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PV
July 4th 03, 03:31 AM
I am looking for a mirror that should be transparent from one side and
reflective from other side. Could anyone please help me where I can find
it? My application is to illuminate the object from the transparent side
and reflected image will be used from the other side for image collection.
Thanks in advance
PV

Tony Spadaro
July 4th 03, 03:35 AM
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.

--
http://www.chapelhillnoir.com
home of The Camera-ist's Manifesto
The Improved Links Pages are at
http://www.chapelhillnoir.com/links/mlinks00.html
New email - Contact on the Menyou page.
"PV" > wrote in message
...
> I am looking for a mirror that should be transparent from one side and
> reflective from other side. Could anyone please help me where I can find
> it? My application is to illuminate the object from the transparent side
> and reflected image will be used from the other side for image collection.
> Thanks in advance
> PV
>

Luke
July 4th 03, 07:15 AM
This is true, but you could get around the problem by shining a very
directed, parrallel, beam of light through the glass and then taking the
shot at an angle to the half silvered side.

I think you would find it easier to just shoot the subject in a normal
mirror, at an angle. If you possition the lights carefully you will be albe
to light the subject with light reflected off the mirror without the light
source appearing in the image. You can start with a light just next to your
cammera, a standard flash for example.

Luke

"Tony Spadaro" > wrote in message
. com...
> 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.
>
> --
> http://www.chapelhillnoir.com
> home of The Camera-ist's Manifesto
> The Improved Links Pages are at
> http://www.chapelhillnoir.com/links/mlinks00.html
> New email - Contact on the Menyou page.
> "PV" > wrote in message
> ...
> > I am looking for a mirror that should be transparent from one side and
> > reflective from other side. Could anyone please help me where I can find
> > it? My application is to illuminate the object from the transparent side
> > and reflected image will be used from the other side for image
collection.
> > Thanks in advance
> > PV
> >
>
>

David Lee
July 4th 03, 11:21 AM
Use a thin sheet of glass between the light source and the object at 45
degrees to the line between them. A fraction of the light will pass
through the glass and illuminate the object whilst the rest will be
reflected at 90 degrees. Likewise with light returning from the object -
part will be transmitted and will return along the same path to the light
source whilst the rest will again be reflected a 90 degrees but in the
opposite direction to that directly from the light source. Thus you divide
the light into two at each pass through the mirror and separate the light
from the object from the 'waste' light from the lamp. It's easier to
imagine than to describe. I'm not good at ASCII art but the sketch below
may help. This is the closest you will get to an "optical check valve".

The mirror can either be a piece of plain glass, in which case you will
maximize the illumination of the object, or else a partially silvered
mirror in which case you will get less illumination but maximize the
collection of light from the object. Plain glass is usually fine.

This is a standard way of illuminating objects for microscopy or macro
photography and also the principle of operation of the autocue and Pepper's
Ghost.

HTH

David


mirror (at 45 degrees)
^ /
| /
| /
| /
|/
LIGHT - - - - - - - >/<-------> OBJECT
/
/
/
/
/

V
CAMERA

PV > wrote in article
>...
> I am looking for a mirror that should be transparent from one side and
> reflective from other side. Could anyone please help me where I can find
> it? My application is to illuminate the object from the transparent side
> and reflected image will be used from the other side for image
collection.
> Thanks in advance
> PV
>
>

Don Stauffer
July 4th 03, 03:07 PM
Right. If it were possible to truly do this, one could make Maxwell's
demon, and end up with a really need solar power system.

Joseph Meehan wrote:
>
> As the others have said, I don't think you can do exactly what you are
> suggesting.

--
Don Stauffer in Minnesota

webpage- http://www.usfamily.net/web/stauffer

H.J.P. Vink
July 4th 03, 03:15 PM
Using polarised light you could get quite close to using all the light. But
probably the reflected light from the object of not completely polarised
anymore....

Rob.

"PV" > wrote in message
...
> I am looking for a mirror that should be transparent from one side and
> reflective from other side. Could anyone please help me where I can find
> it? My application is to illuminate the object from the transparent side
> and reflected image will be used from the other side for image collection.
> Thanks in advance
> PV
>

Don Klipstein
July 4th 03, 07:17 PM
In >, Charlie D wrote:

>Here's one.
><http://www.scientificsonline.com/ec/Products/Display.cfm?CategoryID=1928
>41>

This is a beam splitter. It does not work even to a partial extent as a
check valve. The "loss" (diversion) along/from any given path would be
the same in both directions.

Optical check valves appear impossible to me because:

Suppose you have a closed system which consists of a blackbody chamber -
say, some isolated completely closed room. This room is at some
certain temperature and blackbody radiation exists in it.
Now, suppose you had an "optical check valve" wall and divided this room
with it. There would be a net flow of energy from one side of the wall to
the other. The temperature would rise on one side and fall on the other
side. You would be able to string a pair of thermocouples in this room,
one on each side, and have that turn en electric motor to do work. Or you
could set up some other heat engine in this divided room to do work. Have
the heat coming from friction in whatever work you are doing go into the
hot side. Maybe the motor could just turn a fan or something decorative
to demonstrate the principle.
Obviously, this would have the entropy in the closed system decrease
from where it was before the optical check valve started doing its thing.
And it would be a perpetual motion machine. As far as I understand it,
the assumption that such things cannot work has been used to prove
Einstein's photoelectric loss, and a similar (bandwidth and intensity
dependent) loss in photovoltaic semiconductors.

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.
But if you had a divided blackbody chamber with blackbody radiation
including some radio frequency, I doubt you can make a perpetual motion
machine by putting an antenna in each side and putting a directional
coupler between them.

- Don Klipstein )

PV
July 5th 03, 04:29 AM
I really like to try this Brewster's Mirror. Could you or someone please
help me get one.
Thanks you all the others for your valuable discussion and ideas.
I wonder, whether I can find a beamslitter which should has one side
with higher transparency and other side with higher reflection.

Where can I find such a beamsplitter? Any help.
Thanks in advance.
PV

David wrote:
> I believe the mirror in question would be a Brewster's mirror. These are
> used in LASER applications and are very common. I have several 2 inch square
> ones now. They came from old grocery store check stand scanners. You should
> be able to find them at a LASER hobbyist store. :)
>
>

Luke
July 5th 03, 08:06 AM
>
> Sorry, this won't work. The simple fact is that there is no such thing as
an
> optical element that is transparent in one direction and reflective in the
> other.
>


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:

(usual physical terminology would be 'emissivity' rather than 'absorption',
but the two are aways equal)

Conservation of energy says that transmission through the entire optical
system must be the same from both directions (as someone suggested look up
'Maxwell's demon' if you want convincing).

Conservation of energy (COE) does not imply that the other optical
properties of the surfaces must be the same. What it does say is:

transmission + reflection + absoption = 1

(it is worth noting the reflection may be diffuse or specular, that doesn't
effect COE but does effect the optical properties.)

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.

Yes there is no such thing as a perfect one way mirror, but they do only
function as a mirror from one direction.

Luke

Chris L Peterson
July 5th 03, 08:22 AM
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

Dave Martindale
July 5th 03, 09:00 AM
(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

PV
July 5th 03, 01:58 PM
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

Daniel P. B. Smith
July 5th 03, 07:42 PM
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)

Chris L Peterson
July 5th 03, 08:48 PM
On Sat, 5 Jul 2003 19:37:31 +0000 (UTC), (Dave Martindale)
wrote:

>Chris L Peterson > writes:
>
>>A Brewster window is very useful when
>>working with lasers, as it provides a mechanism for getting the beam out of a
>>sealed tube without losses.
>
>Isn't it also what's used to force a beam to be polarized in the first
>place? The idea that a piece of glass at Brewster's angle allows one
>polarization to pass back and forth without loss, while the orthogonal
>polarization suffers some loss in each pass and is attenuated to
>extinction. This works with external-mirror lasers, where the Brewster
>window is part of the tube envelope, and with integral-mirror lasers
>where there's just a piece of glass at the right angle inside the tube.

Exactly. And since the stimulated emission (the SE in LASER) maintains the
polarization of the stimulating photon, the polarization selected by the
Brewster window ends up being the only one the cavity will support.

_________________________________________________

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

Chris L Peterson
July 8th 03, 03:09 AM
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

Paul Hovnanian P.E.
February 11th 08, 12:59 AM
"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?

Tzortzakakis Dimitrios
February 11th 08, 05:16 PM
? "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