Fluorescent Lamps and Ballasts
by Ian Philips

There has recently been a lot of discussion on fluorescent lighting and T8 vs. T12 bulbs. I thought that a summary of the discussion would be of value. I have distilled many postings down to a unified article.

The bottom line was that people who have tried both types of lamps prefer:
         T8 bulbs in combination with electronic rapid start ballast.
The benefits being:
         Longer bulb life with lower lumens drop off.
         Significant power ($) savings.
         Cheaper bulbs.

  1. Differences between T8 and T12 lamps
  2. Relative amount of light
  3. Lumens depreciation
  4. Design lumens
  5. Ballasts types
  6. Lamp life
  7. Shop lights
  8. Converting shop lights
  9. Costs
  10. References
     

Differences between T8 and T12 lamps
T8's are 1" in diameter, T12's are 1 1/2". T8's run on an electronic ballast, and will not run (efficiently) in a fixture with a regular ballast. The same length of lamp also runs on less electricity, a four foot T8 is 32 watts, while the four foot T12 is 40 watts.

The commercial lighting industry is rapidly shifting towards the more energy efficient T8 bulbs. The driving force in the US is the new US Federal Minimum Energy Standards for fluorescent lights. The old F40T12CW cool whites will be a thing of the past.

Relative amount of light
Bulb for bulb, T8s give the same amount of light, or possibly slightly more, as equivalent T12s. According to the "Philips Lighting Guide to Fluorescent Lamps" for comparable 48" lamps, a T8 produces 2600 "design lumens" versus a T12's 2520 "design lumens". However, the T8 does this using 32 watts versus the T12's 40 watts. So while a one-for-one replacement of T12s with T8s
won't necessarily increase your lighting it will lower your electricity bill.

Lumens depreciation
All fluorescent bulbs dim as they get older. The rated lumen depreciation of T12 fluorescents is about 10 percent after 3,000-4,000 hours of operation and 20 percent after 7,000 hours, while for T8 lamps, running off electronic ballasts, the loss is even slower, 10% total after about 7,000 hours of use. That's a two-year period if the lights are on 10 hours a day. One reason for the T8's lower lumens depreciation is the use of the electronic ballasts. It is reported that conventional ballasts cause increased lumens depreciation.

Some people are suspicious of the lumen depreciation because they don't see commercial places replacing their banks of lights every six to twelve months. The reply to this is that the human eye is a very poor judge of brightness. As long as the ambient light level is adequate for whatever activity is taking place, the lighting is "fine". Most homes or offices don't replace bulbs until they burn out or start to flicker, while some places do perform regular "re-lamping".

Design lumens
All fluorescents have a "burn-in" period, during which they are much brighter. This burn-in period isn't exact, but their lumens decrease at a high rate for a short time, and then settle in to a steady, lower rate. Fluorescent specifications normally give both "initial lumens" and "design lumens". Thus side-by-side comparisons of lamps should be made after they have passed their
"burn-in" period.

Ballasts types
The Time-Life book "How Things Work in Your Home (and What to Do When They Don't)", contains one of the better descriptions of fluorescent lights. It explains that there are three types of starting technology.

  1. Preheat where an auxiliary starter is used to heat the tube ends and strike an arc across the tube.

  2. Rapid start where a special ballast applies a relatively low voltage directly to special, quick-heating cathodes (tube ends).

  3. Instant start where the ballast supplies a voltage spike of a magnitude up to 4
    times the normal operating voltage of the bulb.

Another factor affecting lumens and power consumption is the "ballast factor". "The ability of a ballast to cause a lamp to produce a percentage of its initial rated lumens is called its ballast factor." Magnetic ballasts have a bf less than 100 percent while electronic ballasts can have a bf of more than 100 percent. This is another reason why T8 / electronic ballast combinations give more light for less power.

Most aquarium hoods are of the preheat type. Shop lights and most 40W systems are of the rapid start type, which Time-Life says requires special cathodes in the bulbs. Bulbs with single pins are instant start.

Lamp life
According to the Valmont Ballast technical guide, short lamp life (which may be different than reduced lumens) is attributed to:

  1. Improper voltage

  2. Improper wiring

  3. Poor lamp to lamp holder contact

  4. Extremely short duration cycles (many lamp starts per day)

  5. Defective starters (applies only to preheat)

  6. Defective lamps

  7. Improper ballast application

  8. Defective ballast

In the "Philips Lighting, Guide to Fluorescent Lamps" there is a graph that shows average life for a variety of different fluorescents based on burn hours per start. Double the bulb life expectancy (in thousands of hours) is possible with a 12 hour burn cycle as with a 4 hour burn cycle. Fluorescents are usually rated on a cycle of 3 hours on and 20 minutes off.

The following is a quote from the same text: "Longest lamp life and best *lumen maintenance* (our emphasis) will be obtained with 1) good voltage, 2) proper auxiliaries, and favorable operating conditions such as are ordinarily encountered in a well-designed lighting system".

"The Lighting Management Handbook" by Craig DiLouie gives additional information. Operating temperature affects fluorescents. The light output of fluorescents peaks at 77F. Higher temperatures greatly reduce lumen output and lamp life, with a 10% loss from peak at 100F. The lesson here seems to be to try to keep our lamps as close to 77F as possible. High-output
lamps seem more sensitive to higher temperatures as their light output peaks
at 60F. At 100F, their light output is down almost 20% from peak.

Lamp/ballast combination also plays a role. For best lumen output and lamp life, the lamp needs to be matched to its ballast. With all the different types of ballasts and lamps, who really knows whether their combination is optimal. Note that there are instant start and rapid start circuits for
both electronic and magnetic ballasts. An instant start on an electronic ballast can cut lamp life in half.

Shop lights
The experience on "the net" with the inexpensive shop lights has not been good. The fixtures are not especially long lived, and even worse, they appear to burn out bulbs faster than normal. This was reported by two people who had shop lights burn out tritons and vitalites in less than one year. It has also been reported that bulbs in shop lights do not produce as much light as
they normally should.

Converting shop lights
Fortunately, converting shop lights to use T8 bulbs is relatively simple. Both types use the same lamp holder. All that is required is to replace the ballast in the shop light with a T8 rapid start ballast.

Most fixtures are held together with screws. Just remove the screws. The ballast is in the space above the bulbs if the fixture is right side up.  When you have it upside down to work on it, the ballast is under the reflector. There is normally a schematic drawing right on the ballast, and the
wires are usually the same on the old ballast as on the new one. Mark all connections so you'll know how to put it all back together. That's all there is to it!

Costs
Compared to the price you can get T12 cool white and warm white bulbs for, T8 bulbs are more expensive. However when you compare the specialized spectrum bulbs, T8s are cheaper. Specific prices depend upon where you are. Call suppliers in your area and find out specific prices for bulbs and ballasts.  Another point to consider, while the specialized T8s are a couple of dollars more expensive than the ordinary T12s, due to the cheaper operating costs and longer life, T8s end up saving money in the long run. I leave the calculations to others.

References
The references used by the people on the net include "The Lighting Management Handbook" by Craig DiLouie, a technical guide to retrofitting lights to save energy, the Time-Life book "How Things Work in Your Home (and What to Do When They Don't)", the "Valmont Ballast Technical Guide", and the "Philips Lighting Guide to Fluorescent Lamps". You can find other similar references in your public library.