Photosensitive glass is a crystal-clear glass that belongs to the lithium-silicate family of glasses, in which an image of a mask can be captured by microscopic metallic particles in the glass when it is exposed to short wave radiations such as ultraviolet light.[1] Photosensitive glass was first discovered by S. Donald Stookey in 1937 [2][3][4]
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Photosensitive glass was invented in November 1937 by Dr. Donald Stookey of the Corning Glass Works.[2][3] It was not announced publicly until ten years later on June 1, 1947.[2][3] It was officially patented in 1950 by Stookey as U.S. Pat. No. 2,515,937 and U.S. Pat. Nos. 2,515,943 with gold microscopic particles.[2]
When the glass is exposed to UV light in the wavelength range 280–320 nm, a latent image is formed, the glass stays transparent after this stage, however. the only difference that occurs is that the absorption of the glass increases in the uv range of the spectrum, this absorption increase is only detectable using uv transmission spectroscopy.[5] The reason behind this is suggested to be an oxidation reduction reaction that occurs inside the glass during exposure in which cerium ions are oxidized to a more stable state and silver ions are reduced to silver.[5]
When the glass is heated to temperatures in the range 550–560 °C [6] for several hours the latent image is converted to a visible image through photoexcitation.[2][4] Exposure through photographic negatives permits the development of three dimensional color images and photographs.[2][4] This heat treatment is done in two stages: the temperature is first raised to about 500 °C to allow for the completion of the oxidation-reduction reaction, and formation of silver nanoclusters. In the following stage, when the temperature is raised to 550–560 °C, a new material (lithium metasilicate) with the fomula (Li2SiO3) forms on the silver nanoclustors, this material forms in the crystalline phase.[6]
The lithium metasilicate that forms in the exposed regions of the glass has the unique property of being strongly etched in hydrofluoric Acid (HF). Hence allowing a three-dimensional image of the mask to be produced, the resulting glass microstructures have a surface roughness in the range 5 μm [1] to 0.7 μm.[6]
Photosensitive glass is used in printing and reproducing processes.[7] Photosensitive glass is like traditional camera film except that it reacts to ultraviolet (UV) light, where camera film responds to visible light.[4][8] The ideal wavelength to use for exposure should be between 300 and 350 nm, with 320 nm being optimum.[7]
Photosensitive glass contains microscopic metallic particles.[8] These microscopic metallic ion nanoparticles are made of gold or silver which is responsible for the refractive index change.[8] Photosensitive glass is similar to photographic film. Photographic film uses chemicals, while photosensitive glass uses gold or silver ions in the material that will respond to the action of light.[7] The process is to pass light of the ultraviolet wavelength through a negative on the glass.[8] Photographic resolution can be obtained with adhesive polyester as a reverse negative, however anything which resists UV light can act as a "negative."[7]
The glass is sensitive to light that when passes through a mask can ultimately turn it into a permanent picture with a heat process "fixing" permanently the image.[4][7][8] Silver glass "latent images" will develop in 3–4 hours at 886–976 °F.[7] Gold glass "latent images" require a higher temperature of 968–1058 °F and over a similar period of time for postbaking.[7] Postbaking hastens the occurrence of the particles with the shadow areas of the negative, permitting deeper penetration into the glass than the highlighted areas.[7][8] This gives the picture three dimensions and color.[7][8]
The photograph is developed by heating the photosensitive glass around 1000 °F for several hours after exposure.[8] The glass itself is photosensitive and produces a three dimensional image.[2] Particles that are invisible to the naked eye (i.e. gold or silver) are in the glass. These microscopic particles move and grow when heated to form the photographic image itself.[2] The process is similar to camera film, however a "negative" is placed on top of the photosensitive glass and then exposed to ‘’’ultraviolet’’’ light. Camera film, of course, would be exposed to ordinary visible light. Then there is a special process for the exposed photosensitive glass. The glass is reheated then in a kiln and postbaked for several hours. The image then "appears" within the special exposed glass as if by magic. The heated piece of photosensitive glass is then allowed to cool down and the process is done. The positive images produced within photosensitive glass comes in a variety of colors.
As a material for the hot glass studio artist, an additional method of producing imagery in an object using photosensitive glass is to first blow an extremely thin rondel (cased or otherwise) which is annealed in the typical manner. That rondel is then cut into sections which are exposed under a negative. Next, those sections (containing the latent image) are warmed and applied to the surface of a gather of hot glass on the blowpipe. As the object is completed over several furnace reheats, the heat develops the image as the object is being created. This method specifically eliminates the need for the reheating of the object in a kiln for development, which consumes considerable oven time, energy, and the risk of loss or damage due to shattering on the way up to temperature, or more importantly, slumping while being held at temperature. The timing of the glassblower determines the final degree of development, and simple choices of form minimize distortion in the image.
Since the image is inside and actually a part of the glass itself, photosensitive glass is the most durable photographic medium known.[2][8] It is claimed that a photo image within photosensitive glass is the most durable form of photography and will last as long as the glass itself.[2][3][8] The photographic image is not on the surface of the glass, but internally.[2]
Fluorescent photosensitive glass makes it possible to make fluorescent photographs and fluorescence holography.[9]
Photosensitive glass is different than photochromatic glass.[4] Photochromatic glass is used in self-darkening sunglasses which darkens when exposed to bright daylight.[4] It then returns to see-through transparency when strong daylight is removed and can then be used indoors as regular glasses.[4]
Little has been done to develop the product since its patent.[2] It is labor intensive and has a high cost.[2] Only large commercial glass factories produce it.[2] In the 1980s photosensitive glass was created to a small degree to be used in "hot glass" work.[2] Then individual artists owned smaller studios and created works in blown glass and began experimenting with photosensitive glass.[2] Going into the Twenty-First Century only a few glass artists know the technique of achieving good results with photosensitive glass.[2] In the present time, the only photosensitive glasses produced are Foturan and APEX. Foturan os profuced by Schott Corporation and APEX by Life Bioscience.[5]
One of the reasons between the time photosensitive glass was invented and the delay why it was announced approximately ten years later to the public is because of its military applications.[7] It is possible to burn images and words that are hidden in photosensitive glass until heated at a high temperature.[2] The military used this fact during World War II to send secret messages to allied troops in pieces of what looked like "ordinary glass."[2] At the other hand, the person who received the "ordinary glass" just had to heat it up to read the hidden message.[2] Because of this application photosensitive glass was kept secret until the end of World War II.[2][8]
The United Nations Building is faced with hundreds of square feet of photosensitive glass.[10]
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