Re: Help with DARKFIELD MICROSCOPY

From:Paul Bradbury


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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