also used.  Coating a bulb’s interior with phosphors to modify a lamp’s spectral signature is one example.  The selection of fill and ionizing gasses and electrode and diode materials are also forms of chemical control.
            Controlling the distribution of light is so important that its arguably redundant inclusion in the basic definition of a luminaire is acceptable and justified.  It also warrants its own section in this presentation.

Light Distribution
            The distribution of light is controlled by physical means.  The first step in light distribution control is the selection of direct or indirect lighting, or a combination in specified proportions.  CIE classifies luminaires according to the proportions of direct and indirect light emitted.  The six CIE classifications, summarized in Exhibit 5, are based on light distribution as well as directionality.

            Further control can be achieved with diffusers, reflectors, refractors, and louvers.  As discussed previously, light can be diffused at the source – the lamp – by way of bulb treatments.  A diffuser can also be external to the lamp, such as a translucent glass or plastic covering.
            A reflector can also be integral to a lamp.  In lamps with an internal support structure, a formed and polished metal sheet can be attached.  Alternatively, a portion of the bulb can be aluminized to create a reflective surface that redirects light for a more-advantageous distribution.  It is more common, however, for reflectors to be external to the lamp, eliminating the need for precise orientation of the lamp in its socket.
     Refractors are used to modify a luminaire’s light distribution to create a pattern better-suited to the application.  A refractor could narrow an emitted light beam, increasing the illuminance of a reduced area.  It could also be used in the reverse, increasing the area illuminated, though with reduced illuminance.  These are basic refractors, typically consisting of a glass or plastic cover with a prismatic array formed into one side.  More-complex refractors can be developed, but simpler solutions to the lighting challenges they would solve are typically available.
            Louvers are used, primarily, to reduce glare by limiting the angle at which a lamp can be seen.  Louvers are simply small baffles, often arranged in a grid pattern, that block one’s view of a lamp at certain angles without reducing the direct downward emission of light from the luminaire.  They can also be configured to reflect incident light, modifying the distribution.  The choice of material and finish depends on the purpose of the louvers and aesthetic concerns.
            A luminaire may be said to include a lens; however, this is often use of “loose” terminology.  It is most accurate when referring to a prismatic refractor, but it may also be used to identify a diffuser or transparent cover.  It is best to assume it is being used generically to identify some type of cover for the luminaire’s opening.  If the true nature of the “lens” is important, it should be verified.

Physical Structure
            In order for a luminaire to perform any function reliably, it must have an appropriate physical structure.  The first requirement of the physical structure is to provide a stable and secure mounting mechanism.  It must support the weight of the luminaire, withstand any shock or vibration to which it may be subject, and limit the effects of temperature extremes.
            A luminaire housing provides physical protection for the lamp.  In some applications, such as a medical or manufacturing clean room, the environment may require protection from the lamp in the case of a catastrophic failure.  In such applications, the luminaire must contain glass fragments and any internal components of the lamp that separate, preventing debris from escaping.
            A mechanism may be required to facilitate aiming the luminaire as a first step in creating the desired light distribution.  To effect further control, the structure must include provisions for attachment of a diffuser or other light control device.
            Luminaires may be components of a facility’s heating, ventilation, and air conditioning (HVAC) system as well as its lighting system.  In this type of application, pathways are designed into the structure that allow air to flow through the luminaire to attached HVAC ductwork.  This arrangement may facilitate installation in space-limited applications or in maintaining stable conditions conducive to lamp operation and service life.  It is often used to prevent heat generated by lamps from adding to the heat load of an air-conditioned space.
            Less-stringent applications may employ open luminaires that allow free-flow of air to limit dust buildup.  The opposite extreme is occupied by environments that require sealed luminaires to prevent dust, debris, or liquid from entering.  The extent to which a luminaire can prevent such infiltration is described by the ingress protection (IP) rating system.
            IP ratings are cited in the format “IP##,” where the first “#” represents a device’s level of protection against ingress of solid material and the second of liquid (i.e. water).  A “0” in either position indicates that no protection is provided against infiltration of the corresponding material type, while an “X” indicates that the level of protection is unknown (i.e. untested).  A summary of the basic IP rating system is provided in Exhibit 6.  IP rating systems have been defined in several national and international standards with some variation.  For example, some sources include a water-ingress rating of 9, indicating protection against high-pressure, high-temperature streams.  The summary shown is a useful reference; however, current versions of applicable standards should be consulted for critical applications.

            Additional rating systems are available to identify devices appropriate for use in dangerous environments.  Flameproof and explosion-proof luminaires can be specified for installations in the presence of combustible dust or gas.  Applicable standards and manufacturer data should be consulted before specifying luminaires for such an environment.

Electrical Connection
            A luminaire must provide the means to safely connect to an electrical power supply.  The exact nature of the connection may vary depending on the luminaire’s configuration.  For example, connection must be made to remotely-located control gear, if used.  A luminaire with all required control gear housed internally is connected to supply power directly.
            Provisions for safe connection include properly-sized wires, terminals, and circuits, proper grounding, and protection against physical damage, such as wire chafing or mechanical stress.  Equipment purchased from reputable suppliers can be expected to adhere to high standards of electrical safety; however, installation and maintenance must be performed in a manner that ensures a high level of safety is sustained.
 
            Luminaires can be classified or ranked in various ways.  Several of these relate to characteristics of the lamp contained within.  Typical properties of a number of lamp types are summarized in Exhibit 7.  Among them are CCT and CRI; both were presented in Part 7 of the series and are relevant to applications where color properties are important.

            Luminous efficacy and lamp life are important components of financial analyses.  Luminous efficacy relates the amount of electrical power required to the luminous flux generated; it is a direct input to operating cost estimates.  Lamp life drives maintenance needs and associated costs, including a relamping schedule (timeline of lamp replacement).
            Warm-up time and restrike time of a lamp influence the operation of a lighting system.  Warm-up time refers to the lag between powering on a lamp and it reaching its maximum output of optical radiation.  Whether due to power interruption, thermal overload, or other cause, some lamps require a cool-down period to re-establish conditions conducive to its operation.  Restrike time is the amount of time that must elapse after a lamp is extinguished before it can be restruck, or “re-lit.”
            Other classifications refer to the application for which a luminaire is intended.  These include designations for high-bay and low-bay installations, spotlights, floodlights, area lights, task lights, and so on.  Discussion of these differentiators is best pursued in the context of their applications, lest it be too generic to be useful.  It is the aim of future installments of this series to explore various applications, preferred luminaires, and other aspects of lighting systems.


     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).
 
References
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000.
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] “Lighting.”  Hugh King in Plant Engineer’s Reference Book, 2ed (Chapter 25).  Dennis A. Snow (ed).  Butterworth-Heinemann; 2002.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
[Link] “ANSI/IES RP-7-21 Recommended Practice: Lighting Industrial Facilities.”  ANSI.
[Link] “CFL Light Bulb.”  Energy Education.
[Link] “IP (Ingress Protection) Ratings.”  Blue Sea Systems.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
[email protected]