-snip
>so it is not multiple layers of coating on a single element that makes
>a lens multicoated. then, what is multicoating? there are other substances
>that are more efficient at passing light through, passing about 990f
>all light through, but they don't do this throughout the visible
>spectrum. instead, they might pass 990f the light through but only
>in a narrow band of the visible spectrum, whereas they might only pass
>50% through elsewhere in the visible spectrum. what Pentax discovered
>in the 70's was that these substances, sometimes in combination with the
>coatings used for single-coating, could be layered together in very
>thin layers in a very special way to cause interference between the layers.
>this interference would enable the coating to pass about almost all (99.5%)
>of light through across the visible spectrum.
Actually I do not think that it quite happens this way if you mean that the
coatings are going going to increase the transmission of light through the
lens to 99%. No coating can make glass transmit more light. The effort to
increase the transmission of light through the coating is a valid effort so
that in reducing the light reflected from the lens you do not do it with a
several layers of substances which greatly reduce the light that ever gets
to the lens. If my memory of my old text book on the physics of optics is
correct anti-reflective coatings work by creating a duplicate of the
reflected light component that is 180 degrees out of phase and which
cancels the reflection. So if a lens element, uncoated, passes 970f the
light hitting its surface, with an anti-reflective coating it will pass the
same amount minus any light absorbed or reflected by the coating. The
previously reflected 3 0lare just disappears, speaking in gross terms. All
the action takes place between the glass surface and the inside surface of
the coating. So in multicoating the first layer acts on the reflection from
glass surface and each coating layer after that acts on the reflection from
the layer next to it.
Winsor
Winsor Crosby
Long Beach, California
mailto:wincros@xxxxxxxxxxx
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