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One sided mirror



 
 
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  #11  
Old July 5th 03, 09:00 AM
Dave Martindale
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Default One sided mirror

(Don Klipstein) writes:

This brings me to directional couplers (a radio frequency device) which
I have heard of but I don't know exactly what they do nor how they work.


There are a number of different ways to build directional couplers, but
(as you surmised) they don't provide a perpetual motion machine, nor are
they a one-way valve. They just control where a signal goes to.

One type of directional coupler is the splitter/tap used in cable TV
systems. A signal that arrives at the "input" is distributed to each
of the "outputs" in a specified power ratio. If the splitter is on
the pole in front of your house, there's one output that receives
almost all the signal, which is the main cable feed continuing down the
street, and one or more house-feed taps that may get a signal that's 20
or 30 dB down from what's on the main cable. But it's a passive device,
so the sum of the signal power in all of the outputs is always less than
the input power.

Its use as a splitter is pretty mundane, but any passive splitter also
works as a combiner when the signals arrive from the other direction. A
signal coming in one of the "outputs" is passed through to the "input",
without also being fed to the other "outputs". In other words, a signal
headed "upstream" continues upstream, without the other "downstream"
outputs seeing it. That's why it's called a directional coupler:
direction of the signal matters.

Dave
  #12  
Old July 5th 03, 01:58 PM
PV
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Default One sided mirror

Thank you all. I am going to try both the Brewster's mirror and 5the
0/50 beam splitter. If anyone (mmm..Laser people:-) has any idea where I
can get the Brewster's mirror, please let me know...

I really like this thread to go on and discuss further, since I am
interested in knowing any other solutions for this problems/or similar.

It is really great to read all your valuable ideas...and Thanks YOU ALL
againa and again.
PV

Chris L Peterson wrote:
On Sat, 5 Jul 2003 07:06:29 +0000 (UTC), "Luke" wrote:


This is true, but you can get an element which has the same transmission


from both directions but also high reflectivity on one side and low


reflectivity but high absorption on the other:



Sure, the best example being a plate of matte black material that is aluminized
on one side. [Approximately] 100% reflective on one side, 100% absorptive on the
other. But it sure doesn't do what PV is looking for!

I think the best he's going to get is 50% reflectivity, 50% transmissivity.



So it is possible to have a sheet of glass that is say 50% transmissive in
both directions and 50% reflective from one side but 0% reflective and 50%
absorbative from the other side. This is what people try to achive when
making one way mirrors.



I think typical one-way mirrors are more like 75% reflective. And they don't
have any preferred side- you can install them either way and they work just the
same.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


  #13  
Old July 5th 03, 07:42 PM
Daniel P. B. Smith
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Default One sided mirror

In article ,
"Tony Spadaro" wrote:

SO called one way mirrors are actually half silvered. THey are partially
transparent on both sides (which if you think about it, is the only posible
way). The side that is lit is visible from the side that is dark. Change the
lighting so the dark chamber is light and the light one goes dark and the
"transparancy" will shift.
All those movies with one way mirrors are very inaccurate unless the
viewing room is shown as being dark.


Such a mirror plays a role in Arthur C. Clarke's novel "Childhood's
End." The protagonists have interviews with an alien creature who will
only talk to them from behind a pane of "one-way" glass. So they
smuggle in very bright battery-powered light concealed in an attach
case. The light has a hood arrangement; by jamming it against the
one-way glass and turning it on, they light the room on the far side
brightly enough to see what's in it.

Another thing to be noted about "one-way" mirrors is that in real life
they're usually easy to spot, for a couple of reasons. First, if you
start paying attention to how _bright_ the reflection in a mirror is,
you'll see that for a real mirror, the reflection is _almost_ a match
for the scene being reflected, looking only _slightly_ dimmer. In a
"one-way" mirror the reflectivity is usually low enough so that the
reflection looks suspiciously dim, noticeably dimmer than the scene
being reflected. Other giveaway, of course, is that such mirrors are
often in locations that aren't very logical places for mirrors. And
frequently are mounted in a frame that looks much more like a
windowframe than a picture frame. I remember an airport in Venezuela
that had a whole row of "mirrors" that were mounted on a wall about
fifteen feet above the floor. Of course, it's quite possible that they
didn't care at all whether people thought they were mirrors; the
important thing was that you couldn't tell whether or not you were being
watched.

--
dpbsmith at world dot std dot com
(replace "at" with at-sign and "dot" with period and remove spaces)
  #15  
Old July 8th 03, 03:09 AM
Chris L Peterson
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Default One sided mirror

On Tue, 08 Jul 2003 00:44:22 -0000, James Horn wrote:

By the way, non-reciprocal optical devices exist and are available. For
instance, if you have a two polarizers in parallel with their axes rotated
45 degrees from each other and put a polarization rotating medium between
them that rotates the light 45 degrees (a solution of sugar for home
experimenters), light travelling one way will only have the ganged
polarizer loss (about 50%) but the other way will see *crossed* polarizers
(over 99%). Small units for directional control in fiber optics are
available off the shelf.


Unfortunately for the original poster, however, such devices won't do what he
wants. I don't know of anything that can do this with white light that has
scattered off a random object (like a person) of varying color and texture.
There might be some exotic non-linear optical element like this, but certainly
not something the size of a normal mirror!

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #16  
Old February 11th 08, 01:59 AM posted to sci.optics,sci.engr.lighting,sci.astro.ccd-imaging,rec.photo.digital
Paul Hovnanian P.E.
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Posts: 23
Default One sided mirror

"Daniel P. B. Smith" wrote:

[snip]
I remember an airport in Venezuela
that had a whole row of "mirrors" that were mounted on a wall about
fifteen feet above the floor. Of course, it's quite possible that they
didn't care at all whether people thought they were mirrors; the
important thing was that you couldn't tell whether or not you were being
watched.


Post offices use these quite often. They are used by postal inspectors
looking for theft. The employees know what the mirrors are for but have
no way of knowing when inspectors are watching them.

--
Paul Hovnanian
------------------------------------------------------------------
Where am I going, and what am I doing in this handbasket?
  #17  
Old February 11th 08, 06:16 PM posted to sci.optics,sci.engr.lighting,sci.astro.ccd-imaging,rec.photo.digital
Tzortzakakis Dimitrios
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Posts: 1
Default One sided mirror


? "Paul Hovnanian P.E." ?????? ??? ??????
...
"Daniel P. B. Smith" wrote:

[snip]
I remember an airport in Venezuela
that had a whole row of "mirrors" that were mounted on a wall about
fifteen feet above the floor. Of course, it's quite possible that they
didn't care at all whether people thought they were mirrors; the
important thing was that you couldn't tell whether or not you were being
watched.


Post offices use these quite often. They are used by postal inspectors
looking for theft. The employees know what the mirrors are for but have
no way of knowing when inspectors are watching them.

--

Of course, George Orwell thought of that first:_)Big Brother is watching
you!



--
Tzortzakakis Dimitrios
major in electrical engineering
mechanized infantry reservist
hordad AT otenet DOT gr


 




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