Re: tube lens and spherical aberration

Posted by Shalin Mehta on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Southeastern-Microscopy-Society-meeting-tp4640594p4643682.html

Hi Michael,

I think your misgivings are true. If I understood correctly - you are wondering whether illumination-side spherical aberration is different from imaging-side spherical aberration. I think they are different. Plus they do different things when one uses confocal scanning geometry vs. widefield geometry.  Perhaps different sources of spherical aberration can be explained thus:

- The case considered in most of the microscopy text is of RI mismatch between embedding and immersion media in confocal imaging. The mismatch causes the focused illumination spot to be worse than the diffraction-limited hour-glass, reducing the signal that can be collected by PMT after the pin-hole. So in confocal, spherical aberration = bad SNR. Plus, the signal appears to be coming from the wrong place as spherical aberration shifts the peak intensity in the focal volume.

Since in confocal, both illumination and detection perform imaging and we pick-up the intensity in the center of the imaging volume by using a pin-hole --- I wonder what does the PSF of spherically aberrated confocal look like. I think there are papers by Sheppard and Min Gu that need to be consulted. 

- In widefield imaging, one uses Kohler type of arrangement to bring light from mercury filament onto the specimen. Otherwise, one will see filament imprinted on the specimen. The source of widefield illumination being incohernet (unlike coherent laser illumination of confocal), the aberrations seen by it on the way to specimen are unimportant. The PSF of the widefield system is strictly that of the imaging path - the 'mere' use of illumination is to excite fluorophores uniformly. 

Consequently, as you say, the above RI mismatch will give rise to converging or diverging wavefront from the focal volume between the tube-lens and the objective *in the detection path*. The imaging path's PSF will be therefore have distorted look like the standard spherical aberrated PSF that we see in Born and Wolf or Hecht.

best

Shalin

On Sat, Feb 27, 2010 at 7:52 AM, MODEL, MICHAEL <[hidden email]> wrote:

Andreas, thanks for your response. I understand that the path can be reversed for a ray but it seems to me that the size of the blur in the image will depend, for example, on the distance between the objective and the tube lens. Which is not part of the standard formula.

 

Mike

 

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From: Confocal Microscopy List [[hidden email]] On Behalf Of Andreas Bruckbauer [[hidden email]]
Sent: Friday, February 26, 2010 5:06 PM

To: [hidden email]

Subject: Re: tube lens and spherical aberration

Just go the other way, from a perfect spot in the image plane to the diffuse spot in the object plane, then you are back to the standard situation. You can always reverse the light path in linear optics.

best wishes

Andreas

-----Original Message-----
From: MODEL, MICHAEL <[hidden email]>
To: [hidden email]
Sent: Fri, 26 Feb 2010 20:54
Subject: tube lens and spherical aberration

(I apologize if this is a second post, I got some strange errors from the first 

one)



Dear List



It's not a confocal question, it's strictly a wide-field question... When people 

talk about spherical aberration they typically start with a parallel beam 

falling on an objective, and then, for whatever reason, the objective fails to 

bring all the rays into a common focus. In wide-field, one has a luminous spot 

in the object space, and the effect of spherical aberration would be a failure 

of the objective to collect all the rays into a single parallel beam. Some rays 

will form a converging (or diverging) cone instead. Then, this cone will be 

received (I suspect that in the case of a diverging cone, some light may even 

get lost on its way) by a tube lens and form a blurry spot on the image plane. 

My question is, Is this situation really equivalent to the standard one 

considered in all books? (I suspect, it's not because even the distance to the 

tube lens should make a difference). But I would be very interested in the 

opinion of those who understand optics better than I do. Thanks!



Mike Model



Michael Model, Ph.D.

Confocal Microscopy,

Dpt Biological Sciences,

1275 University Esplanade,

Kent State University, Kent, OH 44242

tel. [phone]