Some reasons for light loss

I've done some digging around and found several possible reasons we may
need to replace our fluorescent lamps more often than usual. The rated
lumen depreciation of fluorescents is about 10 percent after 3,000 hours of
operation and 20 percent after 7,000 hours, but several of us have found a
20 percent drop-off after only two months of use. Below, there is a summary
followed by an elaboration.

In part, what follows is a synthesis of what several others have said. I
hope I've added some interesting information. My data came from "The
Lighting Management Handbook" by Craig DiLouie; some of his charts come
from the U.S. Department of Energy.

Keep temperatures around lamps near 77F. Go for "rapid start electronic

Long version
1. Temperature. Heat may be the main reason our lamps don't last as long.
The light output of fluorescents peaks at 77F. Higher temps greatly reduce
lumen output and lamp life, with a 10 percent loss from peak at 100F. The
lesson here seems to be to try to keep our lamps as close to 77F as

High-output lamps seem more sensitive to higher temperatures as their light
ouput peaks at 60F. At 100F, their light output is down almost 20 percent
from peak.

2. On-off periods. Fluorescents are usually rated on a cycle of 3 hours on
and 20 minutes off. They should last longer as the on period gets longer.
In fact, leaving some lamps on continuously can double their rated life!

3. Lamp/ballast combination. For best lumen output and lamp life, the lamp
needs to be matched to its ballast. Ballast factor is one thing to consider
but there are others, including the quality of current the ballast

"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; electronic ballasts can have a bf of more than
100 percent.

Look for "rapid start electronic" ballasts. Rapid start refers to the type
of circuitry in the ballast. Pre-heat circuitry, which requires a starter,
represents obsolete technology. Instant start circuitry lights lamps with a
burst of voltage and the jolt takes its toll. Rapid start circuitry keeps
the cathode in the lamp warm all the time so it can start the lamp quickly
and more smoothly.

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.

It seems that there are electronic ballasts for lamps other than T8s. They
are hard to find although one list member found one recently.

4. The lamp itself. Different lamps of course are designed differently. It
would be interesting to get information about the rated life and lumen
depreciation under standard testing conditions of the aquatic plant lamps
we use.

Greg. Tong
San Francisco, CA, USA
gtong at sirius_com

"Every infinity is composed of only two halves."