LED Lighting Design Approach

Luminaires or Lamps?

Designing LEDs into general illumination requires a choice between designing either a complete luminaire based on LEDs or an LED-based lamp meant to install into an existing fixture. Generally, a complete luminaire design will have better optical, thermal and electrical performance than the retrofit lamp, since the existing fixture does not constrain the design.

It is up to the designer to decide whether the total system performance of a new luminaire or the convenience of a retrofit lamp is more important in the target application.

Target Existing Luminaires

If the target application is better served by creating a new LED luminaire, then designing the luminaire’s light output to match or exceed an existing luminaire has several advantages. First, an existing design is already optimized to target a known application, and can provide guidance for setting the design goals around light output, cost, and operating environment.
Second, an existing design is already in an accepted form factor. Switching to the LED luminaire is easier for the end user if the form factors are the same.

Unfortunately, some LED luminaire manufacturers are misreporting or inflating claims of LED luminaire efficacy and lifetime characteristics. The lighting industry saw similar problems during the early years of CFL replacement bulbs. The lack of industry standards and wide variations in early product quality delayed the adoption of CFL technology for many years. The United States Department of Energy is aware that the same standards and quality problems may exist with early LED luminaires and that these problems may delay the adoption of LED light in a similar fashion. In response, it launched the DOE SSL Commercial Product Testing Program (CPTP) to test the claims of LED-luminaire makers. This program anonymously tests LED-based luminaires for the following four characteristics:

  • Luminaire light output (lumens)
  • Luminaire efficacy (lumens per watt)
  • Correlated color temperaure (degrees Kelvin)
  • Color-rendering index

DOE’s CPTP sets a good precedent for LED luminaire design by focusing on the usable light output of a luminaire — not just the light output of the light source.

The Idea of a Lamp May Be Outdated

The long lifetime of LED light may make the idea of a lamp outdated. Lighting-class LEDs do not fail catastrophically
like light bulbs. Instead, they can provide at least 50,000 hours of useful lifetime before they gracefully degrade below 70% of their initial light output (also called lumen maintenance). That’s equal to 5.7 years if left on continuously!

However, in most lighting situations, the lights are switched off regularly. This off period can extend the lifetime of the LED well past three decades, as shown in Chart 1 (at right). After the years it will take for an LED luminaire to “burn out,” LED lighting technology will be brighter, more efficient, and probably offer TCO savings over the older LED luminaire.

Hours per day
Chart 1 - What 50,000 Hours Means in Practical Terms

Keep in mind how much environmental impact was avoided over that 50,000 hours of LED luminaire lifetime:

  • At least 25 fewer incandescent bulbs were sent to landfills AND five times less energy used. (About 50% of power in the United States comes from burning coal, which releases mercury into the air)
  • Or, at least 5 fewer CFL bulbs containing mercury sent for disposal.

As mentioned previously, maintenance avoidance is an important benefit for LED luminaires. Therefore, designing the LED luminaire to deliver maximum lifetime and provide TCO savings is an excellent strategy to overcome the hurdle of the higher initial cost of LED-based luminaires.

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