Re: Calibration

From: Andy Resnick (axr67_at_op.cwru.edu)
Date: 12/29/04 

Date: Wed, 29 Dec 2004 09:03:44 -0500

Kristian Ukkonen wrote:

>
> Andy Resnick wrote:
>
>> If, on the other hand, you are acquiring images and need to convert
>> pixels into microns (or some such thing), you need an image of a
>> calibrated ruling. Rulings can be cheap or expensive, depending on
>> the exactitude you need. Again, Edmund carries cheap ones.
>
>
> Out of curiosity, if you happen to know of these systems.
>
> When you have a PC+camera+microscope, and calibrate the measurement
> (for example, distance from point A to point B in image of camera
> at PC software) with a calibrated ruling. When "nothing" changes
> (same microscope, objectives etc.), how often do you re-calibrate - or
> is the calibration ok for 10+ years? Any explanation for the period
> or lack of need of recalibration?
>
> For most electrical measurement devices, you recalibrate every
> year or whatever, and the accuracy is stated as "A% for 24hours,
> B% for 1month, C% for 1 year from calibration date". It seems there
> is no similar statement of accurace of measurements for microscopes..

Interesting question.

Let's also assume the camera and PC are constants. I think that the main
source of error in this case is the user. Since the image will have
some blur, which pixel the user decides is the edge of the micrometer
bar is the source of some variation. Also, there will be some variation
due to non-perfect alignment of the micrometer to the pixel array in the
camera. After that, I would think there are small errors introduced due
to temperature fluctuations (thermal expansion of the micrometer, slight
changes in the optical path due to thermal expansion of the lenses in
the objectives, etc.

Aside from things like bulb output, camera electronics and motorized
stage wear and tear, there probably are sources of long-term drift in a
microscope, but I don't know what they are.

Personally, I rarely re-calibrate. Mostly, it's because our work
doesn't require extremely high levels of precision. If a particular
experiment calls for high precision, I'll take a few "calibration"
images just prior to and just after the experiment as a 'best practice'.

Are you asking for a reason, or idle curiosity? 

-- 
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
CWRU School of Medicine
tanspose 'op' for mail

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