LER: Luminaire Efficacy Rating

Have you ever heard of a factor called the Luminaire Efficacy Rating, or LER?

Luminaire Efficacy Rating is exactly what it sounds like – it is a measure of how efficient a luminaire is, which basically means “how much light does it put out based on how much energy it consumes?”  Imagine it as “miles per gallon” for lighting fixtures; that example is pretty oversimplified, but it’s a good comparison of how the LER relates to the overall efficiency of a luminaire.  LER is expressed in “lumens per watt,” which makes sense if you think about it very briefly – how many lumens does a fixture produce per each watt of power that it uses, or how much light does this thing produce when it eat this much power?

The Luminaire Efficacy Rating generally deals with three important criteria:

  • the efficacy of the luminaire, or how much light it delivers per watt
  • the ability for the luminaire to direct light outside of itself
  • the ability of the luminaire’s ballasts to deliver power to the lamps efficiently

The LER is a factor that the National Electrical Manufacturers Association (NEMA) has put into play – fluorescent luminaires are one of the categories being compared in the case of LER, and the figure compares many factors.  There are three major categories of luminaire types that are broken down with the Luminaire Efficacy Rating – fluorescent luminaires, high-intensity discharge industrial luminaires (arc lamps), and commercial, non-residential downlight luminaires.

I put together an image with the breakdown of the terms and basic definitions – I hope it is helpful!

LER-jimonlight

NEMA breaks down the standards for Luminaire Efficacy Rating in the following documents:

  • Fluorescent Luminaires:
    NEMA LE5
  • High-Intensity Discharge Luminaires:
    NEMA LE5B
  • Commercial (non-residential) Downlights:
    NEMA LE5A

The LER factor mostly deals with luminaires using a ballast.  You can certainly calculate the LER for a luminaire using an incandescent lamp – the difference is that you wouldn’t multiply the Ballast Factor into the equation.  Your new equation would be:

LER = (EFF x TLL)/input watts
Luminaire Efficacy Rating for an incandescent luminaire = the product of the luminaire’s efficiency multiplied by the total lamp lumens of the luminaire, divided by the input watts of the luminaire.  Makes sense, right?  No ballast in an incandescent luminaire!

Let’s look a bit at the definitions in this LER equation.  Not everyone might have heard of all of these figures, and some people might be saying “SAY WHAAAT?”

EFF, or Luminaire Efficiency:
This term refers to the output of the luminaire proportionally to the lamp or lamps’ output.  Technically, it is a measure of the amount of luminous flux of the luminaire divided by the amount of luminous flux of just the lamp itself.

(HEY JIM!  What the heck is luminous flux?)

Luminous flux is the measure of the perceived brightness or “light power” – it’s different than radiant flux, which measures all of the light emitted.  Luminous flux is geared towards what the eye can see and the brain can interpret.

TLL, or Total Lamp Lumens:
This term refers to the total measured (rated) quantity of lumens coming from the lamps.  This amount is also multiplied by how many lamps are in the luminaire.  Pretty understandable, right?  So, for example, if I have a luminaire with 3 lamps with a 2000 lumen output each, the total lamp lumens is 6000 lumens – 2000 lumen lamps multiplied by 3 lamps = 6000 lumens.  Cake.

BF, or Ballast Factor:
Ballast Factor isn’t a difficult thing to understand, but there are a few components to understanding it.  Ballast Factor deals with both parts of the creation of light – the ballast and the lamp.  Ballast factor is the ability of a ballast to produce light from the lamp or lamps that it energizes.  A ballast not only fires up the lamp, but after it’s started, it maintains the processes of the lamp.  The Ballast Factor is measured by taking the lumen output of  lamp and ballast combination and dividing it by a reference lamp/ballast combination.

Reference Ballasts are ballasts that are designed to be nearly “perfect” in order to perform under a particular set of conditions.  NEMA has guidelines set forth for Reference Ballasts, which is how we are able to use them to compare other ballast/lamp combinations.

Luminaire Watts Input:
Another very easy thing to understand – Luminaire Watts Input (also called Watts Input, Input Watts, or a number of terms generally related to the idea) is how many watts of power that the luminaire consumes.

I hope this makes a bit more sense if you didn’t know about it before.  Please send me an email through the contact form if you have any questions!

3 replies
  1. Mik Wood
    Mik Wood says:

    Hi Jim,

    IMHO the LER measure is somewhat confusing and over complex for entertainment lighting and arguably so for architectural as well. It’s been formulated this way for good reason to allow the separation of things like luminaire efficiency and ballast factor but in doing so I believe has somewhat obscured what we really want to know.

    1. Look at EFF and TLL – the bottom line of EFF is the same as TLL so these two values cancel leaving you with ‘Lumens coming for the luminaire’ which is a much easier value to measure.

    2. The Ballast Factor is just confusing with modern luminaires for no real gain. As you say it compares the actual ballast efficiency with a notional ‘reference’ ballast. But very often we don’t want to bother with that as what we really care about is the actual power consumed, not the power compared to some notional ‘ideal’ ballast.

    So, take away those items and we end up with a much simpler efficacy figure:

    Luminaire Efficacy = ‘Lumens coming form the luminaire” / “Luminaire watts input” expressed in lum/W

    This to my mind expresses what we really want to know. The actual usable lumens we get for every Watt of power supplied.

    Simple example – if a Source 4 fitted with a 750W lamp produces 14,000 lumens then the luminaire efficacy is 14,000/750 = 18.7 lum/W

    Mike

  2. Anonymous
    Anonymous says:

    Thank you very much for explaining in this complex topic. The simplest way to understand LER for me was the rate of changing in put watt to lumen. Thanks.

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