Request ==

Would love to see an explanation of the differences between first, second, third, and fourth order Fresnel lenses referenced but not explained in this article. There is a photo of a third order lense and a notation in other locations about the detroit river light house ahving donated it's fourth loder fresnel lense to a museum and I now wonder just what the differences might be between these types. If anyone can shed some light (pun intended) it would be most appreciated.

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