Re: Smear

In message <ff2e7n$akl$1@xxxxxxxxxxxxxxxxxxxxxxxxx>, Manuel Kroehl <manuelkroehl@xxxxxx> writes
Hi,

I am trying to uderstand the cause of smear in CCD-cameras. Every
explanation I find tells me it is caused by either incident light or charge
overflow during the image shift. This tells me, the bright line should run
from the bright spot causing the smear to the border of the picture.
Contrary to transport direction.
But when I look for examples:
http://images.google.com/images?hl=en&q=smear+ccd
the bright line crosses the whole picture.
Could someone please help me to understand the discreprancy between my
understanding of the explanation and reality?

I agree this is not obvious at first. As you know, the smear that you are expecting to see occurs because the filled charge buckets, containing the exposed image, pass by pixel(s) which, for various reasons associated with the intense light exposure, cause leakage of extra charge into the passing charge buckets, increasing the exposure and causing the vertical smear.

The smear in the opposite direction, which you are not expecting to see, happens for a similar reason. But in this case, the leakage is into the passing EMPTY charge buckets, which will later be used to transport the NEXT image. So in this case the unwanted leakage occurs FIRST and the image charge is added LATER. Does that make sense, it's a bit tricky to describe without waving my arms around?

An important consequence is that the smear on one side therefore arises from leakage at a different time from the other side. There are schemes for smear compensation that measure the "extra" exposure in the covered pixels above or below the image. Since they are covered, they receive only the charge leaked into the empty buckets. This value is then subtracted from the entire smeared column. It can work quite well. But this gets upset if the bright light source is moving horizontally, as the smear above and below the bright point have occurred at different times, and so they differ in location and possibly intensity - making a simple single correction not so effective.

Hope this helps.

Kind regards,
--
Alan Hall PC and Embedded Systems Design
Databuzz, Ipswich, UK Digital Video Specialist
"Tsunami" Wavelet Compression Products
http://www.databuzz.co.uk
.