Hi everyone! As readers of this blog know, helping our customers use energy wisely is one of our highest priorities. But it isn’t the only priority – safety is tops, as well as other considerations that are important to your business. Effective lighting, of course, plays a big role in all of them.
I want to share some specific things to consider when retrofitting lighting at your facility, especially if you’re thinking about changing out metal halide lighting for CFLs. The results of this swap out could impact your operations in some undesirable ways. But first, it’s probably worthwhile to review some aspects of lighting that are valuable to this conversation:
- The light output of a lamp is measured in a physical quantity called lumens (L), which the manufacturer will identify.
- Where the facility needs light for the task at hand – places like floors and desks – is called the working surface.
- Light at a working surface is quantified in foot candles (FC), measured with a light meter.
- The height between the lamp fixture and the working surface is measured in feet.
- The foot candles at the working surface can be found by taking the lumens of a lamp and dividing this number by the distance from lamp to working surface in feet squared. This formula is called the “Inverse Square Law.”
Here’s another way to look at it:
Foot candles at desired surface = Lumen output of lamp / Distance in feet squared
As most of you are aware, when replacing an incandescent lamp with a CFL, you typically can reduce the wattage by about 75 percent and still get the same light level at the working surface. That’s because the lumen output of a 100-watt (for example) incandescent lamp is equal to around 1,700 to 1,750 lumens, while the lumen output of a 29-watt CFL is about 1,750 lumens. For this nice energy reduction, we get the same light at the working surface because their lumen outputs are very close. So far, if we’re swapping incandescent lamps for CFLs, it’s a great idea. However, if you chose to swap CFLs for metal halide lamps, the same 75 percent energy reduction rule will not work. Here’s why:
- A 300-watt metal halide bulb will produce around 19,000 lumens.
- A 400-watt metal halide – depending on if has pulse initiation – will produce somewhere between 23,500 and 33,000 lumens.
Let’s use the inverse square law on another example. We’ll consider a facility that is a warehouse with forklifts constantly pulling products off of shelves for distribution. The 
height of the current fixtures is 25 feet, and we’ll use the 300-watt metal halides to make this a worst-case scenario. Using the inverse square law of 19,000 lumens divided by 25 feet squared gives us 30.4 foot candles at the floor. Now let’s swap these out for the 7,000-lumen CFL lamps. The new foot candle reading would be 11.2 foot candles. This reduction would be very apparent to the operators – and may be a safety issue. Fortunately, recommended lighting levels for warehouses can serve as a guide:
- Warehouses that are totally storage-focused, with very little activity, are recommended to be lighted at 5 foot candles.
- Warehouses with very large items (ship containers, large crates, etc.) that are rarely moved are recommended to be at 10 foot candles.
- All other warehouses where heavy activity such as shipping and receiving takes place, or ones that move a lot of products, should be at least 30 foot candles.
The bottom line is that while pursuing energy savings is outstanding, CFLs may not actually be the best option in all scenarios. But by knowing the characteristics of your facility, as well as the inverse square law, you should have the information you need to make sure the lighting in your spaces is appropriate – with safety as the top priority and let’s not forget about employee productivity and efficiency. And, for these and other questions you might have about the best lighting for your business, we’re here to help.
–Mark Roche CEM, BEP, CEP, CEA
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