[3dem] Current measurement with viewing screen

[Liang Jin]

Hi, Fred

In our camera, we have an integrated Faraday plate to measure electron beam current. Therefore, I got a chance to calibrate the screen readings of many different microscopes including FEI and JEOL ones. Unfortunately what I have found out is that the calibration factor is different for different scopes, ranging from 0.6 to 0.9. The only accurate way is like you mentioned to use a Faraday cup and calibrate your instruments. Maybe you can simply request FEI to do a calibration? 

Another easy way of calibrating the beam current is to use your camera. If you know the exact count per electron, you would be able to take an image and use the mean count to estimate the actual electron dose. This method relies on an accurate measure of the sensitivity of your camera in terms of count/electron. The conversion factor for fiber-coupled cameras is determined by the scintillator and the CCD paramters. Given that every scintillator is unique, you will have to check with your camera manufacture to figure out if there is a number measured on your camera.

Best regards, 

-Liang

Dr. Liang Jin
Direct Electron, LP (http://www.directelectron.com/)
13240 Evening Creek Drive S., Ste. 311
San Diego, CA 92128
Tel: (858) 384-0291  x104
Email: ljin at directelectron.com

-----Original Message-----
From: "Bob Grassucci" [rg2502 at columbia.edu]
Date: 11/09/2010 09:16 AM
To: "Fred Sigworth" <fred.sigworth at yale.edu>, 3dem at ncmir.ucsd.edu
Subject: Re: [3dem] Current measurement with viewing screen

Hi Fred,
We have been using a BS factor of 0.85at 200 kV and 0.75 for 300 kV.  It 
can be set so that it is used in low dose  if you are comfortable 
editing the registry.by the following settings.
regedt32
Hkeylocalmachine
     software
         FEI
             Lowdose
                 BS factor    200 0.85
                                     300 0.75
After you do this the low dose options tab should have the correct dose 
readings,  I am told that this is routinely done with the newer scopes 
but was not early on.  Of course a faraday cup is the most accurate way 
to go but not everyone has access to one.  I hope this helps.
Bob


On 11/9/2010 12:01 PM, Fred Sigworth wrote:
> To calibrate our CCD camera we have been using the viewing screen to measure the beam current in our Tecnai F20.  Of course this measurement is in error due to backscattering, secondary electrons, etc. and the right way is to use a Faraday cup.  But, has anyone compared the value from a Faraday cup with the screen current?  If the conversion factor is known, even approximately, that would help us a lot.
>
> Fred Sigworth
>
> Fred J. Sigworth
> Department of Cellular and Molecular Physiology
> Department of Biomedical Engineering
> Yale University
> New Haven, CT  06520
> tel.  203 785 5773
> fax 203 785 4951
> fred.sigworth at yale.edu
>
>
>
> _______________________________________________
> 3dem mailing list
> 3dem at ncmir.ucsd.edu
> https://mail.ncmir.ucsd.edu/mailman/listinfo/3dem
>
>


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