🚀 go-pugleaf

In article <3093@ariel.its.unimelb.EDU.AU>, mark@sunrise.sli.unimelb.edu.au
(Mark Shortis) writes:
> Would anyone like to comment on the problem of calibrating the
> radiometry of individual pixels in a CCD array?  
> 
> Essentially the problem is that for applications which require accurate 
> radiometry (ie accurate grey levels are needed) the response of each pixel 
> becomes important.
> 
> This is the classic issue of "flat fielding".  That is, if you supply a
> perfectly uniform input, the response of every pixel can be calibrated.
> Do this at two light levels and an offset and gain for each pixel can be
> computed.

An alternative approach is to produce a calibration image for each
exposure which will not only remove the effects of CCD response
but nonuniform illumination.   

In my case, I have to deal with nonuniformities in the light box
when digitizing x-ray radiographs.  Thus, for each acquisition
session, I digitize the lightbox with no film and a neutral density
filter on the lens.  A calibrated image is then produced by dividing 
the image of interest by the calibration image (after bias and
dark current subtraction from both images).

In any case, if your illumination is constant over your measurements
and calibrations, there is no need for it to be perfectly uniform.
The illumination nonuniformities will be removed along with the CCD
nonuniformities.
 
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|Dick Wallingford      | Internet: dickw@iastate.edu                 |
|Center for NDE        | Packet  : wd0anb@n0an.ia.usa.na             |
|Iowa State University | "This river don't go to Aintree" - S.H.     | 
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