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>I'm still confused about several other applications.
> I've seen Fresnels 10 inches on a side, sold for affixing to windows
>or glass doors, to create fisheye views. That's image quality, not
>collimation quality.
Correct, but you have been fooled into thinking that the whole lens
forms the image you see with your eye. As a matter of fact only the
bundle of rays that enters your pupil forms that image, and that
bundle comes from a very small region of the lens.
Aperture ratio is the critical parameter in burning glass
function, not sharpness of focus.
The burning glass example may make my point a bit better. If you
hold a burning glass by hand you can certainly light paper (or burn
ants, etc.) with great ease. If you now take a mirror and a second
burning glass, and you form a hot spot (an image of the sun) at the
point of the first burning glass's image, but using the mirrored
Sun as your object, you will find that you deliver roughly twice
the power to the spot. The two lenses, at least one of which is
hand held, probably are not phased to within a fraction of a
wavelength,
but the superimposed images of the Sun are certainly
mutually coherent.
> I've seen quite a few lighthouses, green/white airport beacons, and>
>suchlike that would IMHO benefit from sharper focusing. One would think
>that a shorter-and-brighter pulse would be much easier to notice.
I have yet to see a hard-to-notice lighthouse, so it doesn't seem to
be a problem.