"For the second time in art history, the manufacture of artist color has made a difference that developed into an art movement. The first time was in the 1870s with the Impressionists and later the Post-Impressionists, when oil paint was mass-produced in tubes in a great variety of colors. No longer was it completely necessary for the artist to make and mix the color he wanted. Paintings could be made fast, with passion and a quick squeeze on the tube of paint. Now 100 years later, with the invention of dichroic color, artists have a whole new genre of color never seen before- colors that shift and change with changing light. Dichroic color is not a pigment or a dye, but controlled wavelengths of light similar to colors cast by a prism in a beam of light. The brilliant dichroic colors are unforgettable." - Ray Howlett, from "Glass Art" Sept/Oct 2000 issue.
Dichroic glass is a new product being used by glass artists. Beadmakers, Fusers, Blowers, and Casters alike covet it for the unique iridescent properties it adds to their work. In fact, before dichroic color was developed, there was no man-made material that possessed true iridescence in the technical sense. Only Nature's creations could sport the true iridescent color as in the iridescent blue wings of the Morpho butterfly, the fiery play of light in the opal, or the iridescent green of the peacock's feathers. It was not until the space industry became interested in producing iridescent coatings for optical applications, that man-made iridescence became a possibility. And then, it wasn't until the last twenty years that iridescent color or dichro color as it is colloquially called, became widely available to the artistic community. The result is a very seductive medium available to the artist in which colors glimmer and even change at different angles. However, with the increasing popularity of dichro, there has developed an increasing gap between the creative use of this medium and the scientific understanding of this medium by the artist. The following attempts to bridge this gap by explaining the basic observed properties of dichroic color, and the scientific theory behind these observed properties. What does one see when viewing a piece of dichroic glass? An observer might first notice that there are two different colors dependent upon whether the glass is front-lit or back-lit. For example a piece of dichro that appears green when front-lit will appear red when back-lit. This is the origin of the term "dichroic", which literally means "two colors". These two colors are always identifiable with each other. A blue front-lit piece for example will always appear yellow when back-lit, just as the green front-lit piece will always appear red when back-lit. Technically speaking, one refers to the front-lit color as the reflection color, and the back-lit color as the transmission color. These colors are very unusual, as the observer might note, because they are very pure like a laser's color and they glimmer like a bright mirror, unlike the colors of most objects which are due to dyes or pigments. Colors having the former's properties are said to be saturated. Finally, an observer will notice that the colors shift with the angle at which they are viewed. (see our quick video of color shifts) A piece of dichro with a green reflection color when viewed straight-on, will change from green to blue and then to purple as it is tilted with an increasing angle. Likewise, the transmission color will change from red to orange to yellow with the increasing angle. In summary, the properties that define dichroic color are: (1) a saturated reflection color, (2) a saturated transmission color, and (3) color that changes with the angle of view. These properties collectively define what is known as iridescence. What causes the iridescence of dichro? The color of dichro is not due to any dyes or pigments. The surface of a dichroic piece itself actually has no color but rather consists of extremely thin multilayers of semi-reflective material. One might compare the dichroic surface to a stack of micro-thin mirrors layered one on top of the other. Iridescence occurs when light waves reflected from all of the different reflective layers are exactly in-phase with each other and combine constructively. What is meant by "in-phase" and "combine constructively"? Figure 1a shows what is meant when two light waves are in-phase. Figure 2a-c shows these same light waves combining with each other to form a single wave. Notice that the waves that are perfectly in-phase combine to form the wave with the biggest amplitude, and therefore the biggest brightness (brightness is related to amplitude). Waves spaced 180 degrees apart are exactly out-of-phase and produce a wave with zero amplitude- the most destructive result possible.Referring now to Figure 3 which shows a cross-section of a dichroic surface, light wave(1) incident on reflective surface(A) will partially reflect away as light wave(2), and will partially be transmitted into the glass as light wave(3). This transmitted light will then reflect-back from underlying reflective layer(B) as light wave(4) and then will exit-out as light wave(5). In a given dichroic surface, the reflective layers A and B are exactly spaced apart such that by the time light wave(5) exits out, it is exactly in-phase with light wave(2). It is this thickness between each reflective layer which determines what color, or more technically speaking, what wavelength of light will come-out perfectly in phase and therefore be the brightest color. Different wavelengths of light will either be too long or too short to fit the geometry of the system in Figure 3 to come-out in-phase, and will therefore not be enhanced. This enhancement of one wavelength and the suppression of others, is why the reflection color is so pure and saturated, thus explaining the first observation of dichroic color. Figure 4 diagrams what occurs in producing a saturated transmission color, the second of the observations discussed above. Light wave(1) is incident on a surface(A). It passes through into the layer where part of it is reflected back from surface(B) as light wave (2), and part exits-out as light wave(3). Light wave(2) then reflects back from surface(A) and exits-out through surface(B). The layers are spaced apart exactly so that by the time light wave(2) exits-out, it is in-phase with light wave(3). Different wavelengths of light will either be too long or too short to fit the geometry of the system in Figure 4 to come-out in-phase, and will therefore not be enhanced. This enhancement of one wavelength and the suppression of others, is why the transmission color is so pure and saturated, thus explaining the second observation of dichroic color. The third and final observation of dichroic color is that the color changes with the angle at which it is viewed. Referring to Figure 3, if the light wave(1) incident on surface(A) were tilted to a larger angle, the paths of light waves(3) and (4) would be longer and the orientation of light waves(2) and (5) to each other would be different. As a result of this, the geometry of the system changes and light of the depicted wavelength of Figure 3 is no longer in-phase. A different wavelength, and therefore a different color, is needed to fit the particular geometry of this orientation to end-up in-phase. Thus for every shift in angle there is a shift in which color is amplified, which explains the third observation. Man-made iridescence through the production of micro-thin layers of reflective material, as exemplified by dichroism, is thus possible. However, there are many examples of iridescence occurring in the natural world as well. The iridescent blue wings of the Morpho butterfly are a result of micro-projections of multi-layered chitin structures on the butterfly's wing. Similar projections create the iridescent colors on the peacock's feathers, the iridescent green on the necks of male mallard ducks, and the metal-like sheen from a number of species of beetles. The green reflection of a cat's eyes at night are due to a layered structure in the cat's retina. Pearls and mother-of-pearl get their iridescent glow from multi-layers of calcium carbonate. Oil slicks and soap bubbles get their iridescent sheen from a single layer of oil or soapy water. And finally, opals owe their fiery play of light to micro layers of colloidal water suspended in their bodies of silicon dioxide. Dichroic color thus has three general properties; saturated reflection colors, saturated transmission colors and the ability to shift color with the changing angle of view. These properties are due to the interaction of light waves reflecting along different points within a system of extremely thin multi-layers of reflective material. The spacing between these layers of reflective material is what determines which wavelength of light and therefore which color will be amplified to the viewer. The brilliant colors of dichro are man-made versions of the iridescence present in Nature. And like iridescent objects in Nature, dichroic color is stunningly unique from the type of color produced by dyes or pigments.
Figure 2	Figure 3 Figure 4