Re: How to measure the actual numerical aperture of a microscope objective?

Posted by Guy Cox on
URL: http://confocal-microscopy-list.275.s1.nabble.com/How-to-measure-objective-transmission-curves-tp590172p590198.html

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In principle it's remarkably easy to measure the NA of an
objective.  We get students to do it with nothing more
sophisticated than an old microscope, a board marked
in degrees, and a piece of string.
 
The idea is that if you look through an objective using a
phase telescope, you are imaging the back focal plane
which is where the lens will form an image of an object
at infinity.  So if the lens has an unobstructed view (which
of course you have to arrange) you'll see an image of the
room around you.  
 
The edge of what you can see (it will be quite a fish-eye
view) is determined by the maximum acceptance angle
of the objective, and with a suitable target you can measure
this.  With your NA 1.45 lens it should be 73 degrees from
the optic axis (=straight ahead).
 
Remarkably unsophisticated - and surprisingly accurate.
 
                                                    Guy  
 
 
 
 
 


Optical Imaging Techniques in Cell Biology
by Guy Cox    CRC Press / Taylor & Francis
    http://www.guycox.com/optical.htm
______________________________________________
Associate Professor Guy Cox, MA, DPhil(Oxon)
Electron Microscope Unit, Madsen Building F09,
University of Sydney, NSW 2006
______________________________________________
Phone [phone]     Fax [phone]
Mobile [phone]
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________________________________

From: Confocal Microscopy List [mailto:[hidden email]] On Behalf Of John Oreopoulos
Sent: Tuesday, 11 September 2007 6:29 AM
To: [hidden email]
Subject: How to measure the actual numerical aperture of a microscope objective?


Search the CONFOCAL archive at http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal Hello all again,


In my research, I am now faced with a situation where I need to obtain an actual measurement of the numerical aperture (NA) of an Olympus oil immersion objective. Can anyone tell me or point me to a reference that explains a simple method to determine the actual NA of a microscope objective? If the measurement is difficult and not simple (ie requiring sophisticated optical instruments and opto-mechanical apparatus), can someone tell me how reliable the number written on the barrel of the objective is or what it's actual uncertainty is?
The NA written on my objective is 1.45. How accurate is this number?

Thanks in advance for any help!


John Oreopoulos, BSc,

PhD Candidate

University of Toronto

Institute For Biomaterials and Biomedical Engineering

Centre For Studies in Molecular Imaging




Tel: W:416-946-5022




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