Re: brightness vrs. power

[Leigh Palmer <palmer@SFU.CA>]

> Hello,
>
> I have a question on bulbs in a DC-circuit. What is a relationship between
> power dissipated in a bulb and brightness? I have seen some conceptual
> exercises which seem to assume that brightness is directly proportional to
> the power dissipated. I found a definition for îbrightnessî: it is called
> luminance. Luminance is connected to the luminous intensity which is
> directly proportional to radiant energy.

That is a misconception. "Luminous" and similar words refer to visible
light, while "radiant" and similar words refer to electromagnetic energy
of all wavelengths. The former quantities are associated with photometry,
and the latter with radiometry. You should seek a competent source of, er,
enlightenment. The web has some promising URLs. I haven't evaluated this
one, but try http://arapaho.nsuok.edu/~salmonto/VSII/Lecture2/Lecture2.html
for a start. The keywords should lead you to others.

> Radiant energy depends on temperature of a filament. For instance when two
> similar bulbs are connected in series the net current is (approximately)
> halved. The power dissipated in both bulbs is reduced by factor 1/4.

I don't know how you calculated this, but it is incorrect. Light bulbs
do not obey Ohm's law. They are extremely nonlinear.

But has brightness reduced by the same factor? The current decreases and
hence the temperature decreases. Then intensity and position of the peak
wavelength changes more or less like in case of black body radiation. This
curve (intensity against wavelength) is *not* linear. Would this mean that
brightness is not directly proportional to the power dissipated? Or is
there a mistake in my reasoning?

Your reasoning is correct; the brightness increases more rapidly than
with the diddipated power.

Leigh