Re: Fwd: Re: Importance of the tube lens NA

Posted by Patrick Van Oostveldt on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Fwd-Re-Importance-of-the-tube-lens-NA-tp2436781p2450504.html

Dear,

I like to praticpate to this discussion in public because in concerns  
some basic set-up in the microscope.
I already send the following respons to Garbor, and I hope if I am  
wrong, somebody like to correct me.

Dear Gabor
The fact is that by approximation  F/D=1/2NA where F is the focal  
distance and D is the diameter. If the focal length ( related to  
magnification) is fixed you only can increase NA by increasing the  
diameter of the lens. Collection efficiency = (1-sqr(1-(NA/r)**2))/2  
with sqrt: function square root, r refractive index of the medium.  
Applying this formula indicate that based on pure geometry a decrease  
of the NA from 1.4 to 1.2 reduce collecting efficiency about 40%.
On the other hand we are limited by the standards.
Application of the RMS standard also defined as  ISO standard 8038, it  
is stated that the parfocal distance (= the distance between focal  
plane and the flange of the objective) of the objective is  45 mm, and  
the tube diameter is set to  about 20mm (rev Royal microscopy society  
handbook 15).

Decreasing linear magnification by a factor 5, will increase  
light/unit area by 25.

So these rules will advise you to use minimal magnification and  
maximal aperture.  High aperture and low magnification is only  
possible if your tube lens is close to the objective or if you reduce  
the infinity path of the system.

For that reason, NIKON and Mitutoyo left off  the RMS standard and now  
use a parfocal distance of 60mm and 95mm and a flange diameter of  
nearly 30mm. The benefit of that is an increased infinity space  
without the loss of intensity. Zeiss objectives, which are very  
similar to Olympus, have an infinity space between 110 and 130 mm.  
NIKON between 100 and 200mm.

So in the end, part of this question is also: should we leave the RMS  
standard?

Bye

Patrick Van Oostveldt


Quoting Mark Cannell <[hidden email]>:

> I have a hard time understanding this.  If overall system magnification
> is the same and the NA is set by the objective there is no way to
> increase throughput (see Lagrange invariant) -unless the original
> system had introduced losses such as an aperture stop somewhere in the
> system (the old tube lens?) -but why would one do that in the first
> place? All the tube lenses in infinity systems I've seen look big
> enough to pass the marginal ray. The highest efficiency is obtained by
> using prime focus but that is often too close to the microscope body
> for the camera chip. You then add a lens (or two) to relay the image to
> the camera. Now losses occur if these lenses don't capture the marginal
> ray for the FOV (seen in severe case as vignetting). So perhaps it's
> not the tube lens but a relay lens? From their description it sounds
> like they've made prime focus available to the camera and thereby saved
> a few optical losses.  But shame on them if their  other fluorescence
> microscopes throw light away!
>
> Regards Mark
>>
>>> 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
>>> *7101 Royal Glen Trail, Suite A
>>> McKinney TX 75070
>>> *P: *(972)924-5310  *Skype: *fostermme
>>> *W: *www.MicroscopyEducation.com <http://www.microscopyeducation.com/>
>>>
>>> *NEWS! Visit the NEW and IMPROVED www.MicroscopyEducation.com  
>>> <http://www.microscopyeducation.com/>! And don't forget:  MME is  
>>> now scheduling customized, on-site courses through March 2009.    
>>> Call me for a free assessment and quote.
>>>
>>> *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



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