Posts Tagged ‘Lighting’

Saving Lighting Energy … Scotopically

December 3rd, 2009

Have you ever gone over to your local electrical supply house and bought a new light meter? If you have, it probably cost you somewhere between $100 to $150 … assuming you bought it for use at work. In case you were not aware, those meters measure what is called photopic foot-candles (or lux for those of you that use the metric system). The light receptors in our eyes are made up of cones and rods, but I’m getting ahead of myself.

So what is scotopically enhanced lighting? Lighting is said to be scotopically enhanced if it contains more blue in its spectrum. The added blue content activates a visual response within our eyes that heightens the sensation of brightness and adds to visual clarity. Scotopically enhanced lighting is more like natural daylight than traditional lighting.

Why is scotopically enhanced lighting more energy efficient than traditional lighting? The color of the lighting produces the sensation of brighter space and better visual clarity. Newer lamp technology allows greater efficacy (lumens per watt). Couple this with lamps that have a higher color temperature … most fluorescent lighting used in offices, shops and even warehouses, use lamps with a color temperature of 3500 Kelvin. These are closer to the old “warm white” lamps than are 4100 Kelvin lamps, which more approximate “cool white” lamps.

We can use this to our advantage. By dimming scotopically enhanced lights so they use less energy, we can achieve the same visual perception and visual performance.

So let me take you to a real-world project. The images below are from a warehouse that we retrofitted about five years ago. The first image shows the high pressure sodium (HPS) lighting that was installed when the building was built in the early 1990s. We measured the foot-candles (fc) of the HPS lighting using a standard light meter and it averaged 35  fc. Which system do you think produces more light?HPS_Ltg

T5_Ltg

The retrofitted lighting system utilizes four T5 high output (T5HO) fluorescent lamps per fixture, and the fixtures were replaced one-for-one. Have you determined which system produces “more” light? Using the same meter used to measure the “before” image, the T5HO system averaged 25 fc … 10 fc less than the old system. How can this be?

Remember the meter we purchased when we began this story? It was a photopic meter, right? Photopic light is closer to the yellow part of the color spectrum. So where can you get a meter that measures scotopic light. I am only aware of one company that makes a meter that measures scotopic fc; Solar Light located in Glenside, PA, which is where I bought the meter I have about three years ago. By the way, the lowest price I’ve seen for this meter is about $2,500 here.

However, there is another way of calculating what the scotopic lumens are. Dr. Sam Berman, a pioneer in lighting science, published a chart of scotopic/photopic ratios (sp ratios) for several different lamps. Multiplying the foot-candles readings you take using your photopic light meter by the sp ratio associated with the lamp in the chart that most closely matches the lamp you are using gives you a good approximation of the scotopic foot-candles.

Scotopic_Conversion_Chart

Some lamp manufacturers publish the sp ratios for their products in their lamp specification catalogs and more are beginning to do so each year.

Now let’s go back to the part where I talked about how scotopically enhanced lighting saves energy. Often when retrofitting an older lighting system with scotopically enhanced lighting you will be able to use fewer lamps than what was in the original system. For instance, if you have three-lamp parabolic troffers in your office, you may be able to retrofit the fixtures to a two-lamp configuration. I say retrofit, as you will most likely have to change the position of the lamp holders to avoid the appearance of a missing or burnt-out lamp. If your retrofit doesn’t include replacing the ballast, more of which later, just make sure your ballast can drive two lamps.

What if you are unable to reduce the number of lamps when retrofitting? This is where the discussion about ballasts becomes important. Electronic ballasts were first introduced in the mid 1980s. The first generation electronic ballasts, as so often happens with new technology, enjoyed limited success. Failure rates were fairly high and they were quite costly.

Second generation ballasts solved many of the reliability issues and costs were low enough to cause them to become de rigueur. However, if you needed the capability to dim the lamps you had to purchase special dimming ballasts that were sometimes three of four times the cost of standard electronic ballasts. Enter the third generation electronic ballast.

In actuality, third generation (3G) ballasts are more than electronic; they are actually micro-processor based. Using a special program from the manufacturer or coupled with a compatible control system, you can actually program these ballasts to perform many dimming functions such as personal control, lumen maintenance, daylight harvesting or used to participate in an electric utility demand response program. For our discussion here, I’ll limit this discussion to task dimming.

So you’ve retrofitted your lighting using scotopically enhanced lighting, but you couldn’t reduce the number of lamps in each fixture. Chances are that the scotopic light levels are above the lighting needed for the largest number of tasks performed in the space. With 3G ballasts, you can dim the lighting down to the level necessary, thereby reducing your energy use.

Remember, increased brightness perception + improved visual performance = improved visual efficiency. Scotopically enhanced lighting is energy efficient because it is visually efficient. So what about those cones and rods? We’ll do another post later and get deeper into the science of vision.

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