Light Guide:
Light Source and Fixture Selection

The lighting design process in its most basic form entails identifying a task and then providing a light source that will provide proper quantity and quality of light for the task. The fixture protects the light source, connects it to the power source and distributes its light. In this article, we will review the basic factors that go into specifying a light source and a fixture.

Light Source Specification Checklist

The light source is the actual light-producing component of the lighting system. It may operate simply as a lamp (incandescent/halogen) or as a lamp powered by a ballast/ driver (fluorescent, LED and high-intensity discharge [HID]).

Below are considerations for specifying four basic lamp types:

Incandescent Lamps

  • Do not require a ballast
  • Warm color appearance with a low color temperature and excellent color rendering (CRI 100)
  • Compact light source
  • Simple maintenance due to screw-in Edison base
  • Less efficacious light source
  • Shorter service life than other light sources in most cases
  • Filament is sensitive to vibrations and jarring
  • Bulb can get very hot during operation
  • Must be properly shielded because incandescent lamps can produce direct glare as a point source
  • Require proper line voltage as line voltage variations can severely affect light output and service life

Fluorescent Lamps

  • Require a ballast
  • Range of color temperatures and color rendering capabilities
  • Low surface brightness compared to point sources
  • Cooler operation
  • More efficacious compared to incandescent
  • Ambient temperatures and convection currents can affect light output and life
  • All fixtures installed indoors must use a Class P ballast that disconnects the ballast in the event it begins to overheat; high ballast operating temperatures can shorten ballast life
  • Options for starting methods and lamp current loadings
  • Requires compatibility with ballast
  • Low temperatures can affect starting unless a "cold weather" ballast is specified

HID Lamps

  • Require a ballast
  • Ambient temperature does not affect light output, although low ambient temperatures can affect starting, requiring a special ballast
  • Compact light source
  • High lumen packages
  • Point light source
  • Range of color temperatures and color rendering abilities depending on the lamp type
  • Long service life
  • Highly efficacious in many cases
  • Line voltage variations, possible line voltage drops, and circuits sized for high starting current requirements must be considered

LEDs

  • Requires a power supply (driver)
  • Range of white color temperatures and RGB color mixing
  • Energy efficiency (savings range of 82% to 93%)
  • Long life (up to 100K hours)
  • No UV emissions / low infared
  • Durable
  • Small size and design flexibility
  • Instant on
  • Dimmable
  • Silent operation

 

Below is a checklist for specifying the right lamp for the application:

  • Light output
  • Input wattage
  • Efficacy (lumens per watt)
  • Rated service life
  • Size
  • Surface brightness
  • Color characteristics
  • Electrical operating characteristics
  • Requirement of additional equipment such as ballasts
  • Compatibility with the electrical system
  • Suitability for the operating environment

See also Fluorescent Light Sources, Incandescent Light Sources, HID Light Sources, Demanding Environments, Color Metrics, Industrial Light Source and Fixture Selection, Lighting Metrics

Light Fixture Specification Checklist

A luminaire, often called a light fixture, is a complete lighting unit that produces and distributes light. It contains the light source, a ballast if the lamp is fluorescent or HID, components designed to diffuse or distribute the light in a controlled pattern, components to protect and position the lamp(s), and a connection to the power source.

The light fixture's basic function is to produce and distribute light to fulfill the design goals for the lighted space. Below is a checklist for specifying the right fixture for the job.

Characteristics of The Space

First, the specifier must fully understand the demands of the application and conditions in the space that will affect the operation of the lighting system:

 

  • Tasks to be performed in the space
  • Desired light levels based on the tasks performed in the space
  • Room size and dimensions
  • Structural obstructions such as beams
  • Layout of furniture and obstructions such as partitions
  • Room and object surface colors and reflectances
  • Special concerns such as safety and security
  • Hours of operation
  • Assessment of normal operating conditions
  • Possibility or known existence of abnormal operating conditions
  • Cleanliness of the area during operation
  • Maintenance schedule
  • Availability of daylight

See also Lighting Audits, Demanding Environments, Planned Lighting Maintenance, Lighting Design: Basic Principles

Characteristics of Lighting Components And The Fixture

Now the most appropriate light source can be selected, followed by the fixture. The specifier must understand the factors affecting fixture selection:

 

  • Electrical, physical and operating characteristics of the light source selected
  • Electrical, physical and operating characteristics of appropriate ballasts
  • Electrical, physical and operating characteristics of controls to be employed
  • Fixture efficiency (% lamp light output transmitted out of the fixture)
  • Distribution pattern
  • Glare control
  • Finish
  • Appearance
  • Size
  • Accessibility of interior components for maintenance
  • Ability to handle abnormal as well as normal operating conditions
  • Aesthetics

 

See also Light Fixtures: Classifications, Lighting Metrics, Fixtures: Optical Systems, Lighting Design: Basic Principles, Industrial Light Source and Fixture Selection, Controlling Glare

 

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