Fwd: Re: Importance of the tube lens NA

Posted by Barbara Foster on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Fwd-Re-Importance-of-the-tube-lens-NA-tp2436781.html

Date: Fri, 06 Mar 2009 10:25:52 -0600
To: [hidden email], Confocal Microscopy List <[hidden email]>
From: Barbara Foster <[hidden email]>
Subject: Re: Importance of the tube lens NA

Hi, Gabor

This is an interesting concept.  If you use the expanded version of the Rayleigh criterion [1.22 lamda/(NA obj + NA cond)] and also take a look at the impact on the diffraction image, a number of things emerge which might answer your question.

For example:  if you use a simple ruled grating aligned N-S on your stage, the diffraction pattern will be a series of horizontal dots (emails don't permit a full discussion of the physics... see any basic physics review book).  The bright, central zero order spot carries information about the background.  The other spots carry information about orientation, spacing, and edge information.  To convey spacing and orientation from the object to the image, the receiving lens (the one that FORMS the diffraction pattern) only needs to capture 2 adjacent diffraction spots.  However, the larger the NA, the greater the ability to capture neighboring spots (ex: moving from the center of the pattern to the right: 0, +1, +2, etc.).  The more spots collected, the greater the edge definition.  Also, the greater the NA, the greater the summed intensity of the entire pattern.  (Again, emails don't permit much discussion of all the physics).

It is not clear to me why putting the tube lens near the objective is important... It is more likely that they have put the SAMPLE near the objective, setting up the condition for infinity corrected optics. However, there is a rule in physics that says that intensity falls off as the square of the distance, so perhaps putting the tube lens nearer to the objective allows them to maximize intensity collected from the diffraction pattern.  If you are using a high NA tube lens, by default, you need to move the detector closer, since the distance to the image plane will be shorter. 

Finally, even though you did not mention it, a higher NA lens is often engineered with greater aberration correction.  That extra engineering is also likely to increase the throughput, enabling the observer to detect more. 

All of this might add up to 10x improvement in detection and, actually, resolution and edge information.  You've piqued my curiousity... Looks like time for a chat with Olympu to learn more!

Hope this was helpful,
Barbara Foster

Barbara Foster, President and Sr. Consultant

Microscopy/Microscopy Education
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At 08:19 PM 3/5/2009, you wrote:

Dear All,

We just had a presentation from Olympus about their LV200 bio-luminescence microscope. As they claim (and demonstrated with some images) this system is significantly (approx 10x) times more sensitive then a conventional microscope used with the same objective and camera (and pixel size/resolution). Olympus argues that the "secret" is that they put the tube lens close to the objective (probably less important) and put the camera very close to the tube lens meaning that they use a high-numerical aperture tube lens. Now I simply don't understand why this should result in a significantly higher detection intensity (and the Olympus representative was also unable to give a detailed explanation).
Does anyone of you have an idea why a high NA tube lens would be advantageous? And if this is so nice - why it is not applied in conventional microscopes?


Thanks     Gabor