'Making Darkfield Illumination is easy'

 by Mike Samworth and Wim van Egmond
The advantage of darkfield illumination is that you can see details that are normally not resolved by the microscopes objective. You can't see the actual detail but because it reflects light you can see it. A nice analogy here is that of dust in a room. In a well lit room you do not see the very small dust particles. However, if the lights go out, a beam of light from an acute angle makes these same particles visible. Besides the optical advantages darkfield illumination is very beautiful and gives an almost science fiction like image.

 

 A remarkable symbiotic diatom colony, photographed by Mike Samworth.

It is very easy to make darkfield illumination yourself. What you have to do is place an opaque round stop in the condensor. An easy way is to cut a piece of black paper and put it on a filter in your filterholder. You can put the stop on a piece of clear acetate sheet. You can even try to draw the stop on it with black paint. The most important thing is to have it big enough to stop all light going directly into the objective. Only the light that is reflected by the objects in the sample reaches the objective then. Stronger objectives are more difficult because their NA is often too high. The NA of your condensor should always be higher then the NA of the objective.

 If patch-stops of 8, 10, 12 and 15mm are made you can't go wrong really. For objectives of around x10 the middle sizes prove best.

If you like to make the patchstop as precise as possible: The best way is to set up as normal (brightfield), remove the eyepiece and close/open the substage iris until it is *just* visible. Then, either bending your neck over double, or carefully removing the condenser, measure the diameter of the iris diaphragm as it is now set. A pair of calipers is useful here. This diameter is that for the patch stop. Very often, to be on the safe side it is best to add about 10% to this figure, this avoids leakage, especially if you have no means of centering the stop in the filter holder. If you have a phase contrast condensor, the largest phase contrast annuli often make excellent patch stops for darkfield!

The real connoisseurs must have recognized the skills of Klaus Kemp in the arranged (cleaned) diatom slide photographed by Mike Samworth.

 

 

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Copyright All Material: Mike Samworth and Wim van Egmond