Classifications

• • G02B1/105—
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
  • G02B1/10—Optical coatings produced by application to, or surface treatment of, optical elements
  • G02B1/14—Protective coatings, e.g. hard coatings

Definitions

• the present invention relates generally to durable windows required in multis ⁇ pectral imaging systems for high-speed aircraft. More particularly, the present inven ⁇ tion is directed to durable windows transmitting the visible and near infrared band- passes ranging from 0.4 to 0.9 micrometers in wavelength, the 1.06 and 1.54 microme ⁇ ter wavelength laser bandpasses and the medium wave infrared (MWIR) bandpass ranging from 3 to 5 micrometers in wavelength.
• the visible and near infrared band- passes ranging from 0.4 to 0.9 micrometers in wavelength
• the 1.06 and 1.54 microme ⁇ ter wavelength laser bandpasses and the medium wave infrared (MWIR) bandpass ranging from 3 to 5 micrometers in wavelength.
• MWIR medium wave infrared
• Bulk sapphire meets rain erosion requirements for high-speed aircraft and has good transmission in the visible and near infrared ranging from 0.4 to 0.9 micrometers in wavelength and the 1.06 and 1.54 micrometer laser wavelengths.
• sapphire has poor transmission at the long end of the 3 to 5 micrometer wavelength bandpass due to fundamental lattice vibration abso ⁇ tion.
• abso ⁇ tion in this band ⁇ pass is manifested as emission from the warm window, increasing system background noise.
• Sapphire is also not available in sizes larger than about 7 inches x 7 inches.
• Multispectral zinc sulfide has good transmission at the long end of the 3 to 5 micrometer wavelength bandpass, as well as in the 1.06 and 1.54 mi ⁇ crometer laser wavelengths and is readily available in large sizes with over 20 inch di ⁇ mensions.
• this material also has extremely poor resistance to rain erosion and does not meet rain erosion requirements for high-speed aircraft.
• a window for transmitting infrared radiation comprises a sapphire layer formed on a sub- strate comprising multispectral zinc sulfide.
• the thickness of the sapphire layer is in the range of about 5 to 20 micrometers.
• the window ofthe present invention is durable and can transmit the visible and near infrared ranging from 0.4 to 0.9 micrometers in wavelength, the 1.06 and 1.54 micrometer laser wavelengths and the 3 to 5 micrometer medium wave infrared wave- lengths.
• the window of the present invention provides a higher transmission and therefore higher acquisition range for infrared imaging systems, while still providing the same exterior durability as bulk sapphire. Since there is less abso ⁇ tion, there is also less emission from the window of the present invention in comparison with a bulk sapphire window. Accordingly, background noise is reduced.
• the window of the pres ⁇ ent is also lower in cost than the expensive bulk sapphire counte ⁇ art.
• the present in ⁇ vention provides a readily scaleable window necessary for the large size windows (i.e., 10 to 20 inches) required for multisegmented "greenhouse"-type window assemblies integrated into high-speed aircraft airframes. There is no currently available window or window combination for this application.
• FIG. 1 is a cross-sectional view, depicting an airframe of a high speed aircraft equipped with a multispectral infrared imaging system
• FIG. 2 on coordinates of transmission and wavelength, is a plot ofthe transmission of bare multispectral zinc sulfide prior and subsequent to the 470 mph (miles per hour), 20 minute whirling arm rain erosion test;
• FIG. 3 on coordinates of transmission and wavelength, is a plot ofthe transmission of various thicknesses of sapphire.
• FIG. 4 is a cross-sectional view ofthe composite sapphire-coated multispectral zinc sulfide window ofthe present invention.
• FIG. 1 wherein like reference numerals designate like ele ⁇ ments throughout, an airframe 10 of a high speed aircraft equipped with a multispectral infrared imaging system is depicted.
• the multispectral infrared imaging system is housed in a sensor ball 12 shown in FIG. 1.
• the sensor ball 12 resides within a mul ⁇ tisegmented "greenhouse"-type window assembly 14 comprising an array of flat windows 16 oriented around the sensor ball for the protection thereof
• the "greenhouse"-type win ⁇ dow assembly 14 is so named due to its resemblance and similarity in construction to a greenhouse which also comprises an array of flat windows.
• a multispectral infrared imaging system on a high-speed aircraft requires windows (or a window) 16 that transmits selected visible and infrared wavelengths and that is also resistant to rain erosion
• the window 16 for transmitting the visible and near infrared ranging from 0 4 to 0 9 micrometers, and the 1.06 and 1.54 micrometer wavelength laser and 3 to 5 micrometer medium wave bandpasses comprises either multispectrai zinc sulfide or sapphire
• Multispectral zinc sulfide has extremely poor resistance to rain erosion characterized by an increase in transmission loss with rain exposure
• the transmission curves for multispectral zinc sulfide are shown in FIG. 2 with respect to rain erosion. Resistance to rain erosion is tested using a standard rain erosion test for fixed wing aircraft, which comprises exposing the sample to a 470 mph (miles per hour), 90° angle of incidence, 20 minute whirling arm in rain.
• Curve 18 and Curve 20 show the transmission of multispectral zinc sulfide in the 400 to 2400 nanometer wavelength range prior and subsequent to the standard rain erosion test, re ⁇ spectively.
• Curve 22 and Curve 24 show the transmission of multispectral zinc sulfide in the 2 to 14 micrometer range prior and subsequent to the standard rain erosion test, respectively. Substantial degradation in optical performance of the multispectral zinc sulfide is seen to occur with rain erosion. Bulk sapphire passes the standard rain erosion test with no degradation. As mentioned above, however, sapphire has poor transmission at the long end of the 3 to 5 micrometer wavelength bandpass. Transmission curves for various thicknesses of sapphire are shown in FIG. 3. Curve 26, Curve 28, and Curve 30 show the transmis ⁇ sion for 0.040 inch thick sapphire, 0.125 inch thick sapphire, and 0.247 inch thick sap- phire, respectively.
• the medium wave infrared loss would be even higher than that shown for Curve 30, which depicts transmission for 0.247 inch thick sapphire.
• the abso ⁇ tion in this bandpass is manifested as emission from the window 16, which is warmed by the absorbed energy. This emission increases system background noise.
• a window 16 which is durable and multispectral, i.e., transmitting 0.4 to 0.9, 1.06, 1.54, and 3 to 5 micrometer wave ⁇ lengths, is provided
• the infrared-transmitting window 16 of the invention is shown in FIG. 4.
• the window 16 of the present invention comprises a multispectral zinc sulfide substrate 32 coated with a sapphire layer 34 about 5 to 20 micrometers thick.
• the sap ⁇ phire layer 34 is formed on the multispectral zinc sulfide substrate 32 via conventional techniques which are well-known and thus form no part of this present invention. These conventional techniques include techniques for depositing sapphire such as sputtering, ion-assisted deposition, etc.
• At least one anti-reflection coating is deposited on the multispectral zinc sulfide substrate 32 prior to the forma ⁇ tion of the sapphire layer 34.
• the anti-reflection coating reduces the reflection loss at the multispectral zinc sulfide/sapphire interface.
• the multispectral zinc sulfide substrate 32 provides a substrate with essentially no abso ⁇ tion in the visible and near infrared 0.4 to 0.9 micrometer wavelength range and the 1.06 and 1.54 micrometer wavelength laser and medium wave infrared, i.e., 3 to 5 micrometers wavelength, bandpasses.
• the multispectral zinc sulfide substrate 32 is also available in large sizes, currently as large as 30 inches.
• the sapphire layer 34 provides a durable layer for the exterior of the window 16 which enables the window to survive high-speed rain erosion.
• the sapphire layer 34 also only absorbs a slight amount of MWIR, i.e., 3 to 5 micrometer wavelength, en ⁇ ergy, especially compared to a bulk 0.5 inch thick piece of sapphire; see FIG. 3
• the abso ⁇ tion for 0.5 inch thick sapphire can be extrapolated from FIG. 3. This low ab ⁇ so ⁇ tion is also manifested as low emission, thus reducing background noise.
• the multispectral zinc sulfide windows are available in sizes up to about 30 inch in diameter and deposition ofthe sapphire layer 34 is readily scaleable to these larger sizes.
• the application of a durable sapphire coating allows a straightfor ⁇ ward scalability which can produce the large size, i.e., 10 to 20 inch dimension, win- dows 16 necessary for multisegmented "greenhouse"-type window assemblies 14 inte ⁇ grated into high-speed aircraft airframes.
• a survivable window can be provided for transmitting the 1.06 and 1.54 micrometer wavelength laser and 3 to 5 micrometer medium wave infrared bandpasses for applications such as the large multisegmented windows integrated into high-speed aircraft airframes as well as providing as an op ⁇ tion, transmission in the visible and near infrared range from 0 4 to 0 9 micrometers in wavelength
• the window 16 ofthe present invention provides the following advantages
• the window 16 of the present invention does not possess the birefringence effects common to bulk sapphire windows
• the multispectral zinc sulfide/sapphire coating combination of the present invention is less expensive than a bulk sapphire window
• the present invention provides a window 16 which is readily scaleable and thus necessary for the large size, l e , 10 to 20 inch dimension, windows required for multisegmented "greenhouse" -type window assemblies 14 integrated into high-speed aircraft airframes Further, the upper limit of window size is only dependent on (a) the size of multispectral zinc sulfide substrates available and (b) the deposition technology employed to deposit sapphire

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Glass Compositions (AREA)
• Lenses (AREA)

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• 1996
  • 1996-10-04 AU AU73886/96A patent/AU699028B2/en not_active Expired
  • 1996-10-04 JP JP9514432A patent/JPH10503146A/ja active Pending
  • 1996-10-04 IL IL12092896A patent/IL120928A0/xx unknown
  • 1996-10-04 WO PCT/US1996/015889 patent/WO1997013169A1/en not_active Application Discontinuation
  • 1996-10-04 EP EP96936174A patent/EP0796447A1/de not_active Withdrawn

Non-Patent Citations (1)

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