|
"Dichroic Glass" is
somewhat of a misnomer, since the dielectric
coating that produces all the interesting colors
is not glass at all, but a group of very thin
layers of metal oxides. This stack of thin layers
has a total thickness of three to five millionths
of an inch. The layers produce an "interference
filter", creating the varied and unique color
characteristics we see. Since the filter is so
thin, it has very little mechanical integrity of
its own, and must be supported on a mechanically
stable substrate. Glass is the ideal candidate for
this substrate. Transparent, rigid and stable, it
withstands high temperatures, and is not affected
by moisture, solvents or most acids. The filter
materials are actually more chemically stable than
most glasses used as the substrate. Thus, what we
commonly call "Dichroic Glass", is actually a
piece of dielectric interference filter attached
to the surface of a piece of glass.
Dichroic is defined as the property of having more
than one color, especially when viewed from
different angles. Dichroic glass is a high-tech
spin-off of the space industry. Thin layers of
metallic oxides, such as titanium, silicon, and
magnesium are deposited upon the surface of the
glass in a high temperature, vacuum furnace.
The glass to be
coated is carefully cleaned, and fastened to a
planetary arm in the top of the furnace chamber.
The oxides are placed in a crucible on the bottom
of the chamber. Air inside of the chamber is
removed with a high vacuum-producing cyropump, and
the chamber is heated to 300oF. The metallic
oxides are vaporized by an electron beam, and the
rotating glass target is evenly coated with many
thin layers. The resulting color is determined by
the individual oxide compositions.
Dichroic coatings
transmit certain wavelengths of light, while
reflecting others, thus creating an
interference-effect similar to the iridescence
observed in Nature's fire opal, dragonfly wings
and hummingbird feathers. The transmitted color is
different than the reflected color, and a third
color is produced by viewing the dichroic piece at
a 45o angle. The resulting colors are pure,
saturated, single wavelengths of light, that
appear to originate from within the dichroic
piece. |
