I want to perform a darkfield microscopy, but the commercial darkfield microscopy is not suitable because I want a collimated illuminating light rather than a condensed light. I want to use a filter in the focal plane of the objective lens to stop the ordinary light. Can anyone suggest the configuration or where I can buy them? I especially need the light stop and light filter.
Hm, I'm not so sure about placing a filter in the focal plane. Why? Aberrations!...Inevitably, you'd end up with at least a part of the filter in a converging or diverging beam which will lead to aberrations. I'd place it somewhere in the collimated beam.
P.S. I hope I understood you correctly, and you were talking about a filter indeed and not a pinhole.
If you want to illuminate with a collimated beam, one of the options would be to place a suitable (!) collimator in the focal plane. Then, you'll get what you want from its output. Essentially, all the components will be same as in the picture above but with an additional collimator.
Model this problem as a ray tracing one, i.e. converting a diverging beam into the collimated beam. Then, you can also achieve it with discrete optical components. (Perhaps, you have your reasons of having such illumination conditions in your system...)
Hi Chao,
To perform darkfield, it is essential to collect only scattered light and avoid the illumination light. Illuminating light at angles higher than the NA of the objective is the best way to go. What I find puzzling is that if replace that with a collimated beam how are you going to separate the illumination light from the scattered. Being said here is what I think that can help: if your sample is on the surface you can always do TIRF like illumination. All what you need to do is to focus your illumination on the back focal plane at an offset from the optical axis. The other option (which I think it is commercialized now) is to couple the light directly to the covers glass at an angle larger than the critical angle.
Hope this helps
@Maria Dienerowitz
the figure 2(b) configuration in Expanding the toolbox for nanoparticle trapping and spectroscopy with holographic optical tweezers is just what i need. your work is impressive and there are some common interests between our work. i want to use the high sensitivity of dark microscopy to observe the gas plume. unfortunately i am new in this field and i have a lot to learn. so, i will be very thankful if any help can be provided. where can i buy the center stop? does the shape of the stop matter(circular or rectagular)? could you please offer some link? like thorlabs or edmund optics inc. thank you !
@Maria Dienerowitz
so, if I use the light trace of the figure 2(b) configuration in Expanding the toolbox for nanoparticle trapping and spectroscopy with holographic optical tweezers, and I put a beam splitter behind the objective lens after sample. then the reflected light is passing through the same light trace as the original transmitted one except that no stop is set in the fourier plane. so, will i get the bright field and dark field image at the same time?
@ Christian,
I agree with what you wrote but I think in this case both actions will give you the same result. I mean, it is the higher diffraction orders that shape the details of the image. Yet, the high intensity zero order can overwhelm them. Blocking the zero order will result in a dark background with bright outlines. This is the same result as traditional transmission darkfield.
Also, as far as I understand, usually, the illumination light in darkfield is collimated but not on axis. Can you comment on that?
Regards
Mohammed
@Michael,
Great answer. Still, I would say that the number of cancelled orders depends on the NA of the illumination, NA of the objective and the size of the disk in the back focal plane. Thus, I think the image that is produced is a mix of both.
Mohammed
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