Re: luminous efficiency

[Bernard Cleyet <anngeorg@PACBELL.NET>]

"However, another factor which does change the luminous efficiency is the
variation in emissivity with temperature. Carl"

The emissivity of W (ribbon - by double coiling of filaments the emissivity is increased) is rather flat from 0.3 => .5 micron.  The variation over this range is ~ 5% 2800 K to 1600.  Not until 1.4 micron does the emissivity increase with temperature.  For 2800 K & 1600 the emissivity is: ~

wave length     emissivity

(micrometer)    2800 K          1600 K

0.3             0.46            0.475

0.4             0.465           0.475

0.5             0.45            0.465

0.6             0.44            0.455

0.7             0.42            0.445

estimated from graph (p. 196 Levi, Applied Optics)


bc






Carl E. Mungan wrote:

> 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/