Dear all. I have a problem when I want to build an optical transmission microscope with a halogen light source. The design is shown in figure 1.
The basic idea is a light from a halogen light source is used as illumination. Then an objectice with 4 times magnification and 0.1 NA is used as an condenser before the sample and another objective (50 times NA=0.55). Afterwards, a 2d slit is put on the intermediate image plane to select the area of interest. In the end a bean spillter is used to split the light into the Camera and Spectrometer to get both the image and spectra of the area that we are interesting.
However, when I start to buide the system, something strange to me is happened. The beam in Plane A (which marked by red in Figure 1) is not colimated: The size of the beam is continue increase fast, and I even can see a sharp image before the later lens. What is happened for the setup. Is it due to the magnificaiton or NA of the two objectives are not camparable?
Thanks for you help in advanced.
You would observe a collimated beam in plane A only if the light would emerge from a tiny spot in the plane of your sample, like when it would stem from a focussed laser. As you illuminate a large area, the range of field angles adds to the apparent divergence angle in plane A.
atan(field_angle)=~field_radius/focal_length
Only the object plane that is exactly in focus will be imaged to infinity by the objective lens. Object planes that are slightly more distant are imaged to finite distances behind the objective lens.
Generally one should illuminate the object with the same NA as is used for imaging, otherwise you cannot reach the full resolution, because there is no light in the higher ray angles that form the high NA of the imaging objective lens.
Did you use the concept of a Köhler illumination, which is recommended here? For that you image the light source, the lamp filament, into the back aperture of the illuminating objective, so that it fills this aperture. In other planes before one could restrict the size of the illuminated field.
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