Re: Not a Chance Ralph !

Posted by GES on 2012-03-28 12:46:48 in reply to Re: Not a Chance Ralph ! posted by Phil Ewanicki on 2012-03-28 09:07:32

Phil, Ralph and I had a discussion about an interesting Secchi disk situation, which he had encountered; hence the reference to that. In the case of the leader and sun, the leader behaves like a cylindrical mirror (in the simplest case) which will produce a distorted virtual image of the sun's disk. Along the line, the mirror acts like a plane mirror, giving unity magnification, and perpendicular to the line, it acts like a spherical mirror giving a less than one magnification, depending on the geometry distances, so you get a flattened virtual image of the sun. Then depending on the sun/line/eye arrangement, you will be seeing a somewhat off axis view, unless the sun is behind your head as seen from the line, and then it will also be severely astigmatised, apart from the anamorphic magnification. The intensity will be about 1,000 times down according to my calculations for fluoro, and somewhat brighter for nylon. Then the line also refracts most of the light, and will cause a strongly focussed real sun image (in the transverse direction) on the other side of the line from the sun. This image will be very wide angle, transversely to the line, and if you are within that wide angkle you will see a quite bright image. The point is that despite the high attenuation due to the small index mismatch, the background is even dimmer. Ralph found that he got lower Secchi disk ranges horizontally underwater, and the reason is that in that situation you have a bright background from general illumination and maybe water turbidity; whereas in the normal usage of the Secchi disk, you are presumably looking into a black background situation, so there is no return light from outside the disk, as occurs in the horizontal case. And as Ralph says, the camera doesn't lie, and he actually measured reduced Secchi ranges, compared to verticals in the exact same water. I'll post some photographs of the leader case(simulations), when I get some time to run them. George PS Optical mouses are able to track on ordinary white Xerox paper, because the fibers in the paper form the same reflected artificial stars (fire crackers) off the side of the fiber, and because of the curve the mouse camera can see them in any direction. It also sees low contrast shadow areas from the paper non flatness, and the oblique LED illumination.

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