Dichroic glass

Dichroic glass is glass which displays two different colors by undergoing a color change in certain lighting conditions.

One dichroic material is a modern composite non-translucent glass that is produced by stacking layers of glass and micro-layers of metals or oxides which give the glass shifting colors depending on the angle of view, causing an array of colors to be displayed as an example of thin-film optics. The resulting glass is used for decorative purposes such as stained glass, jewelry and other forms of glass art. The commercial title of "dichroic" can also display three or more colors (trichroic or pleochroic) and even iridescence in some cases. The term dichroic is used more precisely when labelling interference filters for laboratory use.

Another dichroic glass material first appeared in a few pieces of Roman glass from the 4th century and consists of a translucent glass containing colloidal gold and silver particles dispersed in the glass matrix in certain proportions so that the glass has the property of displaying a particular transmitted color and a completely different reflected color, as certain wavelengths of light either pass through or are reflected. In ancient dichroic glass, as seen in the most famous piece, the 4th-century Lycurgus cup in the British Museum, the glass has a green color when lit from in front in reflected light, and another, purple-ish red, when lit from inside or behind the cup so that the light passes through the glass. This is not due to alternating thin metal films but colloidal silver and gold particles dispersed throughout the glass, in an effect similar to that seen in gold ruby glass, though that has only one color whatever the lighting.

Modern dichroic glass is available as a result of materials research carried out by NASA and its contractors, who developed it for use in dichroic filters. However, color changing glass dates back to at least the 4th century AD, though only a very few pieces, mostly fragments, survive. It was also made in the Renaissance in Venice and by imitators elsewhere; these pieces are also rare.

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