Dark/bright field question

kenw232 · Post by **kenw232** » Fri Jul 23, 2010 7:57 pm

these are pics from fpelectronica's latest vid. Is the first/black image a "darkfield" screenshot and the second a bright field? And darkfield seems to show more colour correct?

fpelectronica · Post by **fpelectronica** » Sat Jul 24, 2010 10:28 am

Thanks for your interesting.
I get Darkfield with phase contrast condenser.
Second image was taken with a 4x objective and low quality, that's why colours and definition doens't seems true.
Unfortunately is very difficult to find a darkphase for Nikon Fluophot, but I have in mind darkphase pics with the phase contrast get a very truly colours.
Best regards,

rjlittlefield · Post by **rjlittlefield** » Sat Jul 24, 2010 11:42 am

kenw232, let me see if I can fill in some gaps here.

The key concept of darkfield is that light is beamed in only from the sides, so that it does not enter the objective unless it is somehow deflected by the subject. Light like this can be provided in several different ways. The cheapest is to use what's called a "darkfield stop" in conjunction with an ordinary brightfield condenser or a phase contrast condenser. There are also special darkfield condensers that have an arrangement of lenses and mirrors that is different from ordinary condensers. See HERE for gobs of information.

fpelectronica, I am guessing that you added a darkfield stop to your phase contrast condenser. Is that correct?

Depending on the subject, colors often appear more vivid in darkfield illumination, but this is not always the case.

You may also be interested in a technique called "Rheinberg illumination" that can add colors to an otherwise colorless specimen. Rheinberg illumination is sort of a generalization of darkfield, in which the opaque darkfield stop is replaced by a colored filter. The side light may be filtered also to be a contrasting color. See HERE for more details.

--Rik

kenw232 · Post by **kenw232** » Sat Jul 24, 2010 12:47 pm

Are bright field microscopes available that will do darkfield too with a darkfield condenser? Or should I get a specialized dark filed microscope only if I wanted dark field?

Thanks.

rjlittlefield · Post by **rjlittlefield** » Sat Jul 24, 2010 1:54 pm

Are bright field microscopes available that will do darkfield too with a darkfield condenser?

Yes, many such microscopes are available.

Some further background... At lower magnifications (objectives of 10X and below), the very same image-forming optics are commonly used for brightfield and darkfield. Only the illumination changes. At higher magnifications (40X and above), objectives commonly used for brightfield may not work well for darkfield because their apertures are so wide that it's difficult to keep unwanted light from entering. But even there, getting good darkfield would be just a matter of swapping in a matching condenser and objective. The rest of the scope remains unchanged.

--Rik

fpelectronica · Post by **fpelectronica** » Sun Jul 25, 2010 3:47 am

Hello rik:

Darkfield I get by the ring phase contrast condenser for low magnification (4x & 10x).
My microscope doesn't have "filter holder" in condenser, that's why It's very difficult put any filter for Rheinberg illumination. I have used this method with a Lomo microscope with quite nice results, like this video:

http://www.youtube.com/watch?v=CTJMfasDGow

It's very interesting to do homemade Rheinberg filters with several colours.

Best regards,

-Francisco.