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scott wrote:
>> What are your dubious assumptions?
>
> That all the energy from the star/light bulb comes out equally in all
> directions in the form of 550 nm photons, and your eye has an opening of
> 2mm x 2mm.
>
> Obviously all the energy doesn't come out as visible photons, and you
> could probably do something clever with the spectrum to get a more
> accurate figure.
To say nothing of the absorption and scattering of the Earth's atmosphere...
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Invisible <voi### [at] dev null> wrote:
> scott wrote:
> >> What are your dubious assumptions?
> >
> > That all the energy from the star/light bulb comes out equally in all
> > directions in the form of 550 nm photons, and your eye has an opening of
> > 2mm x 2mm.
> >
> > Obviously all the energy doesn't come out as visible photons, and you
> > could probably do something clever with the spectrum to get a more
> > accurate figure.
>
> To say nothing of the absorption and scattering of the Earth's atmosphere...
Probably not nearly as important as the error introduced by assuming both star
and bulb emit only in the visible.
Otherwise, not such dubious assumptions at all!
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>> To say nothing of the absorption and scattering of the Earth's atmosphere...
>
> Probably not nearly as important as the error introduced by assuming both star
> and bulb emit only in the visible.
>
> Otherwise, not such dubious assumptions at all!
Well, at least he didn't say an LED or a flourescent tube or something
with a complex emission system. A normal light bulb and a star are
(AFAIK) both simple black body radiators, so you just need to know their
colour temperature...
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Invisible <voi### [at] devnull> wrote:
> >> To say nothing of the absorption and scattering of the Earth's atmosphere...
> >
> > Probably not nearly as important as the error introduced by assuming both star
> > and bulb emit only in the visible.
> >
> > Otherwise, not such dubious assumptions at all!
>
> Well, at least he didn't say an LED or a flourescent tube or something
> with a complex emission system. A normal light bulb and a star are
> (AFAIK) both simple black body radiators, so you just need to know their
> colour temperature...
That just tells you the peak power output. The total power output might not be
as much as an order of magnitude different, but it will be different.
In any case, for a lightbulb, less than 10% of that 100W will be visible. An LED
or fluorescent tube will emit mostly in the visible, so the power will be much
more accurate for this comparison.
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>> Well, at least he didn't say an LED or a flourescent tube or something
>> with a complex emission system. A normal light bulb and a star are
>> (AFAIK) both simple black body radiators, so you just need to know their
>> colour temperature...
>
> That just tells you the peak power output. The total power output might not be
> as much as an order of magnitude different, but it will be different.
I was under the impression that the colour temperature tells you roughly
how the signal power is distributed over the whole spectrum. (Initially
just IR, then red as well, and gradually more high frequencies, while
the power at lower frequences decreases.)
> In any case, for a lightbulb, less than 10% of that 100W will be visible. An LED
> or fluorescent tube will emit mostly in the visible, so the power will be much
> more accurate for this comparison.
Yeah, both stars and lightbulbs presumably put out far more IR and radio
waves than visible light...
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Invisible <voi### [at] devnull> wrote:
> I was under the impression that the colour temperature tells you roughly
> how the signal power is distributed over the whole spectrum.
Roughly, in that it tells you where the peak is and the general shape of the
rest of the spectrum.
> Yeah, both stars and lightbulbs presumably put out far more IR and radio
> waves than visible light...
Actually, many stars peak in the visible - Betelgeuse is red. Ours is yellow. I
think maybe lightbulbs peak in the IR (lower temperature), so all we see is a
section of the tail of the spectrum.
http://en.wikipedia.org/wiki/Black_body
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> Invisible <voi### [at] devnull> wrote:
>> I was under the impression that the colour temperature tells you roughly
>> how the signal power is distributed over the whole spectrum.
>
> Roughly, in that it tells you where the peak is and the general shape of the
> rest of the spectrum.
Sure. So if you know how hot the star is, you know roughly the shape of
the entire emission spectrum. :-)
>> Yeah, both stars and lightbulbs presumably put out far more IR and radio
>> waves than visible light...
>
> Actually, many stars peak in the visible - Betelgeuse is red. Ours is yellow. I
> think maybe lightbulbs peak in the IR (lower temperature), so all we see is a
> section of the tail of the spectrum.
The peak is just the single wavelength at which there is the most
energy. I think if you look at *bands* of the spectrum, it's possible
for a wide band with low energy at any specific single wavelength to
have a greater total power and a band containing only a single
high-power (but narrow) peak.
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> for a wide band with low energy at any specific single wavelength to
How can you have a "wide band" at a specific single wavelength? Surely the
definition of a "band" is a range of wavelengths?
Sure, if the peak is at 600 nm, then the energy in the band from 599.99 to
600.01 will most likely be less than the band from 100 to 599.
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scott wrote:
>> for a wide band with low energy at any specific single wavelength to
>
> How can you have a "wide band" at a specific single wavelength? Surely
> the definition of a "band" is a range of wavelengths?
>
> Sure, if the peak is at 600 nm, then the energy in the band from 599.99
> to 600.01 will most likely be less than the band from 100 to 599.
What I actually meant was that a "peak" is just one wavelength with a
lot of energy. The band containing the peak isn't necessarily the band
with the highest total energy.
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Invisible <voi### [at] devnull> wrote:
> Sure. So if you know how hot the star is, you know roughly the shape of
> the entire emission spectrum. :-)
Yes. Except of course it works the other way around in practice.
> > think maybe lightbulbs peak in the IR (lower temperature), so all we see is a
> > section of the tail of the spectrum.
>
> The peak is just the single wavelength at which there is the most
> energy. I think if you look at *bands* of the spectrum, it's possible
> for a wide band with low energy at any specific single wavelength to
> have a greater total power and a band containing only a single
> high-power (but narrow) peak.
What Scott said. But I know what you mean - and yes, of course. I was just
explaining why most of an incandescent bulb's energy output is not in the
visible. Look at the shape of a blackbody spectrum for 3000K. Clearly, most of
the power output is IR and beyond.
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