The bright side: How to determine light quality for led lighting

August 02, 2016
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Measuring light quality can be tricky, because there is no sliding scale or single measurement that determines which light is best for your space. There are multiple factors to consider, including the total output, spread, color, and color rendering potential of an LED light, and each comes with its own unit of measurement and considerations. Below, we detail each of these factors and tell you which metrics to focus on in order to figure out which light fixture is best for your facility.


Measuring Output


Gone are the days when you can pick up a screw-in 60 Watt light bulb and know how bright it will be. With LEDs, the wattage is often much lower with the same light output (measured in lumens), so how do the two translate? It turns out, not perfectly, since different LED fixtures and brands have different levels of efficiency, measured in lumens per watt (LPW).


So how do you find an equivalent LED light fixture to match the output of your traditional fixture? Our advice is to forget about wattage as your go-to output metric and instead look more closely at the lumens and lumens per watt between the original and replacement fixtures.


Lumens v. Foot Candles


So what’s the difference between lumens and foot candles, and can we translate one into the other? Lumens are a measure of total light output, while foot candles are a measure of the ambient light level, or in other words, how brightly a space is illuminated. Lumen output is measured at the light source, while foot candles are measured at a target a certain distance away from the source. This also means that, as the target moves further away from the light source, the foot candle measurement decreases while the lumen output stays the same.


This distinction is important, because it means that you can’t use the same light fixture for your 8-foot office ceilings and your 40-foot warehouse ceiling and expect the spaces to be equally bright. However, it’s also worth mentioning that you may not want all of your spaces to be equally bright in the first place, as not all applications require abundant illumination.


To figure out what light level is appropriate for your application, you can view sample foot candle guidelines here. You can measure foot candles by purchasing a light meter (or have us conduct a Lighting Discovery at your facility for free).


So how does this translate to the lumen rating, and how do you determine which fixture is appropriate for your application? Unfortunately, the lumen output doesn’t tell the whole story, because the same amount of lumens could be either concentrated in one area or spread out depending upon the fixture. It takes an experienced lighting designer working with design software to make sure your space has the light level and spread appropriate for your application. The good news is that we can provide illumination models to businesses considering LED upgrades.


Color Temperature


Color temperature is also important when considering how the look and feel of your facility will be affected by a lighting upgrade. Color temperature is measured in Kelvin, a unit of temperature based on the Kelvin scale. When measuring the color temperature of lighting, all you need to know about this scale is that you aren’t likely to see many color temperature values below 1500 K or above 7500 K. Lower values equate to warmer “yellow” light, while higher values equate to cooler “blue” light. For reference, a typical incandescent light is around 2500 K.

 

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By Poke2001 (Own work) [CC BY-SA 4.0], via Wikimedia Commons

 

LED lights come in a variety of colors, and again, the proper color depends on the application. Indoors, warmer light will have a more pleasing feel than cooler, “clinical” light. Outdoors, cooler lighting will provide better illumination and appear brighter and more inviting. This is because LED light fixtures mimic sunlight, which has a color temperature of around 5500-6000 K. If you don’t believe us, check out this before-and-after aerial video of a Cree exterior lighting retrofit. However, with some cheaper LED fixtures, cooler lighting could actually appear blue instead of giving off a crisp, white light, which is not attractive at all.

 

In fact, two LED fixtures with the same color temperature rating could look slightly different. This is because LED color temperatures are determined by “mixing” different colors of light within one diode, and some manufacturers are better at perfecting this mix than others. For example, Cree’s touts its TrueWhite technology for enabling more precise color management and greater color consistency over the life of its fixtures. For this reason, a side-by-side sample comparison is a good way to figure out whether a particular light fixture’s color temperature will work in (or outside of) your facility.


CRI


The color rendering index, or CRI, is a measurement of the ability of a light source to render the true colors of an object. The standard for comparison is an “ideal” or natural light source such as daylight, and the highest possible CRI value is 100. LEDs benefit from having high CRI values compared with other light sources, typically 80+, but CRI values can vary significantly between LED manufacturers. In short, the CRI is another value worth checking on lighting spec sheets, especially if color rendering is important to you. This is especially true for retail spaces and showrooms, but could apply to a number of other spaces.


To sum up, be sure to take a look at the lumen output, lumens per watt (LPW), color temperature, and CRI on the spec sheets for any LED fixtures you may be considering. Higher LPW and CRI values are best, while lumen output and color temperature depend on your particular space and application. Also, be sure to find out how much illumination in foot candles your application requires. And while spec sheets are useful, sample fixtures are even better, so ask your lighting installer to demo possible fixtures for your space.


Still confused about what values result in the best light for your space? Just reach out to us or call our office at 504-586-0625 for more information, or to schedule a complimentary Lighting Discovery. Our experts are happy to answer any questions you may have and point you toward the LED lighting solutions that are best for you.

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