Re: To David Soderman: A Little Test

[rafe.bustin@xxxxxxxxxxx]

On 10 Apr 2002 at 20:14, Kennedy McEwen wrote:

> >[Kennedy McEwen:]
> >> Miscalibration would result in banding in either case.
> >
> >
> >Miscalibration could cause irregular streaking in a direction
> >perpendicular to the sensor array.
> >
> >It could not cause periodic banding in a direction parallel to
> >the sensor array -- not by any mechanism that I can fathom.
> >
> >If I'm wrong, please explain.
> >
> OK, but first it is certainly true that the type of miscalibration that 
> Paul & I were discussing would result in banding along the scan 
> direction, not along the sensor axis, but that is not the only type of 
> miscalibration.
> 
> In a three channel CCD and it possible that each channel is calibrated 
> to a different reference level - resulting in different brightness or 
> response for each channel.  This would give very noticeable bands 
> running along the direction of the sensor.

<snip>

I must be getting dense in my old age, as I still don't get it.

I trust you have seen the sample JPG on the Nikon site 
(URL in yesterday's post) or maybe Lawrence Smith's 
sample, posted on his website.  In either case, you'll see 
that the bands are widely spaced and quite periodic.  I don't 
have a banded image to refer to at the moment, but as 
I recall, the bands are much, much wider than the line 
gap of any CCD array that I know of (say, 8 or 12 
pixels.)  From vague memory, the bands were, say, 1 
mm apart, which is 160 pixels at 4000 dpi.

The TDI scheme you describe sounds quite a bit like the 
shingling that is used on inkjet printers to smooth the 
output (via integration of errors, more or less.)  But again, 
I still don't see how the "Hamrick" scenario comes into play 
to cause this banding.

There was a long thread on Tony Sleep's listserv about 
this topic when Lawrence Smith first reported the problem 
(and I subsequently reproduced it on my own scanner.)  
Much speculation, but none of it really definitive, as far as 
I could tell.

Here's my other problem with Ed's theory:  If it were so, 
Nikon should be able to offer a firmware upgrade to fix this 
problem -- but they have not.


rafe b.

-
Turn off HTML mail features. Keep quoted material short. Use accurate
subject lines. http://www.leben.com/lists for list instructions.