Following up on yesterday’s post, efficiency pitfall number one: age.
An incandescent or halogen light bulb is a fairly closed system. It can be stuck in a socket and what you see is what you get. A 100 watt light bulb is going to be pretty consistent over time. There will be some decrease in the light output as the tungsten filament gets burned up and bits of it get stuck to the inside of the glass envelope, darkening the glass. However, for the most part the lamp will burn out and the filament break before the decrease in light output becomes much of a factor. Tungsten-halogen lamps do slightly better, since the halogen gas inside the bulb makes the tungsten that gets burned off reattach to the filament and basically “recycle” itself, but even though the lamp will last longer and suffer from less darkening it will still break before those changes tip the efficiency equation too much.
Unfortunately, the same cannot be said for the various discharge sources like fluorescent, compact fluorescent, or the various HID types. While a filament does exist in these lamps, the real work of making light is done by the gas discharge. (Check some of my previous posts about fluorescent lighting for a summary of how it works.) The light bulb does not just “burn out,” but instead slowly degrades over time putting out less and less light.
This means that the efficiency of a fluorescent or compact fluorescent decreases over time if you are just using the lumens per watt calculation. For example, the day you install a brand new CFL 26 watt twist self-ballasted lamp you’ll probably get about 1,700 lumens. However, since the output of fluorescent lamps decreases over time, manufacturers also publish the “mean lumens,” which is the lumen output at about 40% of the lifespan of the lamp, which in our example case is 1,365 lumens. So the efficiency has dropped from 65 LPW when brand new to 52 LPW. The lumen output will continue to drop until the lamp reaches the end of its life.
Granted, the lower efficiency of 52 LPW is still much better than the 17.5 LPW of the 100W lamp we’ve replaced (see yesterday’s example). However, remember that you also have 20% less light. So after a while, the new CFL lamp isn’t providing the amount of light you used to have with the old incandescent lamp. This is the point when some people give up and go back to the old lamp, or they add a reading lamp using another incandescent lamp.
Professionals try to counterbalance the lumen decrease by working depreciation factors into the system when they are planning out what lamps to use in a project. We will intentionally over-light a space from day one knowing that eventually the lamps will decrease in output to the level we want for the longer term. The first 100 hours of a CFL or fluorescent is when the decrease is the most rapid, which is called “seasoning.”
As another age related problem, fluorescents don’t necessarily “burn-out” like incandescents and simply stop working. Sometimes, often times, they will just continue to get darker and darker and darker and don’t simply stop turning on. This is why professionals recommend having a maintenance schedule for replacing your fluorescent lamps based on time, not waiting for the lamps to stop working. (What? No one told you this? Yes, if you are going to use CFLs you should keep track of how old the lamps are replace them on a schedule. And yes, it’s much more of a hassle than just waiting for the bulbs to stop working.)
