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Illuminating illuminance: lumens and lux and footcandles, oh my!

Illuminating illuminance: lumens and lux and footcandles, oh my!

A question we get asked a lot at Leapfrog Lighting is “How bright is your <x> LED light bulb?”.  Our standard response is to cite the number of lumens specific to the lamp in question, or to get more specific, reference the number of lux for a specific distance. But unless the questioner is in the lighting industry, the terms “lumen” and “lux” often lead to confusion.

Manufacturers of the traditional light bulb decided to mass market the term Watts—which actually identifies the amount of power consumed—as a means of describing illuminance level. As a result, most of us are familiar with that metric. So to describe a LED light bulb as providing a certain number of lumens or lux is, well, confusing (especially when a very bright LED light bulb is only 13 Watts!).

So today, we’d like to shed some light on illuminance.

In simplistic terms, illuminance refers to the quantity of light on a given surface.  There are two metrics that are used to measure illuminance: lux and footcandles (which we’ll discuss later). But in order to understand those metrics you must first understand candelas. A candela is the amount of light generated by a candle—an early form of light. The amount of light emitted by one candle is one candela.

Light meters use both lux and footcandles as a light intensity measurement. In essence, lux and footcandles are measures of how many candelas are detectable at a specific distance.

Depending on the measurement system used (metric vs. American), illuminance is either measured in:

  • lux (lumens per square meter)

Lux measurement

Source: Image Arts, Ryerson

1 lux = ~1 candle (at a distance of 1 meter onto a 1 square meter surface)

 

  • footcandles (lumens per square foot)
Foot candle
Source: Continental Lighting

1 = ~1 candle (at a distance of 1 foot onto a 1 square foot surface)

And how do the two compare to each other? 1 footcandle is approximately 10 lux.

Lighting designers often suggest different light levels for a given application. For example, a living room usually requires lower illuminance levels to produce a relaxing environment, whereas a hallway usually requires higher illuminance levels to provide safe navigation.

Note: The IES “Lighting Handbook” provides recommendations for lighting levels—even based on age and activity—if you are unsure of what constitutes a satisfactory lighting level in a specific environment.

Understanding illuminance measurements will help you to select the right LED lighting for your environment. Most manufacturers provide datasheets for their lighting products that include metrics such as lumen, lux and footcandles (for example, see the Leapfrog Lighting ENERGY STAR PAR30 LED lamp datasheet).

Once you’ve selected the right luminance level, don’t forget to account for CCT (correlated color temperature)!

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Sarah Bailey

Sarah Bailey

Manager of Marketing at Leapfrog Lighting
Sarah Bailey is the Manager of Marketing at Leapfrog Lighting. She has an interest in all things SSL, the environment, and, being a Canadian, hockey. She is an active participant in several LED lighting groups on LinkedIn, where many of Leapfrog's blog posts are discussed.

One Response to Illuminating illuminance: lumens and lux and footcandles, oh my!

  1. [...] Manufacturers of the traditional light bulb decided to mass market the term Watts—which actually identifies the amount of power consumed—as a means of describing illuminance level. As a result, most of us are familiar with that metric. So to describe a LED light bulb as providing a certain number of lumens or lux is confusing. Read more… [...]

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