LED LPW - Lumens Per Watt

LED LPW - Learning Objectives

1. What is a lumen?
2. How to compare lumens and footcandles?
3. Why are LPW important?
4. What is the range of LPW of LED compared to other light sources?
5. How many lumens are produced by a 100 watt incandescent light bulb?
6. Which state has the most stringent lighting efficiency codes?

What does the DOE (Department of Energy) and California Title 24 Energy Code require for efficiency in light sources?

Since light sources vary in efficiency, how do you compare them?

What is the difference between efficiency and efficacy?

The basic question is how much light is generated from a source per watt? This gets confusing when we need to add current controlling devices such as ballasts and drivers that also consume energy, so the end result is that  the number needs to include the total amount of watts needed to create the lumens of light emitted from a complete light fixture. A light fixture, called a 'luminaire' includes the light source, the current controller, and the housing.
EFFICIENCY = LPW or Lumens Per Watt of the complete luminaire.
EFFICACY = the light output from the source, before the addition of the fixture, electronic current controllers, housing, and lens.
As you can see from the definitions below, this is a confusing metric that appears to go in circles.

The IES (Illuminating Engineering Society provides the following definitions:
Efficiency:
efficiency[9.4.10]  (See luminaire efficiency and radiant efficiency.)

radiant efficiency[2.6.2]  (of a source of radiation) (ηe)
Ratio of the radiant flux of the emitted radiation to the power consumed by the source. Unit: 1.36
Note: It must be specified whether or not the power dissipated by any auxiliary equipment such as ballasts, if any, is included in the power consumed by the source.
36. International Lighting Vocabulary, Third Edition CIE Document 17.4, Vienna: Commission Internationale de l’Eclairage, 1987.

luminaire efficiency[9.4.8]
NOTE: This term’s definition was changed in RP-16-17 Addendum 1. Deletions have a strike-through. New text is bold.
The ratio of luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used therein.
The luminous flux emitted by a luminaire, divided by the luminous flux emitted by the source(s). Sometimes called light output ratio (LOR).

Efficacy:
efficacy[3.11.1]  (See luminous efficacy of a source of light and spectral luminous efficacy of radiant flux.)

The luminous flux emitted by a luminaire, divided by the luminous flux emitted by the source(s). Sometimes called light output ratio (LOR).

light output ratio:See luminaire efficiency.

Lighting Facts - Comparing the numbers

• Lumens Per Watt = Efficacy of light source
  
• Watts = Power Consumption
• CRI = Color Rendering Index
• R9 = Red light component
• CCT - Correlated Color Temperature -K = Degrees Kelvin, or color temperature of the light
• Flicker = dimability and stability
• TM-30 = New Color Rendition Metric by IES

DOE - Department of Energy
Lighting Facts

 Standard Label with Optional Metrics Anatomy of the LED Lighting Facts® Label Light Output/Lumens

• Measures light output. The higher the number, the more light is emitted.
• Reported as “Total Integrated Flux (Lumens)” on LM-79 test report.

Watts

• Measures energy required to light the product. The lower the wattage, the less energy used.
• Reported as “Input Power (Watts)” on LM-79 test report.

Lumens per Watt/Efficacy

• Measures efficiency. The higher the number, the more efficient the product.
• Reported as “Efficacy” on LM-79 test report.
• A typical pear shaped incandescent light bulb, 100 watt A-19, produces 1,600 Lumens.

IESNA LM-79-2008

• Industry standardized test procedure that measures performance qualities of LED luminaires and integral lamps.
• Allows for a true comparison of luminaires regardless of the light source.

LED Lumen Maintenance

• Listed as a percentage, this metric estimates the amount of light the LED light source is projected to emit at 25,000 hours at a given ambient test temperature, compared to its initial light output. This percentage is based on LM-80, in-situ performance, and TM-21 projections.

Model Number

• Unique manufacturer’s model number for the product.

Type

• Specific type of solid-state lighting fixture.

Brand

• The brand under which each product is available.

• If a label indicates that the product has a warranty, a URL with more information has been verified by the program and is available from the product summary. [2]

Related Topics

Manufacturer's Educational Links, LED for your B&B, Color in Paint - Why they Fade

Footnotes

1. Professor David Heeger - Light/Dark Adaptation
2. www.lightingfacts.com
3. IES RP-28-16 Seniors and Low Vision http://www.ies.org/
   

Photo Credits

• Lighting Facts Card - DOE - Department of Energy
• Test Colors for CRI & Light Source Color Rendering Chart - EYE Lighting
  
• TM-30 Test Colors - IES TM-30-15 Errata

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