Re: Lumen Output of Incandescent Bulbs

In article <d4adneRTmaq4ai7bnZ2dnUVZ_sytnZ2d@xxxxxxxxxxx>, Jeff Wisnia wrote:

<EDIT FOR SPACE>

Two times 75 equals 150, right?

<EDIT FOR SPACE>

I got a reply asking me if I'd thought about the difference in light
output between a pair of 75 watt bulbs (1190*2 = 2380 lumens) and a
single 150 watt bulb of the same brand and type (2850 lumens). That's 20
percent more light output from the 150 watt bulb when running at full
voltage. I hadn't, and put a 150 watt bulb in one of a pair of lamps and
compared it's brightness to the two 75 watt bulbs in the other lamp. The
difference was so noticable that I put 150 watt bulbs back in the other
three lamps.

Which brings me to the subject question....

What are the reasons for the differences in lumens per watt output
between the 75 and 150 watt bulbs? All I can think of is:

The filament of the 150 watt bulb is further away from its base, so
there's a longer thermal path along the filament supports to suck heat
away from the filament, and because it's further away from the filament,
the base obscures and absorbs a smaller percentage of the radiated
visible light.

Can those be the reasons?

There are some economies of scale for higher wattage (and especially
often higher current) filaments:

1. A thicker filament takes longer to suffer from evaporation to point of
failure than a thinner one does, at a given filament temperature. This
means that for equal life expectancy, a thicker filament is operated at a
slightly higher temperature, which slightly increases the percentage of
its radiation being visible light rather than infrared.

2. When lamp wattage increases and lamp cost increases less (or not at
all, as is often the case up to 100 watts for 120V lamps), the percentage
of bottom line cost being from power consumption increases and the
percentage of bottom line cost being from lamp cost (including any labor
for lamp replacement) decreases. This favors making higher wattage lamps
more efficient to improve energy efficiency since energy cost matters more
there, and lower wattage lamps lasting longer since lamp replacement cost
matters more there.

3. In the case of gas-filled incandescent lamps, making the filament
thicker makes the "boundary layer" of filament-heated gas around the
filament accordingly thicker, though not necessarily proportionately.
This means that with a thicker filament, temperature gradient in the gas
immediately surrounding the filament is less than it is with a thinner
filament. This means that with a thicker filament, heat conduction by the
gas per unit area of filament is decreased when the filament is thicker.

==================================================

With efficiency advantages specifically of thicker filaments, would
lower voltage incandescent lamps be more efficient than higher voltage
ones of same wattage and life expectancy? This is largely true until the
filament gets so short and fat that heat conduction through the ends of
the filament gets to be a major loss.
Among incandescent lamps of equal life expectancy and specified wattage,
efficiency as a function of design voltage tends to be maximized when:

For wattages of a fraction of a watt - design voltage is around/ballpark
5-6 volts, the filament is singly coiled, and the bulb has a vacuum.

For wattages of a few watts - design voltage is around/ballpark 6 volts,
the filament is singly coiled, and the bulb is gas-filled.

For wattages in the 10-100 watt range - design voltage is around/ballpark
12 volts, the filament is singly coiled, and the bulb is gas-filled.

If the lamp is designed for a much higher voltage, then if the design
current is still high enough to be better with gas than with a vacuum
(rough ballpark near/over .25 amp) and a gas fill is used, then it helps
to make the filament doubly coiled.

For wattages of a few hundred watts to kilowatt ballpark - design voltage
is a few dozen volts, the filament is coiled-coil, and the bulb is
gas-filled.

- Don Klipstein (don@xxxxxxxxx)
.

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