Re: Mirror position in bended optical path
- From: dudi24 <sven.dudeck@xxxxxx>
- Date: Sun, 5 Apr 2009 12:29:57 -0700 (PDT)
On 31 Mrz., 16:51, Phil Hobbs <pcdhSpamMeSensel...@xxxxxxxxxxxxxxxxxx>
wrote:
dudi24 wrote:
Hello,
I am trying to build a magnifying lens system consisting of an
infinity-corrected objective (L1) and a focussing lens (L2) (f=200mm)..
Between the objective and the lens is a beamsplitter (BS) to include
an illumination. I think in principle this is pretty much a standard
setup.
| () \ () |
O L1 BS L2 I
The problem I have: The overall length of the system is much to long.
Therefore I would like to bend the optical path one or two times but I
am not sure where to put the mirrors to do that.
There is the option to put them within the (quasi) parallel part of
the optical path, somewhere between L1 and L2, or within the focussing
part behind L2.
Option 1:
| () \
\ / () |
O L1 M1
M2 BS L2 I
Option 2:
| () \ () \
\ |
O L1 BS L2 M1
M2 I
I suppose that the position of the bending-mirrors will affect my
optical imaging, but I am not sure in what way. Therefore my question
is, What will be the better choice of mirror-position.
Thank you very much,
Sven Dudeck
Depends on the mirror and the wavelength, as well as the image quality
you're looking for. A perfect first-surface mirror, i.e. a perfectly
flat, perfectly conducting surface, introduces no aberrations at
all--the folded system will perform exactly like the unfolded one.
In discussing the effects of real mirrors, it's worth making a
distinction between two kinds of aberrators--those that depend on
position and those that depend on angle. (There are some that do both,
e.g. an irregular glass plate, but that's a fine point.)
Consider a plate of glass sitting at some angle to the axis of a
converging light beam. If you think of rays, the glass leads to a
sideways shift that depends on angle, which makes the oblique rays miss
the focus. If you think about waves, it leads to nasty phase shifts
that depend on incidence angle (since different plane waves traverse
different distances in the glass), which makes the waves fail to add up
all in phase at the focus. The physics is just that different
components take different paths through the glass.
In parallel light (e.g. between your infinity-corrected lens and the
focusing lens), purely angle-dependent aberrations are generally of no
consequence, since light from a given object point is all going in the
same direction when it gets to the aberrator, e.g. your beam splitter.
Light from different positions in the object will experience different
phase shifts, but that's usually OK because in thermal light they're
mutually incoherent anyway, so they don't interact. (Laser systems are
a bit more complicated, but this is still generally OK.) Thus if you
fold the system in the parallel-light region using good quality
first-surface mirrors, you won't notice any change in the image quality.
So Option 1 is always fine.
None of these nice properties apply in converging or diverging light,
and they get worse rapidly as the numerical aperture(*) and the field
angle(**) increase, so you can't go putting a hunk of glass in there
without calculating its effects.
Second-surface mirrors (like the one you shave with) have the reflective
surface on the back of the glass instead of the front. If you trace the
rays through a mirror of thickness d, there are two effects to worry
about: (1) the effect of a glass plate of thickness 2d (in the unfolded
system), which we've already talked about; and (2) ghost reflections
from the front and back surfaces. You're fighting those with your beam
splitter already, but there's no sense in making them worse.
Mirror surface errors will cause wavefront aberrations, and (more
subtly) even very flat mirrors produce angle-dependent phase shifts on
reflection. These phase shifts are more important with fancy
dielectric mirrors than with metal. For practical purposes, and
certainly as long as your NA is low (say below 0.05), phase shifts from
metal first-surface mirrors are no problem whatever, and you can fold
your system any way you like. At very high NA, and any time you're
using dielectric mirrors, you do have to start worrying.
Cheers,
Phil Hobbs
(*) Numerical aperture (NA = sin theta, where theta is the half-angle of
the cone of rays).
(**) For a simple thin lens, if you draw a line from the centre of the
lens to the object point, the angle it makes with the lens axis is the
field angle. (In a real system you draw the line to the centre of the
entrance pupil, but it's the same idea. We loosely talk about field
angle elsewhere in the system because the analogy is obvious and useful.)
eAeeeeeeeeeeeeeeeeeee*F
e/ \ cccr
ee/ \ ccc rr
e | | ccc r
ee | | ccc rr
e | |ccc r
ee | ccc| rr
--------------e-----|---cCc---|-----r-------- X ---------------------
ee |ccc | rr
e ccc| | r
ee ccc | | rr Field angle = <FCX
e ccc | | r NA = sin(<VFA/2)
ee ccc \ /rr
eccc \ /r
OrrrrrrrrrrrrrrrrrrrrVr
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot nethttp://electrooptical.net
Thanks a lot for all the answers - Ive been away the last days, so I
couldn`t read them up to now.
So, if I get it right, I don't have to worry too much where to put the
mirrors within the setup.
Thanks,
Sven
.
- Prev by Date: Re: Removing surface treatments
- Next by Date: Re: Removing surface treatments
- Previous by thread: Re: Mirror position in bended optical path
- Next by thread: Re: Mirror position in bended optical path
- Index(es):
Relevant Pages
|
Loading
