Infinity corrected lens - expanding before tube lens?

In summary: Also, in the optical image, you mentioned that there is a region that appears white, is this due to the laser light or the white light from the bottom?I'm not sure how you would check the beam alignment, but in the optical image, you would expect to see the sample reflected in the center of the image. The white light coming from the bottom should be attenuated by the dichroic filters, so only a small amount should be visible in the optical image.
  • #1
donroy81
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Hello folks, I am on a project that involves combining Raman microscopy + optical tweezers, and I made a setup over the past year. All of this is home built on minimal funds so its not the neatest of all setups. It involves an upright microscope (coupled to trapping laser) and collecting the back reflected Raman signal at 180o and also the optical image along the same axis.

To make things easy i converted to a NIKON infinity corrected 100x objective and added a tube lens. While the back reflected laser beam from the laser seems to be collimated when the sample is at focus, the white light (illuminated from the bottom) diverges very fast so I catch only 25% of l the light on my tube lens (which needs to be 200 mm away anyway). Should not the white light collected be the same size as the back aperture of the objective (8mm) : I need to catch all the light on 25mm dichroic filters (which eliminate the laser).

Why is the white light expanding out of the objective - Is something wrong? We are a spectroscopy group and not a microscopy one, so all this is pretty new to me and my research group. Any help is appreciated.
 
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  • #2
How are you using the white light? Is there a condenser (a lens that focuses the light onto the sample)?
 
  • #3
Hi Andy, thanks for replying. I have a 10x (.25NA)objective as a condenser, I was using a 1.25 NA condenser before with the same effect.
I know its not Kohler illumination (since the objective is 1.25 oil) but this is also what Thorlabs uses in their "optical trapping kit".
 
  • #4
I'm not surprised that the white light is diverging- it has to, in order for the objective to image objects that are off-center. Some of the diverging light may be 'spurious', in that it would normally be rejected by the tube lens- objectives have a limited field of view. Have you (without the trapping beam) simply looked at the image your setup produces?

I'm not sure how much control you have over the divergence angle (does it change between using the 0.25 NA and 1.25 NA condensers? Can you ring up a field stop?), so one solution may simply be to move the tube lens closer (or use a larger diameter tube lens). AFAIK, the tube lens on my Leica is about 100mm off the back aperture of the objective.
 
  • #5
Andy, sorry for the delay, I wanted to review everything once. I also went over your conversation with Kubikat. I would need to show you some pictures to explain. Slide1 is a closeup of the problem region.I have 1" optics, the filter and the mirrors before the tube lens. DSC09636 gives you another take on it. Everything was fine when I was using a 160mm objective and a complete Nachet, we decided that the infinity will help with the raman and hence this. 2" dichroics run too many $$. In anycase, I don't see why a parallel flux of light from an infinity objective with a back aperture would expand so fast to 2".

Additionally, if I may, I would like to know a few more things, for starters : how do you check that the trapping beam is on center of the back aperture of the objective and perfectly perpendicular to it?
 

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FAQ: Infinity corrected lens - expanding before tube lens?

1. What is an infinity corrected lens?

An infinity corrected lens is a type of microscope objective lens that is designed to produce a parallel beam of light, allowing for precise imaging of objects at high magnification. It is used in conjunction with a tube lens to create an intermediate image of the specimen.

2. How does an infinity corrected lens work?

An infinity corrected lens works by using a complex system of lenses to produce a parallel beam of light. This allows for more precise focusing and imaging of the specimen, as well as minimizing aberrations and distortions in the image.

3. Why does an infinity corrected lens need to expand before the tube lens?

The infinity corrected lens needs to expand before the tube lens in order to create an intermediate image of the specimen. This intermediate image is then magnified by the tube lens to produce the final image that is viewed by the user.

4. What are the advantages of using an infinity corrected lens?

Some advantages of using an infinity corrected lens include improved resolution, reduced aberrations and distortions, and the ability to work with a wider range of illumination techniques. It also allows for more flexibility in the setup of the microscope.

5. Are there any limitations to using an infinity corrected lens?

While infinity corrected lenses offer many advantages, they also have some limitations. These lenses are typically more expensive and complex than other types of lenses, and they may require specific tube lenses and other components in order to function properly. Additionally, they may not be suitable for all types of microscopy techniques.

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