GB2230347A

GB2230347A - Multi-spectral transmission coatings

Info

Publication number: GB2230347A
Application number: GB8711554A
Authority: GB
Inventors: Paul William Walland; Christopher John Howard Wort; James Cyril Alexander Lewis
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1987-05-15
Filing date: 1987-05-15
Publication date: 1990-10-17
Also published as: GB8711554D0

Abstract

A lens or window having resistance to erosion or rain impact comprises a substrate transparent to radiation in at least two spectral wavebands (visible, near infra-red, 3-5 mu m, 8-14 mu m, microwave) and coatings 11, 12 thereon transmissive in different ones of the two bands. Coatings 11, 12 may be partially yttria-stabilised zirconia and germanium-carbon, on a zinc sulphide substrate. Anti-reflection coatings 14 can be provided. If zone plates or a halographic plate window are used, specifically sensitive detectors appropriately located behind the lens or window may detect and differentiate between the transmitted radiations. The coating 11, 12 may be formed on opposing faces of each adjacent pair of teeth on a saw-tooth substrate. <IMAGE>

Description

MULTI-SPECTRAL TRANSMISSION COATINGS.

This invention relates to coatings and support materials transmissive to radiation in a plurality of wavelength bands. In certain cases, it is essential that the coatings have a high resistance to impact or erosion damage. Although multi-spectral coatings are known, in infra-red optical systems, there is now a requirement for a coating or coatings, and a support material, transparent (ie. having a high transmissivity) in a combination of two or more of the following radiation bands:- visible; near infra-red (~1.06 rim); 3-5tm; 8-14em; and microwave. In addition, the materials used must have mechanical properties such as to provide satisfactory resistance to erosion or high velocity rain impact damage.For some of the above mentioned combinations, no suitable substrate material or coating exists.

It is an object of the present invention to provide a lens or window of novel form having transmissivity at two or more of the above specified wavebands, and simultaneously, having satisfactory mechanical properties.

According to the present invention, a lens or window comprises a substrate of appropri at mechanical properties and transparent in at least two spectral .ivebands, having a coating on the surface thereof of a first material having a high trar,smissivitr in a first spectral band, and a coating of a second material having a high transmissivity in a second or the second of the spectral bands, the coatings being deposited upon the substrate so as to provide areas of said lens or window coated only with a respective of the first and second materials.

Areas of the lens or window coated with the first material may be contiguous with areas of the lens or window coated with the second material.

The coated areas may be in the form of concentric rings.

The sum of the areas coated with the first material may equal the sum of the areas coated with the second material.

The first material may be substantially transparent to radiation of wavelength 1.06rum.

The second material may be transparent to radiation of wavelength 8 to 14ism.

The substrate may be of zinc sulphide.

The first material is advantageously of partially stabilized zirconia in which the stabilising additive is Yttria and the second material may be of a germanium-carbon.

Anti-reflection coatings, of the appropriate transmissivity and reflection properties, may be provided on top of and/or underneath the coatings of the first and second materials.

In another embodiment. the coatings of the first and second materials may be in the form i zone plates or be arranged holographically. In such an crnbodiment, thc intensity of transmitted radiation at predetermined points behind the window can be controlled whilst a constant ratio of intensity of the two or more spectral wavebands is retained.

In a preferred embodiment, the coated surface of the substrate provides a saw-tooth section, the opposing faces of each adjacent pair of teeth being coated with the respective first and second coating materials and respective anti-reflection coatings. In such instance, the effective transmission area of the lens or window to each of the spectral wavebands approaches 100%.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a dome-shaped window for the transmission of two spectral wavebands, and constructed in accordance with the present invention; Figure 2 is a plan view of a planar zone plate constructed in accordance with the present invention; Figure 3 is a sectional view, to an enlarged scale, of a fragmentary part of the window of Figure 1 or Figure 2; Figure 4 is an optical diagram of the manner of operation of the zone plate shown in Figure 2; Figures 5 and 6 are exemplary representations of segmented and checkerboard patterns of windows or lenses in accordance with the present invention;; Figures 7 and 8 illustrate embodiments of the present invention in which the transmissive areas apportioned to each of the spectral wavebands effeclively approach 100% of the total area of the window or lens.

In the drawings, like reference numerals refer to similar parts.

Referring now to Figure 1, a dome-shaped window 10 of a radiationdetecting apparatus comprises a substrate of sufficient mechanical strength and mouldable or otherwise formable into the required dome shape. When used in connection with an apparatus for detecting radiation in the near infra-red to microwave radiation, the window 10 may be formed of zinc sulphide which is substantially transparent to radiation of wavelength in the range 0.3otlm to 14 > m and also in the microwave ie. it transmits radiation in the visible spectral range, near-and far-infra-red and some micro-wave radiation.

The window 10 has on its surface, alternate annular rings. The rings are of a first material 11 substantially transparent to radiation of a first spectral range e.g. O.3 m to 2r m and of a second material 12 substantially transparent to radiation of a second spectral range e.g. 8 m to 1 4 m. Each material 11, 12 is substantially opaque to radiation in the respective other spectral range.

The materials 11 and 12 are coated onto the surface of the window 10 by appropriate masking and deposition techniques. The material 11 may be partially stabilised zirconia in which the stabilising additive is yttria and the material 12 may be a germanium-carbon. Throughout the figures of the accompanying drawings, areas of the material 11 are shown shaded whilst those of the material 12 are shown unshaded.

It will be seen that approximatel! half the area l the window has a coating of the material 11 and the other half has .t coating of the material 12. The material il is provide with a 'uper-coat 13 constiiuted by a +/4 anti-reflection surface layer for the radiation to be transmitted thereby and the material 12 is provided with a similar super-coat 14 constituting a r /4 anti-reflection surface layer for the radiation transmitted by the material 12.

The materials above specified have appropriate mechanical strength to resist erosion and to withstand the impact of, for example, rain at super-sonic velocities.

The window 10 may be backed by a lens (not shown) and image intensifier (also not shown) or may be backed by appropriate detectors (also not shown) for the radiations transmitted thereby.

In this respect, it is preferable if the radiations transmitted by the window 10 can be separately detected. This is rendered possible by the use of zone plates in the transmission areas as shown in Figures 2 and 4 or by the use of a holographic plate window as described in relation to Figure 4.

Referring now to Figures 2 and 4, a window 15 of circular shape comprises a plurality of the coatings of the first and second materials 11 and 12 in the form of annular alternate rings 16, 17.

Each of the rings 16 is of equal area and each of the rings 17 is of equal area.

The radial distance of each ring 16 or 17 from the centre of the window 15 is given by ihe formula.

Sm = (m /2 Where Sm is the radial distance of the zone plate forming the mth ring; m is the order of the zone; is the wavelength; is the separation of the resultant high intensity region from the window 15 for radiation sources at infinity; Figure 4 is an optical diagram indicating the impingment of radiation of two spectral wavebands > 1 and on the window 15 of Figure 2. It will be seen that the use of zone p]ates as transmissive areas of the window 15 results in the radiation of first waveband 1 being brought to a "focus" at a location A#1 from the window 15 whilst that of the waveband ss 2 is brought to a "focus" at a location a.

# 2 from the window 15. Appropriate detectors 18 and 19 located at such positions provide for separate detection of the intensity of transmitted radiations. Alternatively, appropriate reflectors dichroic or otherwise, may be interposed on the optical axis and one or both or the detctors 18 and 19 could then be located to one side of the optical axis.

An equivalent effect can be obtained if the zone plate window 15 is replaced by a holographic plate l5a in which alternate holograpic lenses, respectively coated with the first and second material 11, 12, bring radiation of the respective wavebands to respective foci on the optical axis of the plate l5a.

Figures 5 and 6 illustrates examples of the iTitny forms a window or lens, according to the present invention, may take.

In Figure 5, tbc window 20 is shown , Q circular substrate on which the coatings of the first and second materials 11 and 12 are shown as segmented areas 21, 22 respectively.

In Figure 6, the window 23 is shown as a rectangular substrate on which the coatings of the first and second materials 11 and 12 are shown as forming a checkerboard pattern of rectangles 24 and 25 respectively.

The arrangements shown in Figures 7 and 8 has an advantage in that effectively, all the radiations of the chosen spectral wavebands impinging on the whole area of the window are transmitted.

In Figure 7, the window 26 has its first surface 27, that upon which the radiation impinges, ridged to form a saw-tooth pattern.

The opposite surfaces 28, 29 of each tooth 30 are respectively coated with the materials 11 and 12 and an appropriate > /4 super-coat.

Radiation of a first of the desired spectral wavebands impinging upon a surface 28 of will be transmitted and radiation of a second of the desired spectral wavebands will be reflected to impinge upon the surface 29 of the next tooth where it will be transmitted by the window 26, and vice versa. An angular displacement from the optical axis will be occasioned by this window surface but this can be corrected by the provision of a second surface 31, of toothed form, transmissive to both the desired spectral bands, but serving to displace angularly, radiation displaced at the first surface 27.

Where such angular correction is not necessary, a window 32, as shown in Figure 8, having a planar second surface 33 to the substrate, may be provided.

It will be appreciated that, with the arrangements shown in Figure 7 and Figure 8, substantially all of the radiation in the desired two spectral wavebands impinging upon the window, is transmitted.

The invention is not confined to the precise details of the foregoing examples and variations may be made thereto.

For instance, appropriate coatings may be provided on the surface of a window or lens in accordance with the present invention, to permit any two spectral wavebands to be transmitted. In respect of the embodiments shown in Figures 1 to 6, provision for the transmission of more than two spectral wavebands can be made.

As shown in Figure 3, when adjacent coatings are applied to the substrate of a window 10, the second coated material, in this case the material 11, will overlay, in part, edges of the first coated material, in this case, the material 12, giving rise to a slight loss in the transmissive area of the window 10.

Generally speaking, the impinging radiation will be from a source effectively at infinity but radiations from off-axis sources, at infinity or otherwise, may be transmitted by a window or lens according to the invention.

Claims

CLAIMS:

1. A lens or window comprising a substrate of appropriate mechanical properties and transparent in at least two spectral wavebands, having a coating on the surface thereof of a first material having a high transmissivity in a first spectral band, and a coating of a second material having a high transmissivity in a second or the second of the spectral bands, the coatings being deposited upon the substrate so as to provide areas of said lens or window coated only with a respective one of the first and second materials.

2. A lens or window as claimed in in claim 1 wherein areas of the lens or window coated with the first material are contiguous with areas of the lens or window coated with the second material.

3. A lens or window as claimed in claim 1 or 2 wherein the coated areas are in the form of concentric rings.

4. A lens or window as claimed in any of claims 1 to 3 wherein the sum of the areas coated with the first material are equal to the sum of the areas coated with the second material.

5. A lens or window as claimed in any preceding claim wherein the first material is substantially transparent to radiation of wavelength 1.06clam.

6. A lens or window as claimed in any preceding claim wherein the first material is substantially transparent to radiation of wavelength 1.06 m.

7. A lens or window as claimed in any preceding claim wherein the substrate is of zinc sulphide.

8. A lens or window as claimed in any preceding claim wherein the first material is partially stabilized zirconia in which the stabalising additive is Yttria.

9. A lens or window as claimed in any preceding claim wherein the second material is a germanium-carbon.

10. A lens or window as claimed in any preceding claim wherein anti-reflection properties, are provided on top of the coatings of the first and second materials.

11. A lens or window as claimed in claim 1 wherein the coatings of the first and second materials are in the form of zone plates.

12. A lens or window as claimed in claim 11 wherein the intensity of transmitted radiation at predetermined points behind the window is controlled whilst a constant ratio of intensity of the two or more spectral wavebands is retained.

13. A lens or window as claimed in any preceding claim wherein the coated surface of the substrate provides a saw-tooth section, the opposing faces of each adjacent pair of teeth being coated with the respective first and second coating materials and respective antireflection coatings.

14. A lens or window substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 5, Figures 2, 3 and 4, or Figure 5 or Figure 6 or Figure 7 or Figures 8 of the accompanying drawings.