So how do we know if the colour you see is correct?

Alan Browne wrote:
Chris Malcolm wrote:
Alan Browne wrote:

Possibly not a lash up, but certainly not something one wants to use;
hence "head in droves to electronic flash".

Just a particular example of the more general horror of all manual
modes of operation found in people who were traumatised in childhood
by an arithmetic teacher.

Don't confuse "electronic" with "automatic".

Electronic flash does not eliminate the need to calculate, quite the
contrary, it just eliminates heat, burned bulbs and being forced to use
relatively slow shutter speeds or fast film (or both).

There are a number of pretty expensive flash systems which hope to
have eliminated the need for calculation. Don't know far they achieve
that in practice.

Although I'm quite happy using a handful of cheap old flashes in
manual mode for portable cheap lighting, if I were trying to equip a
photographic studio today I'd be tempted to use big tungsten halogen
lights because they're powerful, long lasting, good colour temp, and
are cheaply and easily dimmable if heat is a problem.

--
Chris Malcolm DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]

In article ,
Colin_D wrote:


Aaahh, Al, that's not quite right, if you'll forgive the intrusion.

The resistance of the filament varies with temperature. The temperature
coefficient (alpha) of tungsten is 0.0044/deg.C. For a photoflood lamp
designed to run at 3,200K, we have a filament running at a temperature
rise of 2900KC (3200K minus ambient at 293K approx (20C = 293K). So the
change in resistance over 2900 degrees is 0.0044*2900 = 12.76, say 13 times.

When you halve the voltage, the temperature drops to some intermediate
figure, which means the resistance drops accordingly, so rather more
current flows at half voltage than if the resistance remained constant,
and therefore the power consumed by the lamp is more than would be
expected if one didn't take tungsten alpha into account. And, as you
say, the light output is approximately proportional to voltage^3.4, but
only over a small range. At half power the light output is probably
less than half, too yellow, and the efficiency is execrable.

Let me ask a beginners question. Is the resistance due to an abundance
of electrons in the chain when the voltage is higher. Seems to reason,
that that is the case.

--
Reality is a picture perfected and never looking back.

____ wrote:
In article ,
Colin_D wrote:

Aaahh, Al, that's not quite right, if you'll forgive the intrusion.

The resistance of the filament varies with temperature. The temperature
coefficient (alpha) of tungsten is 0.0044/deg.C. For a photoflood lamp
designed to run at 3,200K, we have a filament running at a temperature
rise of 2900KC (3200K minus ambient at 293K approx (20C = 293K). So the
change in resistance over 2900 degrees is 0.0044*2900 = 12.76, say 13 times.

When you halve the voltage, the temperature drops to some intermediate
figure, which means the resistance drops accordingly, so rather more
current flows at half voltage than if the resistance remained constant,
and therefore the power consumed by the lamp is more than would be
expected if one didn't take tungsten alpha into account. And, as you
say, the light output is approximately proportional to voltage^3.4, but
only over a small range. At half power the light output is probably
less than half, too yellow, and the efficiency is execrable.

Let me ask a beginners question. Is the resistance due to an abundance
of electrons in the chain when the voltage is higher. Seems to reason,
that that is the case.

Electron 'flow' or movement is measured in amperes, or amps for short.
Voltage is the pressure that drives amperes through the load, and
resistance is an inherent property of conductors to allow a certain
current to flow at a certain voltage. Half the voltage (pressure) will
cause half the current to flow - if the resistance remains constant.
Note that electrons do not flow at the speed of light, but the
'knock-on' effect does move at that speed, or near to it.

Colin D.

--
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Colin_D wrote:

... allow a certain current to flow ...

FERGAWD'SSAKE!!!! Current _IS_ flow!!! You may say, "electrons flow", or
"charge flows", but you can't say flow flows when you mean flow!! Stop it
right now and don't ever do it again!

Thank you. I feel better now.

Wilba wrote:
Colin_D wrote:
... allow a certain current to flow ...

FERGAWD'SSAKE!!!! Current _IS_ flow!!! You may say, "electrons flow", or
"charge flows", but you can't say flow flows when you mean flow!! Stop it
right now and don't ever do it again!

Thank you. I feel better now.


Well, the wind blows, and the tide flows, so where you're coming from,
nobody knows.

Or, in plain prose, you must have been feeling bloody awful, if that
made you feel better.

Or, perhaps pointless pedantry pimps your passion for pinpricking.

Colin D.

--
Posted via a free Usenet account from http://www.teranews.com

Colin_D wrote:
Wilba wrote:
Colin_D wrote:

... allow a certain current to flow ...

FERGAWD'SSAKE!!!! Current _IS_ flow!!! You may say, "electrons flow",
or "charge flows", but you can't say flow flows when you mean flow!! Stop
it right now and don't ever do it again!

Thank you. I feel better now.

Well, the wind blows, and the tide flows, so where you're coming from,
nobody knows.

I bet lots of people (e.g. anyone who has done high-school physics), know
that I'm coming from something like -

cur·rent, adj.
a. A flow of electric charge.
b. The amount of electric charge flowing past a specified circuit point per
unit time.

Or, in plain prose, you must have been feeling bloody awful, if that made
you feel better.

That's a strange idea - I can feel better from anywhere this side of
ultimate ecstacy.

Or, perhaps pointless pedantry pimps your passion for pinpricking.

Nope, but I do prefer the concise expression of reasonable meaning over
sloppy nonsense. :-)

Colin_D wrote:
____ wrote:
In article ,
Colin_D wrote:

Aaahh, Al, that's not quite right, if you'll forgive the intrusion.

The resistance of the filament varies with temperature. The
temperature coefficient (alpha) of tungsten is 0.0044/deg.C. For a
photoflood lamp designed to run at 3,200K, we have a filament running
at a temperature rise of 2900KC (3200K minus ambient at 293K approx
(20C = 293K). So the change in resistance over 2900 degrees is
0.0044*2900 = 12.76, say 13 times.

When you halve the voltage, the temperature drops to some
intermediate figure, which means the resistance drops accordingly, so
rather more current flows at half voltage than if the resistance
remained constant, and therefore the power consumed by the lamp is
more than would be expected if one didn't take tungsten alpha into
account. And, as you say, the light output is approximately
proportional to voltage^3.4, but only over a small range. At half
power the light output is probably less than half, too yellow, and
the efficiency is execrable.



Colin : I did not see your other post, but the factor I mentioned is in
the literature ( light output prop to V ^ 3.4 ).

Cheers,
Alan


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-- e-meil: Remove FreeLunch.

Alan Browne wrote:
Colin_D wrote:
____ wrote:
In article ,
Colin_D wrote:

Aaahh, Al, that's not quite right, if you'll forgive the intrusion.

The resistance of the filament varies with temperature. The
temperature coefficient (alpha) of tungsten is 0.0044/deg.C. For a
photoflood lamp designed to run at 3,200K, we have a filament
running at a temperature rise of 2900KC (3200K minus ambient at 293K
approx (20C = 293K). So the change in resistance over 2900 degrees
is 0.0044*2900 = 12.76, say 13 times.

When you halve the voltage, the temperature drops to some
intermediate figure, which means the resistance drops accordingly,
so rather more current flows at half voltage than if the resistance
remained constant, and therefore the power consumed by the lamp is
more than would be expected if one didn't take tungsten alpha into
account. And, as you say, the light output is approximately
proportional to voltage^3.4, but only over a small range. At half
power the light output is probably less than half, too yellow, and
the efficiency is execrable.



Colin : I did not see your other post, but the factor I mentioned is in
the literature ( light output prop to V ^ 3.4 ).

Cheers,
Alan


Hi Alan,

Ok, I just pointed out that halving the voltage does not result in half
the current, because of the temperature coefficient of tungsten, as
above, but rather more than half the current (because the lamp
resistance drops with lower temperature). This means that at half
voltage, more than a quarter power is being consumed.

Regards,

Colin D.

--
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