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| Science Forum Index » Optics Forum » Please explain this weirdness... |
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| dave fenner... |
 Posted: Sun Oct 04, 2009 7:56 am |
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I am retired cinema projection engineer. Years ago carbon arcs were
ubiquitous, 12" or bigger mirror with central hole for -ive carbon,
arc gap about 3/8", positive carbon co-axial,. hotspot facing facing
mirror about 3-4" away, 2nd focus at the aperture about 32" forward.
When viewed from the side, when smoke present the cone shaped beam
mirror to aperture appears to have curved sides! The beam appears
shaped like an exponential horn, a trumpet.
I have heard but cannot remember a word to describe this. Now I talk
to a current day cinema engineer who knows only xenon lamps (no smoke)
and he wont believe me about this, and of course I do understand why.
Someone please tell me the name of this effect, where I may find
illustration or explanation. TIA.
dave fenner |
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| Helmut Wabnig... |
| Posted: Sun Oct 04, 2009 12:17 pm |
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On Sun, 4 Oct 2009 10:56:50 -0700 (PDT), dave fenner
<dffenner at (no spam) googlemail.com> wrote:
[quote:d177a14b03]I am retired cinema projection engineer. Years ago carbon arcs were
ubiquitous, 12" or bigger mirror with central hole for -ive carbon,
arc gap about 3/8", positive carbon co-axial,. hotspot facing facing
mirror about 3-4" away, 2nd focus at the aperture about 32" forward.
When viewed from the side, when smoke present the cone shaped beam
mirror to aperture appears to have curved sides! The beam appears
shaped like an exponential horn, a trumpet.
I have heard but cannot remember a word to describe this. Now I talk
to a current day cinema engineer who knows only xenon lamps (no smoke)
and he wont believe me about this, and of course I do understand why.
Someone please tell me the name of this effect, where I may find
illustration or explanation. TIA.
dave fenner
[/quote:d177a14b03]
This one?
http://en.wikipedia.org/wiki/Caustic_%28optics%29
w. |
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| AES... |
| Posted: Sun Oct 04, 2009 4:39 pm |
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In article <3IOdnQt3Idq3d1XXnZ2dnUVZ_judnZ2d at (no spam) supernews.com>,
Phil Hobbs <pcdhSpamMeSenseless at (no spam) electrooptical.net> wrote:
[quote:291e664431]I am retired cinema projection engineer. Years ago carbon arcs were
ubiquitous, 12" or bigger mirror with central hole for -ive carbon,
arc gap about 3/8", positive carbon co-axial,. hotspot facing facing
mirror about 3-4" away, 2nd focus at the aperture about 32" forward.
[/quote:291e664431]
I have no previous knowledge of this phenomenon, but I'd note that a
hotspot only 3" or 4" away from a 12" diameter mirror represents one
hell of a small f-number.
Wouldn't it be likely that different annular regions of the mirror would
be imaging that hot spot at substantially different distances away from
that mirror (which was probably more spherical than parabolic), and that
strong convergence of the light coming from the outer regions of the
mirror and weaker convergence of the light from the more central regions
could produce the appearance described by the OP? |
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| Bob May... |
| Posted: Sun Oct 04, 2009 5:21 pm |
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The mirror is usually a parabolic or near barabolic surface, usually with
the silvering on the backside which makes it technically a Mangin mirror.
When the light source is outside of the focus of the mirror, the beam will
shrink until it hits a minimum point and then it starts expanding. Needless
to say, the light path is a straight line.
I have one of those mirors and it has a focal length of about 7.5". If you
put a light source at about another quarter inch further away from the
mirror you would see this shrinking of the beam diameter. Most often, this
shrinking of the beeam diameter is just because the operator of the
searchlight just got "close enough for gubernment work" on the position of
the arc. I'll also note that since part of the light source (a quarteer
inch long arc is really a long line of light), the light won't come to aa
point but rather a long wasited narrow zone and it looks like a caustic
curve as different partss of the light source come to a different focus.
--
Bob May
rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net |
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| Andrew Gabriel... |
| Posted: Mon Oct 05, 2009 3:55 am |
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In article <1254681252.874317 at (no spam) athprx04>,
"I.N. Galidakis" <morpheus at (no spam) olympus.mons> writes:
[quote:19812e0746]
The other phenomenon, is a prominent "bending" of the arc because of convection
currents. I don't think there's a name for this, except maybe "convection
current arc bending".
[/quote:19812e0746]
Jacob's ladder being a well-known demonstration of this effect.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup] |
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| Victor Roberts... |
| Posted: Tue Oct 06, 2009 6:46 am |
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On Mon, 05 Oct 2009 07:34:40 -0700, Salmon Egg
<SalmonEgg at (no spam) sbcglobal.net> wrote:
[quote:feba4dc4c6]In article <hactrv$q5r$2 at (no spam) news.eternal-september.org>,
andrew at (no spam) cucumber.demon.co.uk (Andrew Gabriel) wrote:
In article <1254681252.874317 at (no spam) athprx04>,
"I.N. Galidakis" <morpheus at (no spam) olympus.mons> writes:
The other phenomenon, is a prominent "bending" of the arc because of
convection
currents. I don't think there's a name for this, except maybe "convection
current arc bending".
It is possible that the banding described is why the arc is called an
"arc."
[/quote:feba4dc4c6]
That is exactly the explanation used by one of my colleagues
to explain why high pressure discharges are called "arc"
discharges. He claims the term originates from the time of
discharges in atmospheric pressure air between two
horizontally opposed electrodes. These would bend upward by
virtue of the convective currents and therefore acquired the
name "arc discharge."
--
Vic Roberts
http://www.RobertsResearchInc.com
http://www.cflfacts.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.
This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission. |
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| aspherix... |
| Posted: Thu Oct 15, 2009 8:52 am |
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"dave fenner" <dffenner at (no spam) googlemail.com> wrote
[quote:7e0db75308]I am retired cinema projection engineer. Years ago carbon arcs were
ubiquitous, 12" or bigger mirror with central hole for -ive [???] carbon,
arc gap about 3/8", positive carbon co-axial,. hotspot facing facing
mirror about 3-4" away, 2nd focus at the aperture about 32" forward.
When viewed from the side, when smoke present the cone shaped beam
mirror to aperture appears to have curved sides! The beam appears
shaped like an exponential horn, a trumpet.
I have heard but cannot remember a word to describe this. Now I talk
to a current day cinema engineer who knows only xenon lamps (no smoke)
and he wont believe me about this, and of course I do understand why.
Someone please tell me the name of this effect, where I may find
illustration or explanation. TIA.
[/quote:7e0db75308]
Spherical aberration should explain at least part of what you are observing. |
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