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Possible to run fluorescent on lower current?

Discussion in 'Lighting' started by Earl Kiosterud, May 10, 2008.

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  1. Hi folks,

    This is tangential to my post about an internally lighted sign, in which Dean and Douglas
    are helping. It appears that we're going to be able to use only about two 48" fluorescent
    tubes to not go over the city ordinance for maximum light. This is all very rough numbers
    at at this stage. But this sign will be about 4' x 8', and about 9" deep, making it hard to
    spread the light out uniformly from only two such lamps.

    I divide the world into two types, politicians and scientists. The politician in me says
    "don't worry about it, just use multiple tubes to get the even light, and put filters in to
    eat up the extra light." But I don't like that side of me. The scientist in me says to try
    to do it efficiently.

    So I wonder if we can use more tubes, and run them at lower current. It would seem that
    some extra inductance in the ballast would give us less current. I think I've read in Don
    Klipstein's pages that fluorescents can fail prematurely in such conditions, but I can't
    find the reference. Any thoughts on running fluorescent tubes at lower current? Bad idea?
    If not, how to go about it.
  2. Another idea: Get a bunch of the 25 watt or whatever is the
    most-reduced-power version of F32T8. Get a ballast rated to handle
    perhaps 4 of them with ballast factor on the low side. You should be able
    to get even light without having to "eat" as much light, and use not too
    much more power than with two "more normal" 4-footers.

    One more idea: Add diffusing filters to make the light more even. You
    probably don't need to worry if you add a bit of loss here if you combine
    this with upping the lamp count.

    - Don Klipstein ()
  3. TKM

    TKM Guest

    It's certainly possible to run the fluorescent lamps at lower current; but,
    yes, do use enough lamps to light the sign faces uniformly. For whatever
    lamps you decide to use, look in the ballast catalogs for the "Ballast
    Factor" A BF=1 means the ballast will operate the lamp at full rated current
    and therefore rated light output (lumens). A BF=0.9 means the lamps would
    operate at 90% of rated value. Ballasts are made in several ballast-factor
    versions these days.

    You can also use circular lamps too, of course, if they match the geometry
    of the sign better.

    Here's the caution: fluorescent HO lamps and sign ballasts will start and
    operate the lamps in cold weather. But, if your sign is located where it
    gets cold, operating the lamps at less than rated current also means less
    than rated watts and therefore less heat to keep the lamps warm. I've seen
    some very dim transilluminated plastic signs on cold winter nights.

    For the sign faces, use a good-quality sign plastic such as Plexiglas(R)
    which is acrylic. The sign versions are UV stabalized, diffuse the light
    nicely with minimum losses and there are lower transmission versions
    available too. As a rule-of-thumb, the distance between lamps in a sign can
    be no greater than 1.5 times the distance between the sign face and the
    surface of the lamps if you want even sign face illumination. Also, as a
    rule-of-thumb, the optical efficiency of the sign "box" assuming a double
    faced sign is about 85% if the interior is painted white. That means 85% of
    the lumens generated by the lamps will reach the sign faces (on the inside).

    What about the sign content? If you have a highly-decorated sign with lots
    of lettering, it's average light output will be less than a relatively plain
    sign. That's also the way to reduce the light output of the sign -- add
    more decoration in the form of a painted surround or details. However, I
    think your first priority is to make the sign visible and comfortable for
    whomever you want to see it and then worry about meeting the sign ordinance.
    The city people are really telling you that they want to see a readable
    sign, not a glaring lighting fixture. From a light pollution standpoint,
    light lettering with a dark background is far better than dark lettering
    against a light background and the sign message is just as visible.

    If you have access to a technical library, find a copy of the Illuminating
    Engineering Society Handbook, 8th. Edition or earlier (not the current 9th.
    Edition). There's a chapter on sign design with all of the design

    Terry McGowan
  4. Terry,

    I suspect that white lettering on a black background is not an option from what little I
    know of the details of this project. I agree that it looks better that way.

    Your 85% figure I'm presuming is just losses inside the box, not counting transmission loss
    in the face, and before the lettering is added.

    The ballast factor I presume relates to Watts. A 40-Watt tube with a ballast factor of 0.9
    would be 36 Watts, etc., and we'd still use a 40 Watt tube.

    Thanks so much for some useful info and good tips. I don't know much of the details of the
    sign. It's Pete's first illuminated sign, and I got involved to help with the light issues,
    partly because I've always needed to take the time to understand lumens, candelas,
    foot-candles, etc.. I'll pass all this on to him.

    I'm still trying to understand why 34,400 lumens doesn't work out to 4.38 foot-candles at 25
    feet. I have that in another post.
  5. If you have access to a technical library, find a copy of the Illuminating
    The "Lighting for Advertising" chapter of the Handbook was combined with
    the chapter on "Retail Lighting" in the 9th edition, so the sign stuff is
    still available in the latest edition, it's just the second half of the
    retail lighting chapter.
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