I remember someone telling me once that the compact fluorescent (CFL) twisty bulb was “sexy.” They were convinced the product would have market appeal. I have yet to meet someone who actually likes the light that comes from CFL bulbs. Now, LEDs are another thing entirely. They do have market appeal and we can see that manufacturers are working hard to develop products that capture consumer interest at prices that make us buy.
The efficacy, controllability, and light quality of LEDs make them a no brainer for new lighting and replacement projects. But beware – these babies are bright and getting brighter. For new construction in commercial applications, the code is no longer a useful metric for lighting efficiency. I’ve blogged on this before and I wish I knew who was influencing the code minimums to be so out of whack with what is achievable with current technology. One of the results of code being out of alignment with the available technology is that spaces are being severely over lit. Yes, the installed LPD might be 25% better than code, but with LEDs that is likely 50% more light than is needed in most spaces. So we are putting in more light and more watts than we need and calling it efficient because it’s better than a code that seems to be based on technology that has not been in use for two generations of lighting products.
Image by Flickr user Bokoko.
“It’s on a dimmer.” “We can control it.” “You can cap the output.” These are all responses I hear when I raise the over lighting concern. Well, sure, we can, but the reality is controls get overridden, they don’t get programmed, they fail, and when those things occur, there will be too much light. The rampant over lighting I’m seeing is akin to the tendency to oversize HVAC equipment. But with lighting it seems almost like the designers are kids in a candy store. Five years ago, their only options were a relatively limited variety of fluorescent fixtures. Now there are so many cool, beautiful, artful, useful and/or ingenious LED lighting products that lighting design is fun again. And, manufacturers want to move these products. When vendors or sales engineers help you with a layout, just cross check your foot-candle levels with what you know works.
How To Avoid Over Lighting
For example, I just looked at two options for lighting a 10x12 office with an 8’ ceiling.
Option 1 uses four 2-lamp T-8-17 lamps per 2x2 direct/indirect recessed fixture. This approach results in 4 fixtures, 22FC and 1 watt per square foot. This design is 10% better than code (I intentionally did not select the most efficacious fixture on the market but rather one I’ve seen in common use for office lighting.)
Option 2 uses 3 2x2 LED fixtures with a similar style. This approach results in 3 fixtures, 25 FC and .5 watts per square foot.
How Many Foot Candles of Light Should an Office Have?
The IESNA recommends a 30FC desktop target for non-reflective computer viewing.
What I see are lighting plans with 4 of the LED fixtures which will deliver an average of 34 FC in the space and come in at 0.66 w/sq ft. But why? The three fixture solution can provide more light than would have been achieved in the base-case fluorescent design. The light level at the desk top is likely well above the room average. In addition, a task ambient approach would likely make the most sense. Use a target of 20 FC average ambient and provide people with task lights for when they need more illumination.
Image by Flickr user Kendra.
Why Over Lighting is a Problem
Over lighting costs more money to install than right lighting. Three fixtures per office instead of four per office times the number of offices yields real money. Over illumination provides a poor visual environment and requires controls which can fail. Over lighting connects more load than necessary to the grid resulting in higher system peaks and higher costs for everyone. While the total lighting load is going down, why not drive it as low as we can during the LED revolution?
This is an exciting time for lighting. LED prices have come down. The product quality is good and a massive, nationwide lighting retrofit could cut commercial building power consumption by 200 billion kWh!
Let’s not squander the opportunity to make this change right by not taking full advantage of the efficacy of LED lighting. Design it right and don’t over light!