From: Michael Mutmansky (psu4ever@ix.netcom.com)
Date: 02/11/00-09:33:22 AM Z
Hello all,
As promised, I have some further information on the light output of BL
and BLB lamps, courtesy of Philips. The output makes a rather dramatic
drop in the first 1000 hours, but then the output stabilizes, around 77%
of initial output. From this point onward, the chart shows the output
decreasing in a nearly linear fashion through 7000 hours, where it has
an output of approximately 67%.
The chart only goes through 7000 hours, and most lamps people use are
10,000 hour lamps, some (like mine) are 20,000 hour lamps. It is not
clear from the data whether the curve will stretch out for other lamps,
but I would expect to see a similar curve for any BL and BLB lamp.
This curve closely matches the lumen depreciation curves of traditional
fluorescent lamps, so I don't believe anything too unusual is going on
with the BL and BLB lamps.
Depending on how you use the exposure unit, with new lamps, you will see
exposures increase every time you make an exposure (if you keep the unit
on continuously). Once the lamps have 1000 hours of burn time, the
exposure will become quite stable, and adjustments may not need to be
made except at about 1500 hour increments.
As I originally stated, the output goes through it's most dramatic
changes at the very beginning, so I would recommend "seasoning" the
lamps for at least 100 hours before trying to establish any type of base
exposure times. In the chart from Philips, the curve is still quite
vertical out to about 300 hours, and then curves to a slight downward
straight line by 1000. When the lamps are young, you are going to have
problems getting a reliable base exposure time.
---Michael Mutmansky
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