Re: Help with DARKFIELD MICROSCOPY
Hi Charles,
The principles of darkfield microscopy are dependent on blocking any light
rays that go directly into the bottom lens of the objective. The only light
rays that are allowed to enter the objective are those that are deviated
from their original path by the specimen. Consequently, the specimen is
brilliantly illuminated on a black background. Darkfield does not have a lot
of application in histopathology, but is a wonderful tool when dealing with
cell suspensions, fluids, etc. The classical use for it is searching for
spirochetes in exudates from syphilitic sores.
There are two ways to achieve darkfield illumination.
The best results are obtained by using a dedicated darkfield condenser that
replaces the regular substage condenser on your microscope. These are quite
costly due to the optics involved; circular parabolic reflective surfaces
are expensive to produce.
A simple and inexpensive (but quite effective) alternative is to simply
block the path of the light rays that enter the central portion of the
objective lens. The easiest way to do this is by placing a small opaque disk
on the upper surface of the substage condenser lens. A disc of thin black
card works quite well.
If you have a "shelf" below the stage, this is probably a slider mechanism
that contains a circular stop that can be moved into the light path to block
the central rays. This is a better system than the disc of card, but way
less expensive than the darkfield condenser. If the slider is removable, you
should be able to see a hole with a solid central portion. You may well find
that you get much better results with a one specific objective than with
others. This will be due to the diameter of the light cone, working
distance, apertures, etc.
The specimen does not need to be stained or coloured, just coverslipped.
Hope this helps, let me know if you need any more info.
Paul Bradbury, FIMLS, ART
Kamloops, BC Canada
----Original Message Follows----
From: Charles Read
To: histonet@pathology.swmed.edu
Subject: Help with DARKFIELD MICROSCOPY
Date: Mon, 27 Jan 2003 15:27:19 -0500
Hey everybody,
I own a LW Scientiic Observer IV (microscope that is) with brightfield
lighting. There is a 'tray' for lack of a better word, where you can put
colored lenses to color the light. On a microscope at school there is a
similar 'tray' that has a few different color lenses and a setting that puts
the scope into darkfield, my question is: Is darkfield lighting
accomplished with a lens? If so, where can I get a lens that would fit my
scope? Thanks in advance!!!!!!
-CR
Hi Charles,
There are two ways to achieve darkfield illumination.
The best results are obtained by using a dedicated darkfield condenser that
replaces the regular substage condenser on your microscope. These are quuite
costly due to the optics involved; circular parabolic reflective surfaces
are expensive to produce.
A simple and inexpensive (but quite effective) alternative is to simply
block the path of the light rays that enter the central portion of the
objective lens. The easiest way to do this is by placing a small opaque disk
on the upper surface of the substage condenser lens. A disc of thin black
card works quite well.
If you have a "shelf" below the stage, this is probably a slider mechanism
that contains a circular stop that can be moved into the light path to block
the central rays.
The principles of darkfield microscopy are dependent on blocking any light
rays that go directly into the bottom lens of the objective. The only light
rays that enter the objective are those that are deviated from their
original path by the specimen.
Paul Bradbury, FIMLS, ART
Kamloops, BC Canada
----Original Message Follows----
From: Charles Read
To: histonet@pathology.swmed.edu
Subject: Help with DARKFIELD MICROSCOPY
Date: Mon, 27 Jan 2003 15:27:19 -0500
Hey everybody,
I own a LW Scientiic Observer IV (microscope that is) with brightfield
lighting. There is a 'tray' for lack of a better word, where you can put
colored lenses to color the light. On a microscope at school there is a
similar 'tray' that has a few different color lenses and a setting that puts
the scope into darkfield, my question is: Is darkfield lighting
accomplished with a lens? If so, where can I get a lens that would fit my
scope? Thanks in advance!!!!!!
-CR
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