Re: luminous efficiency

["Carl E. Mungan" <mungan@USNA.EDU>]

Larry Woolf  wrote:

> > I agree that higher wattage means higher
> > filament temperature, which tends to increase the light output in the
> > visible by Wien's law.
>
> The light output also increases because of the increasing filament
diameter
> and length (higher surface area for light emission) for larger wattage
> bulbs:
>
> 25 W            0.0030 cm dia   56 cm length    ratio=1.0
> 75 W            0.0053 cm dia   55 cm length             3.2
> 100 W           0.0064 cm dia   58 cm length            4.4
> 200 W           0.0102 cm dia   72 cm length            9.0

True, but I was thinking specifically of increasing the luminous efficiency
which is the ratio of visible output in lumens to electrical power in watts.
Note that most of the change in filament size alters both of these values
proportionately. Specifically, I added a fourth column to your table above,
namely the ratio of diameter squared to length, normalized to unity for the
first entry. This ratio tracks pretty closely with the power, as you can
see, the discrepancy indicating some variation in resistivity with
temperature.

However, another factor which does change the luminous efficiency is the
variation in emissivity with temperature. Carl
-----
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
      U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5040
mailto:mungan@usna.edu          http://usna.edu/Users/physics/mungan/