Fluorescent "design lumens"
I looked up the lumen output of fluroescents in my Philips Lighting book
and found that individual T8 and T12 lamps give off similar amounts of
light. Here's some info for comparable 48" lamps: A T8 gives off 2600
"design lumens" versus a T12's 2520 design lumens. Of course, the T8 uses
fewer watts--32 watts vs 40 watts. This suggests that a one-for-one
replacement of T12s with T8s won't necessarily increase your lighting but
will lower your electricity bill.
Karen mentioned that fluorescents have a "burn in" period. That's what
"design lumens" alludes to. When you first light up a fluorescent, they
burn more brightly than after they've settled in. The specs usually quote
both initial lumens and design lumens. (In an earlier post, I covered how
design lumens are determined. Usually, 77F and no shorter than a 3 hour
If you compare lux (or foot candle) readings at the time you first light up
a fluorescent and a month or two later, you will capture the drop after the
burn-in period. This can easily be 10 to 20 percent. After the burn-in
period, lumens should depreciate at a much slower rate of around 20 percent
after 7,000 hours of use.
This suggests we design our lamp fixtures for the design lumens and not the
initial lumens. If we depend on the brighter burn-in period, we could save
money and effort by installing more lamps instead of replacing them as
San Francisco, CA, USA
gtong at sirius_com
"Every infinity is composed of only two halves."