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Natural Science Forum / Physics / Optics / November 2007Koehler illumination in conoscopic observation
Hello, On a Nikon scope of type L-Ke the illumination is truly of the Koehler type. The filament of the tungsten light is imaged onto the diaphragm of the condenser lens and is centered and clearly visible at the back focal plane of the objective when viewed through a Barlow lens. Koehler illumination is considered to be an ad- vantage for most types of observation. It is of apparent disadvantage when one wishes to ob- serve the interference figures formed by bire- fringent crystals using the conoscopic form of observation in polarization microscopy. In that case the interference pattern formed by the crystal is superimposed upon the image of the lamp's filament causing difficulty in clearly seeing it.. Does anybody know of a way to get around this problem? One might think of a diffusing filter someplace in the path of illumination. If this is a feasible approach where would one place this filter and what kind should one use? Thanks for any suggestions GR. use conoscopic observation of the interference figures formed by birefrigent crystals in the manner > Hello, > [quoted text clipped - 28 lines] > interference figures formed by birefrigent crystals in the > manner In the good ole days, Kohler illumination was done with a lamp who's filament was about as wide as your finger. However your just not going to find any of those lamps. A ground glass filter should be used, I'll bet there is a place for one. Other wise I'd put it close to the lamp, if possible directly in front of the lamp. Then, technically, your using critical illumination, not Kohler but who cares? Thanks, Kevin Cunningham SMS Dear Kevin, Thank you for your helpful reply. There is only one easy way to insert a diffuser and that is in a pre- mounted swing-out filter holder between the field lens and the condenser. Any other approach would require mechanical modifications to the scope. A diffuser in the filter holder would work fine, provided it is fine grained and does not rob too much light. The filter holder accepts round glass 33mm in diameter. Do you have any idea where to get a suitable diffuser with this dimension? Thank you again GR. > > Hello, > > [quoted text clipped - 42 lines] > Kevin Cunningham > SMS Dear Kevin, I forgot to raise a point: Is opal glass or ground glass the choice for a diffuser? TIA GR. > Dear Kevin, > [quoted text clipped - 3 lines] > TIA > GR. It depends on how much diffusion and for what purpose. In this case ground glass would be better. This is one of those "Don't get me started" kinda things. If you want the next couple of hours filled up with discussion of who, what and how many I can do it, if you want to make your Nikon work, ground glass. Thanks, Kevin Cunningham SMS > I forgot to raise a point: Is opal glass or ground glass the > choice for a diffuser? http://www.luminitco.com/ > > I forgot to raise a point: Is opal glass or ground glass the > > choice for a diffuser? > > http://www.luminitco.com/ Dear Dr. Kinch, Most interesting! I have looked at Luminit's pdf on Light Shaping Diffusers (LSDs) and found a description of such a diffuser which exactly fits my needs. A pertinent image and text can be found under "Homogenized Light". The advantage of their diffusers seems to be that they cause very little loss of light. There is of course a question not answered by this brochure: Is the probably very high price of Luminit's LSDs justified by the gain they make in light throughput compared to cheap ground glass? Kevin is right: modern scopes all use light diffusers to hide the filament structure of the illuminating lamp. I wonder whether Luminit like diffusers are used in these scopes. Another interesting fact seems to be that centering screws for the condenser no longer are needed and provided. I wonder whether it has been found that the mounting can be manufactured accurately enough to avoid the centering mechanism altogether. Fact is that modern and expensive scopes no longer offer what is known as Koehler illumination. Thnanks for your interest. GR. > There is of course a question not answered by this brochure: > Is the probably very high price of Luminit's LSDs justified by > the gain they make in light throughput compared to cheap > ground glass? Well, if you just need one, they'll be glad to send you a free sample. They're used in a lot of consumer gadgets, so I don't think they're that expensive. > > There is of course a question not answered by this brochure: > > Is the probably very high price of Luminit's LSDs justified by [quoted text clipped - 5 lines] > They're used in a lot of consumer gadgets, so I don't think they're that > expensive. I hope you guessed right. I already asked them for a free sample. We shall see whether they are generous enough to part with a sample. GR.. > Dear Kevin, > [quoted text clipped - 59 lines] > > Kevin Cunningham > > SMS I'd contact Nikon first, they Olympus of course the microscope div. In the back of the Olympus price list in the old days there used to be tons of filters. They both use lots of ground glass filters and one should have the size you need. If not try an on line photo shop although, this is my personal view, you don't need less resolution in any camera. Thanks, Kevin Cunningham SMS <snip> > A ground glass filter should be used, I'll bet there is a place for > one. Other wise I'd put it close to the lamp, if possible directly in > front of the lamp. Then, technically, your using critical > illumination, not Kohler but who cares? I don't think that's correct- critical illumination is the Fourier transform of Kohler illumination: the filament is focused onto the sample plane rather than the entrance pupil of the condenser. And actually- that's the better solution. Using a ground glass plate is a kludge to destroy the image of the filament in the back aperture of the objective lens; it may work, but it isn't critical illumination.  SignatureAndrew Resnick, Ph.D. Department of Physiology and Biophysics Case Western Reserve University > <snip> > [quoted text clipped - 11 lines] > filament in the back aperture of the objective lens; it may work, but it > isn't critical illumination. Hello Andrew, Thanks for your clarification. It would be interesting to try critical illumination. Unfortunately I can't think of a way to place the light source or its image into the plane of the field diaphragm on my Nikon scope. Should there be a way I would be happy to try it. Please let me know. G.R. >><snip> >> [quoted text clipped - 19 lines] > Nikon scope. Should there be a way I would be happy to try it. > Please let me know. If your source has a focussing knob, you could try using that. In all honesty, critical illumination is rarely used, because when the image of the source is superimposed on the sample, viewing the sample means looking at a magnified image of the bulb filament. SignatureAndrew Resnick, Ph.D. Department of Physiology and Biophysics Case Western Reserve University > <snip> > [quoted text clipped - 16 lines] > Department of Physiology and Biophysics > Case Western Reserve University Dr. Resnick, Good points! I've been having this discussion over a few years with some buddies in Germany and Austria. I should have known better. What NoSpam and I were discussing was whats commonly called either critical or Kohler illumination. In fact no one uses either exactly. The usual start point is a ground glass assembly, take a look in the base of a modern microscope there are between 1 and 3 ground glasses. This is because designers know that if you use a specially designed lamp with a thick filament they are a pain to get, expensive and in a few years very few will be used. Its simpler to use ground glass and a lot cheaper. Thanks, Kevin Cunningham SMS >><snip> >> [quoted text clipped - 28 lines] > are a pain to get, expensive and in a few years very few will be > used. Its simpler to use ground glass and a lot cheaper. For home use, I sort of agree with you- a diffuser allows a hobbyist to quickly and easily quasi-align the microscope for even illumination. I think it's important to remember that Kohler/critical illumination is more than just simply focussing and centering the condenser field stop onto the sample- it means that all the aperture and field stops (condenser and objective) are at Fourier transform planes, and that the source is also at one of those planes. Which plane the source is at (field planes or pupil/aperture planes) determine if the microscope is "Kohler" or "critical". In the lab, I remove the diffuser (Leica uses some wierd diffraction element as opposed to a ground glass plate). Our users don't use conoscopic viewing, so images of the filament are not really a problem. An alternative solution I use is to pipe the halogen light in with a liquid-core fiber. SignatureAndrew Resnick, Ph.D. Department of Physiology and Biophysics Case Western Reserve University |
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