Re: Home-brew beam splitter question.

[P3D Greg Erker <erker@xxxxxxxxxxxxxx>]

>
>                        +-+      /             /       \
>                        |A|>   /                subject
>                        +-+  /                 \ here  /
>                              +-^-+
>                              | B |
>                              +---+
>
>    My question is, does the plate have to be a `half silvered' mirror, or
>    is an ordinary sheet of plate glass sufficient?  (I realize that for
>    best results a black box surrounding the glass/camera lens would be
>    needed as well, etc. etc.)  Of course if a half silvered mirror is
>    needed my next question will be, does anyone know of a good source for
>    the appropriate type.  :-)  Thanks in advance....


  Regular glass will reflect about 6% (IIRC) of
the light, depending on angle. So the light getting
to B will be about 4 stops dimmer (6% ~= 1/16) than
the light going to A.

  A bigger problem is that there will be a reflection
from both the front and back surfaces of the glass,
so B will see a double image. If one surface was
antireflection coated the  double image could be
made much dimmer. (Single coating gets it down
to 1-2% reflection, and Pentax's SMC coating is
0.2%).

  If the mirror is 50% silvered then you will still
get some double image, but the ratios will be better
since the main image is brighter.

  Edmund Scientific sells various BS's. An example
would be #A43,359 which is 50% reflection/50%
transmission. It is 50x50x1 mm and sells for $15.90.
They also have some that are AR coated, which would
presumably reduce the ghosting.

Regards - Greg E.



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