A Division of PJ's
Touch of Glass
2577 Evansville Ave
Henderson, NV 89052
702-897-8274
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A Division of PJ's
Touch of Glass |
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| About the Artist | Whats Gnu? | 'Sup (as in What's Up) | Lost N Found (Site Index) | Gallery |
| I have had so many
requests for my white paper on dichroic glass, I decided to put the information
here. In the meantime, I have found at least one other very good source of
research at www.inspirationfarm.com. If this is too cumbersome to print out, email me and I'll attach a wordperfect 5.1 format file for you. (Warnng -- The type is small on the text file!) Dichroic glass What is it? The technology was developed in Germany over 100 years ago by Dr. Arthur Pfeiffer. The technical definition of dichroism is an optical effect observed in certain crystalline materials, in which two different colors are seen when the crystals are viewed in different directions in plane polarized light (light whose waves are restricted to a single direction of vibration.) Dichroism is observed only in colored materials, -- more so in deeply colored than in pale substances. Optically, crystals belong to two classes; isotropic and anisotropic. Light that passes through an isotropic crystal is absorbed in the same way in all directions. In an anisotropic crystal, the light is absorbed differently, depending on its direction of travel through the crystal. Isotropic colored crystals show no color change as they are rotated in plane polarized light, whereas anisotropic crystals show a change in color when they are viewed in different directions. An anisotropic crystal may produce a number of different colors: known as pleochroism. When only two colors are observed, it is called dichroism. Applications of dichroic coatings range from use in diagnosing diseases (by tracing fluorescent antibodies), to use in heat-seeking missiles, to being a principle component in energy producing solar cells and serving as a quantitative measuring device in research photography. The technology, called "Thin Film Physics", has evolved from our aerospace programs into the glass art field. In the late sixties, a handful of sculptors started using dichroics in their work. The base, or preliminary color of dichroic glass is identified by viewing the glass straight on in transmitted light. When the glass is turned about 45 , the glass shifts to its secondary color. When there is no transmitted light, the coating becomes a third color in reflection. This is the main reason it is difficult to explain or photograph the mysterious beauty of the alternating colors. What is usually perceived as color in glass, and most other things, is the result of rare earths, elements and compounds added to the glass chemistry for the purpose of absorbing the light energy in different portions of the visible spectrum. The energy that is not absorbed is the color that we see. Dichroic Art Glass creates its effect in a completely different way. It is made from materials that are completely transparent, meaning they do not adsorb any appreciable light energy in the visible spectrum. This lack of adsorption is responsible for the clean crisp colors evident in dichroic glass. Dichroic glass actually works with reflections. White light is the sum of all the colors in the visible light spectrum. Dichroic glass selectively reflects away certain wave-lengths (colors) of light and allows the remaining wave-lengths to transmit through. This is the same way nature creates the colors seen in bird feathers, fish scales, opals and even the color seen on an oil slick on water. We emulate the natural effect with the principals of Optical Thin Film Physics. We know that reflections are created when light traveling through a media of one optical index of refraction and encounters a media of a different optical index of refraction, a reflection occurs. The index of refraction can be visualized as a measurement of the resistance to the light as it travels through a media. The intensity of the reflection is going to be proportional to the difference of indices. For example, light traveling through air, with an index of 1.51 will create a reflection of about 4% of the light energy. If the piece of glass or lens is transparent, there will obviously be two surfaces (differences of indices) & two reflections will occur. So in our example, the loss of transmission due to reflection will be approximately 8%. When dealing with dichroics, we refer to either ADDITIVE (red, green & blue) or SUBTRACTIVE (yellow, magenta & cyan) colors. To obtain a green transmission of light, both red & blue rays need to be reflected. The right combination of red & blue can appear pink. When the conditions of light are such that you can see THROUGH the glass, and the color is green, the same piece will appear pink when looking AT the reflection. The Process: First the glass is optically cleaned in a meticulously clean room. The slightest speck of dust can mar the coating. The glass is polished with rouge, then cleaned to the molecular level with several types of solvent alcohol. The glass is then coated with extremely thin films of elements, such as titanium, magnesium, zirconium, etc. The elements are evaporated and vacuum deposited onto the glass to such a fine degree of thickness that certain wavelengths of light will pass through and others will be reflected. The light rays which do get through the coated glass appear to take on the color characteristics of the coating. Up to twenty layers of materials such as Beryllium, Chromium, Selenium, Yttrium and Tin are used. The Laws of Refraction as well as the differences in Refractive Indexes between air, glass, and the coating layers dictate how and why the glass changes color when viewed from different angles. You might wonder how we manage to create coatings that are thinner than the diameter of some molecules. This is where the high-tech magic comes in. When you say "coating", the image that comes to mind is usually spray painting, or some similar process where we apply some sticky, gooey chemical to the surface of the glass. When we dichroic coating we are actually saying "Electron Beam Vacuum Physical Vapor Multi-Layer Optical Deposition". What we are doing is growing very thin coatings or films of crystal on the surface of the glass. This is done in a very clean high vacuum environment. In the bottom of this vacuum vessel is mounted an electron beam gun which provides up to 10,000 watts of power in a 2 millimeter spot. This beam of electrons is electro-magnetically bent and swept over the surface of the material to be vaporized. Some of these materials will vaporize at temperatures as high at 5000 - 6000 F. In the top of the chamber, the glass is suspended upside down and rotating in a double rotation planetary manner. The glass is clipped to the system and, during the coating process, rotates in the chamber for an even coating. While the glass is rotating the chamber is pumping down, the glass is heated slightly via infrared emitters mounted around the walls of the chamber. The chamber is slowly heated to 300 F over 20 to 30 minutes This is done for two reasons: it helps to vaporize off any foreign contaminant and out gas the surface, and provide a little more energy to the surface of the glass to improve the condensation rate. When the glass is at the right temperature, the vacuum is at the right pressure and at least a hundred other things are just right, the shutter over the beam gun is opened and the vapor is permitted to rise in the chamber. The crystalline particles in the vapor will begin to find places where they can condense into a film covering the glass, and anything else in the chamber. The operator will monitor the condensation of the film and when it is at the proper thickness for the structure being applied, will close the shutter and switch to the gun with the other material. This will repeat over and over again until the number of layers required by the structure is achieved. The chamber has to be roughed down to a low vacuum by means of a mechanical pump. When this is complete (approximately 30 minutes), a high-vacuum pump (gyropump) removes all remaining residual gasses from the chamber, creating an environment similar to that of outer space. After the electronically monitored layers are applied, the glass is allowed to cool. When the coating is complete the glass is allowed to cool in the vacuum to a point at which it will not thermal shock, and the chamber is vented up to ambient pressure. The glass is allowed to cool to room temperature and then quality inspected, cataloged and packaged for shipment. The whole process time required in the machine is from two and a half to four hours, depending on the structure. This is the reason dichroic glass is so very expensive. When the artist fuses dichroic glass, several transformations take place. As the material, which starts off as a two-way mirror, heats up, the glass expands at a greater rate than the coating, separating them. The separation characteristics are a result of the formulation of materials deposited onto the glass. When the material cools and the glass contracts, the coating never gets back to its original position. The higher the temperature, the more severe and rugged the surface rise. Obviously creating dichroic art glass is anything but simple. There are literally hundreds of considerations that must be taken when designing a coating structure. Reflections of light are not going to automatically add to each other. Those light reflections which are at the same wavelength and the same vibrational period, or phase relationship, are going to add in intensity. Those light reflections which are at the same wavelength and a different vibrational period, or phase relationship, are going to subtract in intensity. And each reflection will interact with every other reflection. So it becomes evident that the design, or schedule of films,.must take into account the phase relationship of every layer with respect to every other layer. There are many other considerations that must be allowed for, the angle of the light as it strikes the coating structure, for instance will modify the center wavelength of the design. Even things such as the distance the light travels as it passes through the stack of films at 183.6M miles per second must be taken into account so that when the reflections travel back through the stack they will all be vibrating at the same period in the same physical place.
Each dichroic glass color must be individually engineered. There is no magic material that makes precisely the color you want, or some mythical high-tech machine that allows you to press the blue button and get blue dichroic glass off the conveyer belt. Each film structure is different depending on which spectra of light we want to reflect away. As an example: magenta transmission requires us to reflect away all the green light, allowing the blue violet, orange and red light to transmit through. That particular structure requires 19 layers work of reflections to remove the light we wish from the transmitted spectra. Not all colors are so easy to achieve, green transmission is just the opposite of the magenta. To get a green transmission, we have to reflect away all the light except the green. That structure requires 49 layers. This sounds like a big, thick coating, but if you add all those layer thicknesses together, the total coating thickness will still be less than one thousandth of a millimeter. © PJewelry 2000 PO Box 9976, Alexandria, VA 22304-3919 703-370-5291 Contact me for questions or problems |
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