First an explanation of Dichroic:
Dichroism (noun) from the Greek word di-chroos meaning (di) two (chroos) colour – the property possessed by some crystals of exhibiting two different colours when viewed along different crystal axes the property of exhibiting different colors by reflected or transmitted light.
We perceive colours by the way wavelengths (colour) of light are selectively interfered with by matter (absorbed, diffracted, reflected, refracted or scattered) on its way to our eyes and hence to our brain.
But Dichroic glass is unique!
The term “Dichroic Glass" is a bit of a misnomer, as the glass itself does not produce the colors, rather it is the dielectric coating of micro-thin layers of metal oxides and quartz applied to the surface of the glass which produces a “interference filter" that creates the varied colors we see. Similar to what happens when viewing hummingbird or peacock feathers. So when you hear or see the term “Dichroic Glass", you will know it is actually a “dielectric interference filter applied to the surface of glass."
The micro thin layers are clear, there is no color inherent in the material. Due to the incredibly thin layers it has no mechanical integrity on its own which is why it is applied to the surface of glass.
Why glass? Glass is stable, rigid, able to withstand fairly high temperatures, is not effected by moisture or solvents and it is transparent making it an ideal substrate for the coating process.
What makes the dichroic filter unique is that the material absorbs no light, each wavelength striking the surface of the “interference filter" (which acts as a selective color mirror) is accounted for either in reflected light or transmitted light, there is no loss of light energy due to absorption. Depending on the thickness of each layer what metal oxide or quartz was used and the order in which it was applied, allows the filter to selectively reflect certain wavelengths of light while transmitting the remaining wavelengths. Contrast this with colored glass where light enters the glass and part of the wavelength is absorbed, emitting only the part of the color spectrum that was not absorbed.
The Process
Dichroic glass is created by a highly technical vacuum deposition process whereby multiple micro-thin layers of metal oxides, quartz crystals and others are vaporized with an electron beam gun mounted at the bottom of an airless vacuum chamber.
Step 1
It is vital that the glass is thoroughly cleaned. Depending on the application for which the glass will be used, it will be cleaned approximately four times to highly technical specifications which require that the glass be cleaned to molecular level in a spotless environment before being coated. Even the tiniest bit of debris, fingerprints or water spot is magnified by the coating process.
Step 2
Next is the airless vacuum chamber where a technician will electronically monitor each step of the vaporization process.
After preparing the vacuum chamber, making sure the pressure inside is compressed to about 125 tons (which is close to the atmosphere in outer space) the plate glass is at the correct temperature and a number of other considerations, the first material is chosen and vaporized by opening the shutter on the electron gun and sweeping it over the material which reside in a crucible. As the material vaporizes and floats upward it will attach to everything inside the chamber and begin to condense on the surface of the rotating glass plate forming a crystal structure. The technician monitoring this condensation will close the shutter on the electron gun once the thickness for that specific material is achieved and will switch to a new material where the process is repeated until the desired layer/colour is achieved.
Step 3
Once the layers are complete air is slowly fed back into the chamber until the air inside the chamber matches the air outside the chamber and the door opens.
Step 4
The dichroic glass is removed and quality checked before packing and shipping. This process takes approximately two to two-and-a-half hours from start to finish.
In Conclusion:
One final example of how the Dichroic filter works: to achieve the colour green “in reflection" all the wavelengths of light must pass through the surface except the colour green. Primary colours require about fifteen layers of material while other colours require 30 layers! Given all those colours one would assume that the coating would be at least an inch thick, whereas the total thickness is approximately 3 to 5 millionth's of an inch. Truly incredible, isn’t it?
In appreciation
A word of thanks to Paula Radke of Paula Radke Studios for helping me to understand how the Dichroic bead is made and setting me upon the path of learning.
Thanks also go to CBS Coatings by Sandberg for their assistance and expertise in reviewing and critiquing the accuracy of the information. They have the know-how and capability to give any glass item you may have a dichroic coating - this applies to professionals as well as the public at large. www.cbs-dichroic.com.
Thank you to wordsmith Lynn Mortimer of Lynn Mortimer Marketing Services for her assistance with copy and marketing expertise.
By: Bead Realm
Author: M. Fleming
Article may be reproduced as long the complete text remains intact. ©Copyright Bead Realm 2006
tweet this!
