What Your Light Levels Mean

10/01/2014 | By Janelle Penny

How to measure and interpret indoor light levels

A light meter will do much of the heavy lifting in measuring and interpreting workplace light levels. Compare the results with footcandle requirements to determine if illumination is adequate.

Are your occupants complaining about headaches from brightly lit cubicles or tired eyes from a dim interior office?

Before you start looking at retrofits, investigate in-house by measuring light levels. You can get some of the diagnostic and fine-tuning work out of the way yourself with a handful of small devices and some investigation.

Tools of the Trade
You’ll only need a few instruments to start interpreting workplace light levels, but they’re important ones. The starring role will be played by a light meter, which measures the footcandle level on either a horizontal plane (the top of a desk) or a vertical one (the wall). Smartphone apps purporting to measure light levels are available, but they’re not as reliable, cautions Kristin Fedoruk, director of energy sales and solutions for Juno Lighting Group, a lighting fixture manufacturer.

“Apps are only as good as the camera on your device. I could have three different apps to check footcandle levels and they’ll give me three separate results that could be significantly different from each other,” explains Fedoruk. “Sales reps for most reputable lighting manufacturers have footcandle meters, and so do most distributors and wholesalers.”

Corporate specifications are necessary if your company has them, Fedoruk says: “I’ve worked with industrial companies that have their own specifications as to what light levels are needed and the allowable lighting power density. Make sure you’re following what corporate wants internally.”

If your organization doesn’t have this data, look instead at local building codes and the footcandle guidelines from the Illuminating Engineering Society (IES). Compare them with your findings about what tasks are performed in each space to make sure light levels are consistent with what’s typically needed.

Bring a camera and binoculars to help you remember what you see, Fedoruk recommends. This is especially helpful in spaces with tall ceilings – from the ground, a fixture may look like it’s relatively flat with the ceiling, but it could actually be sitting several feet below the ceiling.

Temperature measurements can also be useful, especially in spaces like warehouses where temperature is often poorly controlled. Light levels that deviate from what’s needed may be doing so because you’re using the wrong fixture – for example, the top of a warehouse ceiling could reach up to 130-140 degrees F. when it’s 80 degrees F. outside, so a heat-sensitive fixture could be damaged by the heat.

Best Practices
Fedoruk recommends three rules of thumb to maintain optimal light levels throughout your facility:

1) Understand what’s in your plenum ceiling. After you compare your measured light levels to your corporate specifications or the IES recommendations, Fedoruk recommends examining the lamps and ballasts already in place to see how they’re currently utilized.

“There are different ballasts that can push more or fewer lumens out of that bulb,” says Fedoruk. “You don’t know how much wattage you’re using out of that fixture unless you can find out what lamp and ballast it’s utilizing. There are specs showing how much energy they’ll draw.”

2) Keep your equipment in good shape. “The light meter should be recalibrated at least once a year because of being dropping or left in your car,” Fedoruk recommends. “Make it as accurate as possible. When you measure something, always use the same light meter – don’t measure one day with one person’s light meter and then after your renovation come back with a different one.”

3) Know what the range in the IES guidelines means. “Never go for fewer footcandles than the IES requirements state,” Fedoruk says. “They recommend a low number and a high number – the low one is the minimum you can have for someone who has the best eyesight and the higher level is for someone who is older or has a more difficult task that requires more light. In one building alone, you might have seven or eight different types of light levels – for example, an open office might have 30 footcandles, but the lobby might only need 10.”

Janelle Penny janelle.penny@buildings.com is senior editor of BUILDINGS.

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