Fluorescent lighting and incandescents

There’s a lot of talk about Compact fluorescent Lights (CFLs) in news and politics. Lighting is a topic in engineering that I enjoy. The #1 reason people look to CFLs is the energy savings. Rightfully so, there are energy savings to be had, but not as much as advertised.

The common measurement of lighting efficiency is lumens per watt. Actually, that’s efficacy. Efficiency is unitless. Below is a table of various lighting technologies and their associated efficacies: (higher numbers are better)

Type of light source Color Luminous effectiveness in lumens per watt
Low Pressure Sodium   (LPS/SOX) yellow/amber 80 – 200
High Pressure Sodium   (HPS/SON) pink/amber-white 90 – 130
Metal Halide bluish-white/white 60 -120
Mercury-Vapour blue-greenish white 13 – 48
Incandescent yellow/white 8 – 25

source: http://en.wikipedia.org/wiki/Light_pollution

There is no question about the compact fluorescent’s luminous efficacy. The general range is 50-70 lumens/watt (http://www.energyfederation.org/consumer/default.php/cPath/25_44_784). The best household incandescents can do 20 lumens/watt.

Doing a real world comparison, the 13 watt CFL that is typically found in stores never seems to be as bright as a 60 watt bulb. Its using 21.67% of the power. There are always games that can be played with lumens and color temperatures. From observation, the 23 watt CFL (100 watt replacement bulb) is nearly identical in brightness to a 75 watt incandescent bulb. That’s still using only 30.67% of the power of the incandescent. For saving energy it’s a really good deal, considering how long they last and how much energy they save. The higher cost of the CFL bulb will be offset by these savings.

That said, there is a large list of uses for CFL’s that I would recommend against:

1. It wet locations. This includes bathrooms with showers. Not all CFLs are created equally, but I stopped putting them in the bathroom as moist air messes them up.

2. In rough service conditions. These bulbs present an environmental hazard (mercury) when broken. The mercury isn’t a problem if disposed of properly. I recommend a specific rough service light for these locations.

3. Vibration prone locations. Next to a furnace, evaporative cooler, washing machine; a CFL will lead a shortened life. This makes the higher cost of the bulb not worth it.  Incandescents last longer in these conditions. The electronics in the CFLs tend to be the issue when located in vibration prone locations. Integrated ballasted fixtures can help this problem, but that requires changing out the current fixture.

4. Dimmable fixtures. Incandescents, just plain dim better, (as they function as a “blackbody radiator”) and provide for better mood lighting when doing so. CFLs keep the same color temperature when dimmed. Only a “dimmable” CFL may be used in these fixtures. These lights are expensive, at least for the time being.

5. In the cold. CFL’s have improved greatly in this area, but 0 degrees F still serves as a limit on these lights. Incandescents work fine in the cold.

6. On-off-on conditions. CFLs take time to warm up and are not designed for locations where turning off and on the lights repeatedly is common. This is another reason why I refrain from using CFLs in bathrooms.

7. Hot locations. The heat can damage the electrical ballast components of a CFL, reducing its life.

Keep in mind CFLs save quite a bit during the summer in the form of reduced air conditioning loads. Then again, most of the summer is daylight, so I don’t use lights as much. In the winter, the extra heat from incandescents warms the house. The only loss there is the cost difference between electricity and your heating source; natural gas in my case. Electric heat in my area costs about twice as much as natural has heat for an 80% efficiency furnace. In that effect, using CFLs in the winter will save me only 33% on electricity costs when I get to consider the heat gained from incandescents less the difference of the cheaper natural gas fuel. Another problem of CFLs is that they decrease in efficacy over their life. What once was 60 lumens/watt can decrease down to 80% (48 lumens/watt) of that by the end of its life, with much of that decrease in the beginning of the lights life.

This doesn’t mean that I am anti-CFLs. I like the technology, and I use them in my house. The main floor lights; where I am at are either T-8 tube fluorescent of CFL’s. Those lights are my main use lights, not located next to any vibration sources, or moisture sources. The fixtures are simple with no dimmable features. Not too hot, not too cold in color temperature. The two fixtures, one a T-8 fluorescent, the other using CFL’s account for the majority of my light usage.

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