On Thu, 24 Jun 1999, Arthur Carlson wrote: > Tony Moss wrote: > > > >I found a couple of web sites on signal mirrors. > > >http://www.equipped.com/signal.htm#ReflectionsOfLight > > >describes how they work and how to use them. I also made a sketch of how > > >I think it works. The attached bit map file shows this. > > > > I understood the principle immediately from your diagram. 'One picture > > is worth 10000 words' > > Well, I'm sorry, but I haven't caught on yet. Bob refers twice to an image > of the sun, but I don't see from his diagram how or where any image of the > sun should be visible. The web site explains, "The fireball is produced by > retrodirective reflection from small metalized glass spheres adhered to a > mesh grid or cloth disk with a center hole." I could understand it if the > reflective material consisted of needles or flakes perpendicular to the > plane of the mirror, but spheres don't seem to be direction specific enough. > Wouldn't they look bright regardless of the orientation of the mirror? > Please try to share your insight! > > --Art Carlson >
Well, as best I can make it out, the spheres act as lenses with focal length equal to their diameter. If they are embedded in a reflective background, an incoming bundle of parallel rays is focussed to a point (disregarding spherical aberration) on the rear surface, reflected back (must be along complementary paths!), and recollimated into a parallel bundle again - directly back along the entry path. From the diagram, a small part of the light from the sun passes through the hole in the silvered layer, reflects off the rear glass surface onto the beads, and returns to the same point on the rear surface. Some of the return light is reflected back towards the sun, and some penetrates the rear surface, to the user's eye. He sees a diffuse, much dimmed virtual solar image, aligned with the direction the major part of the reflected light takes, towards the target. Dave
