Given the constraints pointed out so far I'd be tempted to try a simple 
DV video camera (for image quality), and a conical convex mirror on a 
stick. One would put the camera at one end of the tube shooting down it, 
and 'walk' the conical mirror all the length of the instrument. With 
adequate lighting it should be possible to reconstruct the object from 
image analysis which can be done later.

Peter
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