Green Coating?

Here's a newbie question:

When I read reviews of EPs or binoculars on astro pages I often
encounter the author sighing with please at the sight of the intense
green reflection from the glass coatings (I call it "bug-eye green").
This is, apparently, seen as a sign of a high quality optic.

On the other hand, I'm a photographer, and none of my Zeiss, Nikkor,
and Leitz lenses (for 35mm), my Schneider and Nikkor lenses (for 4x5),
my Zeiss lenses (for 6x6), nor my Pentax lenses (for 6x7)--not owned
at the same time!--have ever shown anything remotely like this--all
are multi-coated and perform very well.

I also notice that my higher-priced Takahashi EPs don't show this, but
that the cheaper EPs I own do. My (cheap by bino standards) Oberwerk
binos are green, too.

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?

--Darin

Darin Boville
Fine Art Photography and Video
www.darinboville.com

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

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

Darin Boville 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?

In addition to Roland's informative response, you may also find this web
page to be of value:
http://www.astrosurf.com/lombry/reports-coating.htm


Best,
Stephen

Remove footfrommouth to reply

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Darin Boville 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?

In addition to Roland's informative response, you may also find this web
page to be of value:
http://www.astrosurf.com/lombry/reports-coating.htm


Best,
Stephen

Remove footfrommouth to reply

--
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+ Stephen Tonkin | ATM Resources; Astro-Tutorials; Astro Books +
+ (N51.162 E0.995) | http://astunit.com +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

"Chris1011" wrote in message
...
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.



Roland Christen


Mr. C: Can you recommend any good literature on the whole refractor lense
making process?
it seems that this literature is scarce.

"Chris1011" wrote in message
...
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.



Roland Christen


Mr. C: Can you recommend any good literature on the whole refractor lense
making process?
it seems that this literature is scarce.

Stephen Tonkin wrote in message ...
Darin Boville 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?

In addition to Roland's informative response, you may also find this web
page to be of value:
http://www.astrosurf.com/lombry/reports-coating.htm


Best,
Stephen

Remove footfrommouth to reply


Thanks for the reply and the link. Interesting artcle.

(On a sidenote, I can't help but note that the much maligned Nikon
series "E" lenes were used to illustrate the loewer quality
single-coated lenses. Sort of sad in that these lenses were not at all
bad optically, just that their hardware was not as nice as the
mainstream Nikkors--Nikon, just put cheaper hardware on existing lens
designs--often classic and famous designs--to be able to offer a lower
cost lens.

Also, the caption says this represents the what was offered in the
1950's to 1970's. Actually the 100mm was introduced for the first time
in 1980!)

--Darin

Darin Boville
Fine Art Photography and Video
www.darinboville.com

Stephen Tonkin wrote in message ...
Darin Boville 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?

In addition to Roland's informative response, you may also find this web
page to be of value:
http://www.astrosurf.com/lombry/reports-coating.htm


Best,
Stephen

Remove footfrommouth to reply


Thanks for the reply and the link. Interesting artcle.

(On a sidenote, I can't help but note that the much maligned Nikon
series "E" lenes were used to illustrate the loewer quality
single-coated lenses. Sort of sad in that these lenses were not at all
bad optically, just that their hardware was not as nice as the
mainstream Nikkors--Nikon, just put cheaper hardware on existing lens
designs--often classic and famous designs--to be able to offer a lower
cost lens.

Also, the caption says this represents the what was offered in the
1950's to 1970's. Actually the 100mm was introduced for the first time
in 1980!)

--Darin

Darin Boville
Fine Art Photography and Video
www.darinboville.com

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