Optical waveguides – Having particular optical characteristic modifying chemical... – Of waveguide core
Reexamination Certificate
1999-05-18
2003-02-25
Font, Frank G. (Department: 2877)
Optical waveguides
Having particular optical characteristic modifying chemical...
Of waveguide core
C385S123000, C385S901000, C252S301160
Reexamination Certificate
active
06526213
ABSTRACT:
This invention relates to polymeric compositions having excellent white light transmission that is substantially free of color over long light path lengths. Specifically, the invention relates to polymeric light pipe compositions having trace amounts of dyes that counteract the inherent absorption of red light to provide for the transmission of substantially colorless light when illuminated with a white light source. Furthermore, the invention also relates to light pipe compositions having trace amounts of dyes to provide for the transmission of light having a controlled color when illuminated with a white light source. The invention also relates to light pipe compositions having trace amounts of dyes that reduce the color of light when illuminated with an off-white light source.
Polymeric materials having excellent light-transmitting properties are known in the art. Such polymeric material is generally prepared using high glass transition temperature (“Tg”) clear plastics such as polystyrene, polycarbonate, polymethyl methacrylate, polymethyl glutarimide, etc., and rubbery polymers such as crosslinked polyethylacrylate, polydimethylsiloxane, etc.
Applications requiring low color polymeric materials having excellent light-transmitting properties include light pipes for remote-source lighting systems, large plastic sheeting in which the edge color is minimized or controlled, plastic viewing windows or tanks for aquariums, and plastic block for windows.
One example of light transmitting polymeric materials are polymeric light pipes. Polymeric light pipes having excellent light-transmitting properties generally include a core material with a cladding material covering the core. Light pipes have been prepared using polymeric materials in the pipe core that have a low light absorption and low refractive index polymers for the pipe cladding.
One problem which exists for polymeric light pipes is that the length of the light pipe useful for transmission is limited. This is in contrast to inorganic glass optical fibers which have light transmission characteristics useful in fiber optic communication networks wherein digital light signals are transmitted over distances of tens of kilometers without significant attenuation. Polymers, on the other hand, have much higher absorption of light than do inorganic glasses, thus polymeric materials are limited to applications requiring transmission of light up to distances of tens of meters.
It is known that transparent polymeric materials have a light absorbance at peak maximum at a wavelength between 600 and 650 nm which arises from the vibrational overtones and harmonics of the carbon-hydrogen bonds. The wavelengths and assignments of the absorbance maxima of polymethyl methacrylate are 740 nm (&ngr;
5
), 680 nm (&ngr;
5
+&dgr;), 630 nm (&ngr;
6
), and 550 nm (&ngr;
7
). (Takezawa, et al., Journal of Applied Polymer Physics, vol. 42, pp. 2811-2817, 1991). The absorbance at 630 nm (&ngr;
6
), referred to in the art as the sixth harmonic of the carbon-hydrogen stretch causes white light passing through a meter or more of polymethyl methacrylate to appear green.
It is also known that the significant absorbances in polystyrene arising from carbon-hydrogen bond harmonics and overtones also occur at wavelengths between 600 nm and 650 nm. Thus, it is expected that all polymeric materials having carbon-hydrogen bonds will have an absorbance in the vicinity of 630 nm. Hence, it is expected that all low color polymeric materials having excellent light-transmitting properties and having carbon-hydrogen bonds will cause white light passing through a meter or more of the polymeric material to appear green. Although this is only a very slight color shift, this problem is particularly apparent in applications, such as light pipes for remote source lighting systems, wherein polymeric materials are required to transmit white light through a meter or more of the polymeric material.
Although glass has superior light transmission properties, rubbery polymeric fibers are much more flexible, workable, and lower in weight than glass optical fibers. These characteristics, together with their light transmission limitation of tens of meters, make polymeric light pipes useful for applications such as signs, instrumentation displays, medical devices, etc. Rubbery polymeric materials are especially useful for preparing large-core diameter (greater than about three millimeters) light pipes that are particularly useful in remote-source lighting applications where the large diameter allows the transmission of a large quantity of light from a single light source (illuminator) to multiple points up to tens of meters away.
Accordingly, it is desirable to provide relatively long, highly transparent, polymeric light pipes wherein no such color change occurs as the light path through the polymeric material is lengthened. These light pipes could provide light with a color that is independent of length. In end light applications, light pipes of different lengths can be used since all lengths will provide illumination of the same color. In side light applications, the color of the light will not change along the length.
In certain applications, such as remote source lighting, it is also desirable to provide relatively long, highly transparent, polymeric light pipes in which the color is controlled. Colored light is useful for lighting merchandise in display cases whereby the merchandise appears to be more appealing with light having a slight hue rather than pure white light. For example, light having a bluish hue is useful for illuminating yellowish articles, such as jewelry and the like, to make these articles appear desirably whiter. On the other end of the spectrum, light having a reddish hue provides a “warmer” appearance to furniture and household goods. Also, some light sources produce light that is off white or yellow. In these cases it is sometimes desirable to reduce the color of the light, or make the transmitted light whiter.
One attempt to overcome the problems associated with the absorbance of light arising from the carbon-hydrogen bonds of the polymeric cores in optical fibers is disclosed in U.S. Pat. No. RE 31,868 to Beasley, et al. Beasley discloses several polymeric materials that have substantially diminished absorption arising from carbon-hydrogen covalent bonds, which as a result, have remarkably high transmission of light in the visible spectrum. The suitable polymeric materials include acrylates and methacrylates containing deuterium wherein the carbon-hydrogen covalent bonds have been replaced by carbon-deuterium covalent bonds. Optical fiber cores disclosed by Beasley are copolymers that contain a percentage of deuterium containing methyl methacrylate or deuterium containing polymethyl methacrylate polymer itself. As a result, polymers disclosed by Beasley are not expected to cause white light to turn green over relatively long distances compared to light pipe cores having hydrogen containing polymethyl methacrylate. Unfortunately, compared to commercially-available hydrogen containing monomers, deuterium containing monomers are only available in small research quantities, thereby making large-core optical fibers having deuterium containing polymers impracticable and prohibitively expensive for applications such as remote source lighting.
One attempt to overcome the problem of providing colored light emanating from polymeric light pipes is disclosed in U.S. Pat. No. 5,579,429 to Naum. Naum discloses a large core optical fiber with a fluorescent dye that provides a monochromatic neon-like side emission. The core is a methyl methacrylate polymer crosslinked by polymerization in the presence of allyl diglycol carbonate and a laser dye that fluoresces to produce light of a narrow wavelength (i.e., a single pure color) out the side of the optical fiber. Selection of the dye results in side light emission of the desired color ranging from blue-violet to red. The dye compositions of Naum are useful for light pipes that transmit intense monochromatic light usin
Caro Michael Thomas
Ilenda Casmir Stanislaus
Leonard Brian Francis
Bruzga Charles E.
Fiberstars Incorporated
Font Frank G.
Mooney Michael P.
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