so, the Russian Maks that came out red, and the old MegaVista
eyepieces that were almost bright green were representative of poor
coating application ?
Jerry



Chris1011 wrote:

Is there any real reason to prefer the green, lovely as it is to look
at, in actual use? Why do people praise the green reflection?

The reason to use multi-coatings is to reduce the reflections at each air-glass
surface. A modern high quality multi-coat usually has 3 layers applied to the
surface of the glass. If done right, the reflectivity will be less than 0.3%
average over a wide wavelength range. There are many refractive substances that
can be used for this purpose, the most common being aluminum oxide, zirconium
oxide and magnesium fluoride. These are applied using electron beam coating
machines. The layer thicknesses must be very tightly controlled, otherwise the
end result is not optimum, and higher overall reflection is the result.

Some refractive mixtures will result in two nulls on either side of the central
wavelength with a slight rise in the green, amounting to perhaps 0.5% at 530nm.
This produces the characteristic faint green hue on these multi-coatings. Other
mixtures may produce a broad minumum from blue to yellow with a slight rise at
the red and violet ends of the spectrum. This produces a characteristic grape
color with perhaps a faint reddish hue. In any case, these reflections are on
the order of 10 times less than an uncoated surface.

So what causes intense colors (bright green or red) on some glasses? These can
be the result of using the wrong thickness layers (poor quality control) or
using a cheaper evaporation process, or using many more layers or thicker
layers than the optimum for lowest reflectivity. Take your pick.

Roland Christen