----- Original Message ----- From: "Richard M. Koolish" <kool...@bbn.com>
> I'm not sure I would describe the workings of a zone plate this way. It > doesn't matter where in the zone plate (or pinhole) the light passes > through. It doesn't bend at one place and not in another. The way you > think about a zone plate or pinhole is that every clear point acts like a > spherical wave is generated there. Then from any point in the image plane, > you add up all the contributions to the intensity at that position from all > the individual contributing points of the zone plate or pinhole. Some > waves will be in phase and reinforce each other, and some waves will be out > of phase and destructively interfere. In the "simple" case of a pinhole, > you discover, that a point source of light in a subject does not generate a > true point of light at the film or even a simple disk, as if a beam of > light came through. Instead, you get the clasic diffraction pattern of a > point, i.e. a central disk (the Airy disk) surrounded by diffraction rings. > Note that every aperture produces diffraction. Even the most perfect lens > can't focus a point source into an image point. Normally you don't see > this because the diffraction pattern is so small. An f/10 lens produces > an Airy disk of only .0134 mm for green light. On the other hand, if you > place a very small pinhole, say 50 microns (.05 mm) 100 mm away from the > film and shine a red laser pointer at it, you will get a diffraction disk > of about 3 mm in diameter. > > The zone plate has a lot of chromatic aberration. That is, it can't > focus different colors of light at the same place. That's what causes > the characteristic glow around bright objects in a zone plate photograph. > One color may get focussed sharply and the others will get spread out > into disks of various sizes around the sharp point. I can't speak to the > issue of how much this changes with the number of rings in the zone plate > since I haven't done any research on this. It would be an interesting > and fairly easy experiment to try. > > Dick Koolish (kool...@bbn.com) > > _______________________________________________ > Pinhole-Discussion mailing list > Pinhole-Discussion@p at ??????? > unsubscribe or change your account at > http://www.???????/discussion/ >
