# 75 w light thats 300 w bright! how do these flourescent bulbs work?

Discussion in 'Electronic Basics' started by max slomoff, Feb 12, 2004.

1. ### max slomoffGuest

hi,

you always see the claim that these flourescent lights are really
bright but only use a fraction of the wattage.

obviously the claim that a light is 300w bright if it only uses 75w is
an approximation. how accurate is it?

how do these bulbs work?

and, if you have 2 150w bulbs shining on the same subject is the
brightness exactly equal to that of 1 300w bulb?

thanks for the info,
max

2. ### CCGuest

Hi Max,

It's all radiation. Incandescent lamps emit visible light as well as a
lot of unseen light in the Ultra Violet and Infrared range. The is why
you see mostly frosted lights in a house. The coating changes some of
the Ultra Violet into visible light.

phosphors that convert the light into something visible.

I think the efficiencies are found with the florescent lamps emitting
less infrared (i.e. heat)

Hope that helps.

Chris

3. ### w_tomGuest

Incandescent bulbs are on the order of single digit
efficient if the only useful energy radiated is light - as
measured by lumens. Fluorescent bulbs do light creation with
about 5 times more efficiency. How fluorescent works is
summarized by the other post. Notice how inefficient an
incandescent bulb really is.

That '300 watts bright' is written so that they can make a
numerical claim without typical human eyes 'glazing over'.
Would you notice if they said 300 lumens? Most humans in a
technical society simply and still don't comprehend simple
numbers unless those numbers are 'dumbed down'.

4. ### Bob MyersGuest

Not so. A "frosted" incandescent bulb is most often just
that - the inner surface of the glass has been roughened slightly
to better diffuse the light, so you're not looking straight at the
glare of the filament. The filament itself has relatively little emission
in the UV range (practically none), much more in the IR. The
problem is that incandescence isn't a particularly efficient means of
producing visible light. The "300W of light from a 75W bulb" is actually
the same light out of THIS bulb (fluorescent) which only consumes
75W, as you do from an INCANDESCENT consuming 300W."
The bottom line is that most of that 300W in the incandescent is
just wasted, as heat.

Bob M.

5. ### Christopher.CoxGuest

Interesting info Bob,

While I always considered the problem with incandescent lamps as
excessive IR emission, I could have swore I read somewhere that frosted
lamps where more efficient than none frosted. Simply diffusing the light
certainly would not accomplish this.

"Incandescent light bulbs give off most of their energy in the form of
heat-carrying infrared light photons -- only about 10 percent of the
light produced is in the visible spectrum. This wastes a lot of
electricity."

Leaves allot to be improved on....

Regards

Chris

6. ### Don KlipsteinGuest

Go where there are clear and frosted versions of the same brand, same
wattage and same life expectancy. Usually clear and "standard frost" are
the same in rated light output and "Soft White" is maybe 2.5% less than
clear and "standard frost".

- Don Klipstein ()

7. ### Don KlipsteinGuest

No, generally a 300W of the same life expectancy and filament style
produces slightly more than twice the light of a 150W.

Look at the lumen ratings of various wattages.

Lower wattages are less efficient for three reasons:

1. In higher wattage bulbs, a greater percentage of lighting cost is
electricity cost and a lower percentage is bulb replacement cost. Because
of this, it pays to run the higher wattage filament filaments hotter for
greater efficiency despite lower life expectancy.

2. In higher wattage bulbs the filament is thicker, and at a given
temperature within a given time a smaller percentage of the thicker
filament evaporates. The thicker filament can be run hotter for the same
life expectancy.

3. In gas-filled bulbs, the heat conduction from the filament by the gas
is surprisingly proportional to the visibly apparent filament length and
less than proportional to the visibly apparent overall filament diameter.
A wider filament has a thicker "boundary layer" of hot gas around it and a
correspondingly lower temperature gradient. With a thicker/wider
filament, a smaller percentage of the energy going into the bulb becomes
heat conducted from the filament by the gas.
In fact, below a certain "wattage per centimeter" of overall filament
length of maybe roughly 8-10 w/cm or so, they use a vacuum rather than a
gas because the gas hurts more than it helps. (When the gas is used, it
slows down filament evaporation by having gas atoms "bounce" evaporated
tungsten atoms back onto the filament to permit a higher filament
temperature for a given evaporation rate.)

- Don Klipstein ()