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u0362565
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Hi all,
I have a couple of questions about the process of kohler illumination (optimised condenser alignment) in brightfield microscopy and how this affects the final resolution attained.
Resolution concerns the minimum detectable distance between two objects and the closer to structures are, then a greater degree of diffraction occurs. Ultimately this means that higher resolution information is encoded by light rays that are at a more obtuse angle if the optical axis is zero degrees, hence the wider the acceptance angle of the lens (NA) the higher the resolution.
The light leaving the specimen can be seen as a mirror image of the light cone emanating from the condenser. If the condenser is closed down too far then the angles over which the objective lens can receive light is reduced and thus the full NA of the lens is not realized. Is that an acceptable way of thinking about this?
I'm slightly confused though by the opposite case, if the condenser aperture is opened too far. The acceptance angle range of the lens is fixed, so why does a wider light cone mean that the NA of the lens is not realized? Does closing the aperture force light into the lens, an over simplification i know..
The first image on this page gives a nice illustration of how I'm visualising the light cones emanating from the condenser and specimen.
http://www.olympusmicro.com/primer/anatomy/condensers.html
My second question concerns the field diaphragm. Why do you need a physical barrier to prevent some light from reaching the condenser? The crucial step is controlling the angles of light allowed to pass to the objective lens so why not let the condenser do all the work?
Thanks for your help!
Matt
I have a couple of questions about the process of kohler illumination (optimised condenser alignment) in brightfield microscopy and how this affects the final resolution attained.
Resolution concerns the minimum detectable distance between two objects and the closer to structures are, then a greater degree of diffraction occurs. Ultimately this means that higher resolution information is encoded by light rays that are at a more obtuse angle if the optical axis is zero degrees, hence the wider the acceptance angle of the lens (NA) the higher the resolution.
The light leaving the specimen can be seen as a mirror image of the light cone emanating from the condenser. If the condenser is closed down too far then the angles over which the objective lens can receive light is reduced and thus the full NA of the lens is not realized. Is that an acceptable way of thinking about this?
I'm slightly confused though by the opposite case, if the condenser aperture is opened too far. The acceptance angle range of the lens is fixed, so why does a wider light cone mean that the NA of the lens is not realized? Does closing the aperture force light into the lens, an over simplification i know..
The first image on this page gives a nice illustration of how I'm visualising the light cones emanating from the condenser and specimen.
http://www.olympusmicro.com/primer/anatomy/condensers.html
My second question concerns the field diaphragm. Why do you need a physical barrier to prevent some light from reaching the condenser? The crucial step is controlling the angles of light allowed to pass to the objective lens so why not let the condenser do all the work?
Thanks for your help!
Matt