Lightlover: "please expand on 'rings in image of point source' " I'll give it a try. If I understood it better I would be able to give a better non-technical explanation.
In an ideal world, a camera taking a picture of a point source like a camera would give an image that is just a point. Thinks are not ideal though. Because of the wave nature of light and diffraction effects there is some blurring with the best camera lenses. The better the lens the better the image (the smaller the blurred spot and closer to a point). But, there are fundamental physical limits. A perfect f1.4 lens doesn't yield a perfect image, it just gives the best resolution an f1.4 lens can give. Your $500 Nikon lens won't give you the resolution of the 200 inch Mt. Palomar telescope. An f1.4 lens if it is very good will give a better image at f1.4 than at f16.
Now where do the rings come in. (Here's where I'll lose everybody including myself.)
The diffraction of light and interference of light causes the rings. In a very good lens, the image of a point source will be a single bright spot with very faint rings around it. The worse the lens (or reflector), the worse the the 'aberrations' and the more pronounced those rings become.
In a flashlight, the point source is the filament -- the region where the light is produced. This forms an image or is projected onto what you are looking at. There are complications because the filament isn't really a point source and you probabably don't have the lens or reflector focused perfectly onto what you're looking at.
Mathematically, in one dimension, a graph of the image of a point source would be
( sin(x)/x )**2 or (sin(x)/x)*(sin(x)/x). This oscillates with smaller oscillations as you look away from the center.
Hey, I'm sorry that's not very clear. I'll look for a better explanation on the web.
What I do for a living is computational microscopy: we characterize the blurring of the microscope in 3-D then computationally remove the blurring. Optics can get complicated.
NASA would have a bit of material on designing reflectors
Telescopes are a bit different from flashlights though. If you want to project a really tiny spot of light, put the bulb at the focus of the telescope. Flashlight reflector design is probably weird because you want to design an optimally bad reflector or lens to spread the light out nicely rather than put it all into the smallest possible spot.
Again, sorry I'm not clearer; it's late here (EST). Goodnight all.
Walter