Optical: systems and elements – Having significant infrared or ultraviolet property – Multilayer filter or multilayer reflector
Reexamination Certificate
1997-12-30
2001-09-11
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Having significant infrared or ultraviolet property
Multilayer filter or multilayer reflector
C359S536000, C359S586000, C428S403000, C428S406000
Reexamination Certificate
active
06288837
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to broadband infrared and signature control materials, particularly films and pigmented coatings for reflecting wavebands in the infrared wavelengths of 1-5 and/or 8-12 microns, and improved methods for producing the same.
BACKGROUND
Multilayer thin films and thin film pigmented coatings for use in infrared and visual signature control, as well as solar heat load control (herein referred to generically as “signature control”), are in particular demand.
Such multilayer thin films are conventionally produced by depositing on a substrate, e.g., by vapor deposition or sputter deposition, a plurality of thin alternating layers of at least two optical materials having different indices of refraction and a predetermined differential between their indices of refraction. The larger the differential between the indices the greater the transmission or reflectance characteristic, and the broader the band of transmission or reflectance.
Conventional optical materials have indices of refraction in the order of from about 2 to about 4, which places an inherent limit on the differential between the indices, with a differential of about 2 being considered quite large.
Due to the limitations on the differential, it is necessary, in order to attain the desired degree of transmission or reflectance, i.e., the desired optical density, to build up a great many alternating layers of the two materials, e.g., 15 to 30 or more layers.
The performance of a multilayer optical device is wave band specific. It is controlled by the thicknesses and the number of the respective layers, and is dependent upon the refractive index difference between layers for the magnitude of its effect. If the thickness or the differential is changed, the performance changes. What is effective in the visible wave band may be of no use in the far infrared.
Materials useful in the production of such devices are elements from a small fraction of the periodic table, because most of the elements in the table are unsatisfactory from either performance or feasibility characteristics. Therefore, the same material may earn mention in many disclosures, but in far different contexts. The same materials, produced in different thicknesses and evaluated at other wave bands, may offer special, beneficial and unexpected results.
It is in the context of other domains and unique requirements that the present invention resides.
SUMMARY OF THE INVENTION
An object of the present invention is to provide signature control materials that are not electrically conductive, not reflective in the visible spectrum, and reflective only in particular bands of the infrared, and otherwise transmissive or absorptive in the infrared. Such materials must also be weather resistant and insoluble in fuels and other fluids, including conventional solvents.
Another object of the invention is to provide such materials in particulate form of performance size for use as pigments in coating materials, such as paints.
Yet another object of the invention is to provide methods of producing such materials in an efficient, practical and economical manner.
It is in particular an object of the invention to increase the differential between the indices of refraction of the high and low index materials in a multilayer optical device, thereby to accommodate development of the desired density in a minimum number of alternating layers.
In accordance with the invention, the differential between the high and low index materials is significantly enhanced by utilization as the low index material of a very thin film of a material having a high coefficient of extinction.
Materials having a high coefficient of extinction have a very low index of refraction, usually below 1.0. However, they are classified as complete absorbers beyond 7 microns and are normally regarded as nontransmissive. They have therefore been deemed unsuitable for use in optical filters at wavelengths larger than 7 microns.
In contrast to the prior art, the present invention is predicated on the discovery that if materials having a high coefficient of extinction can be deposited in sufficiently thin film form, e.g., a few microns or less, the extinction is not intolerable, i.e., transmission is adequate.
The present invention therefore provides a significant increase in the refractive index differential between the layers of a multilayer coating by using a very thin layer of a material having a high coefficient of extinction in alternating layer relationship with a conventional optical material having a high index of refraction.
By virtue of the significant increase in the differential between the indices of refraction of the two materials, broadband filters of the desired optical density can now be formed from as few as 3 to 7 thin film layers, in contrast to the 15 to 30 or more layers previously required to achieve comparable reflectance performance.
Moreover, the present invention provides for perturbation of the layer thicknesses from the conventional one-quarter wavelength optical thickness (“QWOT”) whereby to develop reflectance harmonics covering specific wavelength bands.
The invention therefore provides significant advantages in the structure and the methods of fabrication of optical filters, particularly signature control and infrared reflective materials.
The invention further provides for the highly advantageous production and harvesting of multilayer coatings in particulate form of sensible or performance size for use, for example, as pigments in liquid coating materials.
In particular, the invention provides a novel method for producing pigment particles for signature control coating compositions characterized by the step of coating dielectric microspheres with selected coating materials having differing indices of refraction.
The invention further resides in the discovery that for certain applications effective signature control may be achieved by application of a single layer of material of high refractive index to a substrate of low refractive index, e.g., a dielectric microsphere or microballoon.
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General Atomics
Greer Burns & Crain Ltd.
Juettner Thomas R.
Spyrou Cassandra
Treas Jared
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