🚀 go-pugleaf

In article <3093@ariel.its.unimelb.EDU.AU> mark@sunrise.UUCP (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.
>
>How do you create such a flat field?  Perfectly uniform light sources
>don't grow on trees!
>

The way I do it is to put a light diffuser (a piece of white transluscent 
glass or plastic) *between the lens and the ccd*.  Then, point the lens at
a fairly uniformly-lit wall, and put it way out of focus.  If you can't put
the diffuser after the lens, put it right before the lens, and make sure to
focus far from the diffuser.  (The problem with doing it this way is that 
the lens will introduce some non-uniformity, though this should be at very
low spatial frequency, which may be acceptable to you.)

Another way to go is to remove the lens completely, but you can only do this
if the lens and shutter are separable.

-Stephen Blackstock
 Leaf Systems, Inc.