Jan van Wijk wrote: > I probably need to dig into the real construction of CCD's, but this > description of how the 'pixels' read light intensity from a real > 'focal plane' in front of them contradicts my own interpretation!
> I really don't think it works that way, I think the CCD 'pixels' are very > small area's on the CCD surface that build up electric potential when > fotons (light) hits them. The fotons could come from any direction, but > AFAIK the pixels are 'shielded' from each other in a grid pattern, wich > also limits the angle of the incoming light to reach the actual sensor > surface ... > I tend to think of them as small buckets where the 'sensor' is at the > bottom of the bucket. (could be a too simplistic view :-) Hi Jan, I think we're really talking about the same thing here, and I think my conceptual view is essentially identical to the one you describe above. I may not have done a very good job of explaining it. Your analogy of the "electron bucket" is a very good one, and it's the analogy typically used to explain in simple terms how a CCD works -- maybe a bucket full of tranparent doped silicon is even better for the current discussion. The buckets are formed as layers of semiconductor material are sequentially deposited onto the silicon substrate. Now, how tall are these buckets; that is, how thick are the coatings that are deposited? And where does one place the focal plane with respect to these coatings? In other words, is the ideal location for the focal plane the top of the bucket, the bottom of the bucket, somewhere in between, or somewhere else? I'm not sure -- would be nice to hear some technical details, as Rob Studdert suggests. Also, I'm not sure if the pixel-to-pixel shielding is optical shielding or simply electrical shielding. At least for monochrome CCDs, the shielding may only be electrical, but for color CCDs, the shielding may very well be optical as well -- I'm just not sure. I've got a book at home on CCD imaging and CCD camera and electronics construction, written primarily from an astroimaging point of view -- maybe I'll have to dig that out and see if it sheds any light on the matter. Rob mentioned the potential problem of "light spill" or cross-talk between pixels. I've seen this commonly referred to as "blooming", and there are chip fab techniques which mitigate the potential for this problem. Chips fabricated in this manner are marketed as "anti-blooming". As I understand, blooming is not caused by incoming photons with trajectories far off of perpendicular. Rather, blooming is caused when a pixel's "bucket" is overfilled with electrons. The "overflowing bucket" spills electrons into adjacent pixels -- in the same column, IIRC. Blooming can be a problem, e.g., when one images very bright stars and very dim stars in the same field during astro-imaging. Anthony Farr's comments on the Philips chip were interesting -- especially his recollection about the apparent distance between the color mask and the actual sensor array. Sounds like the problem is not so much a fundamental problem of using large-area CCD arrays with 35mm lenses, but rather an issue with the construction of this particular Philips chip -- perhaps vignetting issues? Again, it will be interesting to see the technical references. Ralf, have you found anything yet? Bill Peifer Rochester, NY - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
