Archived posting to the Leica Users Group, 2001/10/17
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]On Wednesday, October 17, 2001, at 03:01 AM, Leica Users digest wrote: > Date: Tue, 16 Oct 2001 18:28:54 -0700 > From: Jim Brick <jim@brick.org> > Subject: [Leica] RE: Re: 24x36 CCD > Message-ID: <5.1.0.14.2.20011016182022.032f0488@pop.alink.net> > References: <5.1.0.14.2.20011016092630.032ea9a8@pop.alink.net> > > I cannot say for sure. But my educated guess is: > > The scanning backs no as single line CCD's can be made very dense, eg; > Leica S1, better light, etc... > > Instant (non scanning) such as Phase 1, (can't remember the others) I > suspect yes. Since you are using the center of the lens (small > CCD) you are > using the best part and the lens frequency would cause fringing, > aliasing, > etc., unless cut to a frequency lower than the lens. Even true > with very > large CCD's as they cannot be as dense as smaller CCD's. Hm. I am skeptical, Jim. I certainly know of no case where a serious optical microscopist has used any kind of "frequency cutoff" filter when projecting an image from a microscope objective onto a scientific-grade CCD. We do generally correct for the overall resolution of a given lens - we try to match the objective's Rayleigh limit to the apparent CCD pixel size - but this is only to avoid the loss of high-frequency data, and we commonly bin 2X2 or 3x3 on the CCD with only a linear loss of spatial resolution. Certainly I'm not aware of artifacts other than linear resolution loss that arise through the use of less dense arrays. As for the divergence issue, I could see that being much more serious. Many of the really hot CCD's on the market now, especially the Sonys, use a tiny plastic microlens (not a fiber-optic) over each well to increase the effective collection area (and thus the apparent quantum efficiency) of the wells. Rays hitting pixels at the corners would come in at acute angles, and therefore be more likely to be reflected and scattered instead of refracted into the well. This would result in both loss of information and increased noise. The presence of a stepdown lens in the body could solve this problem AND protect the CCD surface from contamination. I know that people will screab about additional glass, but they are not screaming about the additional glass required when they use projectors and enlargers - a more serious problem! Finally, another concern is the full well capacity of the sensor wells. The smaller they are, the smaller their dynamic range. There's a tradeoff between spatial resolution and intrascene dynamic range, and I'm not certain how much that tradeoff can be improved upon without radically new CCD designs. But we should not discount how fast the new CMOS chips might improve. Compared to CCDs which are apparently a pretty mature technology, CMOS is just in its infancy. I for one would consider trading some starting resolution for the degradation that inevitably occurs during even the the best analog inaging chain (film to enlarging or scanning). I expect that our Leica optics would still shine, paarticularly at wide apertures. Alexey Merz Department of Biochemistry Dartmouth Medical School, Hanover NH 03755-3844 alexey@webcom.com | alexey@dartmouth.edu 603-650-1702 | http://www.webcom.com/alexey/ - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html