Abstract:
An optical system employs an optical coating on a substrate positioned relative to an optical collection system. The thickness of the coating is tailored such that light perceived by the collection system over prescribed angles of acceptance has a spectral content that is unchanged, or changed in a prescribed manner, from that entering the system when the sensor is oriented in the “look ahead” direction. The thickness of the coating is varied so that the wavelengths transmitted or reflected correspond to those required by a specific application. The invention is adaptable to system requirements that are centered about a single wavelength, or that require multiple wavelengths. The optical coating can take any of a number of forms, including stacked dielectric layers and rugate filters.

Description:
This application claims the benefit of provisional patent application No. 60/507,603 to Hall and Southwell, filed Sep. 30, 2003. 
    
    
     This invention was made with Government support under Contract F33615-97-C-5400 awarded by the Air Force. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of optical coatings, and particularly to optical coatings where wide field-of-view is required. 
     2. Description of the Related Art 
     Optical thin film interference coatings are used in many applications where the spectral content of light is important. When applied to a surface in an optical system, such coatings allow some wavelengths to be transmitted and other wavelengths to be reflected. Common applications include antireflection coatings that transmit all relevant wavelengths with high efficiency, bandpass filters that allow only a specified spectral region to be transmitted and to reflect other wavelengths, and notch filters where specific wavelengths are reflected while other wavelengths are transmitted. Such coatings are well known, and are described, for example, in U.S. Pat. No. 4,756,602 to Southwell et al. and U.S. Pat. No. 4,666,250 to Southwell. 
     All of these optical coatings suffer from an angle-of-incidence (AOI) dependent spectral response. This change in performance is a result of the changing path length, for light propagating in the optical coating, of the light as a function of the incident angle. This applies to all optical interference coatings no matter the application, including antireflection, bandpass, bandstop, and high reflection applications. 
     In many applications, this shift in the spectral properties of the optical coating is undesirable. In these applications, the desire is to have a very wide field-of-view where the spectral properties do not change as a function of incidence angle on the coating. 
     Applications where wide field-of-view are important include: 
     1. Optical sensors on a gimbal placed behind a window. The sensor can change its viewing direction over a wide range of angles, in each case viewing through a different area of the window. A uniform coating on the window will have different spectral characteristics for the light that impinges on the sensor depending on the direction in which the sensor is pointed. Also, light coming through any fixed portion of the window that originates from different locations will impinge on the window from different directions and have different spectral characteristics imposed on it. 
     2. A head mounted display (HMD) with a see-through beam combiner where the projected image is reflected into the users eyes using a fixed (e.g., CRT, LCD) or scanning (e.g., laser, laser diode) image source. These beam combiners are designed to operate at a non-normal angle of incidence and, based on their proximity to the user&#39;s eyes, can be required to provide constant spectral performance over wide viewing angles. The coatings on the beam combiner are designed to reflect specific wavelengths of the HMD display and transmit the external scene to the user. The reflected display wavelengths will have a different spectral response depending on the specific location on the beam combiner where the light is reflected. This can cause the beam combiner reflection band, as perceived by the eye, to shift relative to the display wavelengths, reducing the display intensity perceived by the user and changing the spectral content of the external scene as viewed by the wearer. These changes in intensity and spectral content will vary with location on the beam combiner as determined by the relative angle between any given location and the pupil of the eye. 
     3. Spectacles or sunglasses having coatings that tailor the spectral content of the transmitted light. An example might be optical coatings that are applied to sunglasses to preferentially transmit those wavelengths for which the eye has greater visual acuity. For example, these glasses are important for hunters and sportsmen. Light transmitted at different locations and at different angles would have a different spectral content, which can be perceptible to the user. 
     Many other applications will be readily apparent to those skilled in the art. 
     SUMMARY OF THE INVENTION 
     A solution to the problems noted above is the subject of this invention, where the optical coating on the external window or lens is designed so that the spectral behavior of the coating shifts with location on the window so that when the optical collection system (human eye, gimbaled optical sensor) rotates to allow the system pupil to be positioned to collect light that enters through a different portion of the window or lens, the light entering through the window or lens at “prescribed angles of acceptance”, the optical sensor sees a spectral content that is unchanged, or changed in a prescribed manner from that entering the combined system when the sensor is oriented in the “look ahead” direction. The tailored coatings are also effective when the optical system is oriented in the “look ahead” position. 
     The present invention varies the thickness of an optical interference coating applied to a substrate (e.g., a window or lens) through or on which the optical sensor detects light via transmission or reflection. In one example, the window/lens with the tailored optical coating is positioned relative to the optical sensor (gimbaled sensor or eyeball), such that the sensor is free to rotate on an axis through its center such that the system pupil sweeps out an arc behind the fixed window/lens. Stabilization of the optical coating from the system perspective is accomplished using a thickness-tailored optical interference coating that overcomes the problems noted above, stabilizing the spectral reflectance and spectral transmittance of incident radiation over the prescribed angles of acceptance for predetermined AOI. 
     The thickness of the present coating is varied so that the wavelengths transmitted or reflected correspond to those required by the specific application. The invention is adaptable to system requirements that are centered about a single wavelength, or that require multiple wavelengths (such as blue, green and red that might be used in a display application). The optical coating can take any of a number of forms, including stacked dielectric layers and rugate filters. 
     The tailoring of the optical coating&#39;s spectral properties is accomplished by depositing the coating in a manner such that its thickness, including the individual layers of the coating, vary with spatial position on the window/lens in a prescribed fashion. For example, when constant spectral behavior is desired, the coating thickness increases with radial position on the optic. Radial position is determined from the nominal ‘forward-look’ direction of the optical system. 
     Those skilled in the art will realize that additional applications exist where a nonuniform spectral response is desired, or where spectral variation is desired in only one dimension rather than two. In these cases, the overall principle remains the same, but the variation of thickness, and hence the spectral characteristics with position, may be tailored in other fashions than increasing with radial distance from a central point. 
     The tailored nature of the present coating enables an optical system that makes it possible for a user of a properly-coated helmet-mounted display beam combiner, for example, to accurately perceive the displayed content over a large display area, or for an eyeglass or sunglass wearer to perceive only predetermined wavelengths regardless of the direction in which they are looking. 
     Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the basic principles of the invention. 
         FIG. 2  is a diagram of one embodiment of an eyewear application in accordance with the present invention. 
         FIG. 3  is a diagram of one embodiment of a display application in accordance with the present invention. 
         FIG. 4  is a diagram of another embodiment of a display application in accordance with the present invention. 
         FIG. 5  is a flow chart illustrating a method of applying a tailored optical coating in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The basic principles of the invention are illustrated in  FIG. 1 . A substrate  20 , typically a window or a lens, has an interior side  22  and an exterior side  24 . A tailored optical interference coating  26  is applied to at least one side of substrate  20 ; in the example shown, coating  26  is applied to the substrate&#39;s exterior side  24 . Such a coating is characterized by being highly reflective for one or more light wavelengths, and transmissive for all other wavelengths. However, as noted above, the wavelength at which such a coating is highly reflective varies depending on the angle-of-incidence (AOI) of light impinging on it. 
     To overcome this problem, coating  26  is tailored such that it has different spectral characteristics at different locations on the substrate. An optical collection system  30 , typically a human eye or a gimbaled optical sensor such as a silicon detector or a CCD focal plane array (as in a video camera), receives light  32  from or through the substrate over a limited range of angles—referred to herein as “prescribed angles of acceptance”—which depend on the relative positions of collection system  30  and substrate  20 , as well as the collection system&#39;s aperture size, the number of axes about which it can rotate, and the amount of rotation allowed for each axis. Prescribed angles of acceptance  34  in one dimension are defined in  FIG. 1 ; there would typically also be a prescribed range of angles for the horizontal direction. The prescribed angles of acceptance define a spatial area on the substrate over which light incident on the substrate can be sensed by optical collection system  30 . Coating  26  is tailored such that optical collection system  30  sees a spectral content that is unchanged, or is changed in a prescribed manner, for predetermined AOI over the defined spatial area (when compared with light of normal incidence). 
     The substrate is typically made transparent, to allow incident radiation to pass through from exterior side  24  to interior side  22 . The tailored coating may be deposited on the substrate&#39;s exterior side (as shown in  FIG. 1 ), its interior side, or both. In addition, incoming light may be incident to exterior side  24 , to interior side  22 , or both. 
     One possible optical collection system  30  is an optical sensor mounted on a gimbal  36 . The gimbal may arranged such that the sensor can rotate about a single axis, such as an axis  38  that would permit vertical rotation over prescribed range  34 . In this case, the coating need be tailored to provide spectral variation in only one dimension. The gimbal might alternatively be arranged such that the sensor can rotate about two axes  38 ,  40 , thereby providing horizontal rotation over a different prescribed range of angles (not shown); in this case, the coating is tailored to provide spectral variation in two dimensions. 
     The optical interference coating includes a plurality of layers deposited asymmetrically on substrate  20  to achieve the desired stabilized reflectance and transmittance spectra. The optical coating&#39;s properties are adjusted by varying the thickness of the layers that make up the coating. Deposition of the layers is preferably done asymmetrically, using vapor deposition and a mask. The coating and its formation are discussed in more detail below. 
     The present coating could be tailored in a number of different ways, depending on the application for which it is used. Already noted was tailoring the coating such that an optical collection system sees a spectral content that is essentially constant over prescribed angles of acceptance. The coating might also be tailored such that an optical collection system sees a spectral content that is changed in a prescribed manner over prescribed angles of acceptance. The coating might also be arranged to completely block certain wavelengths, or to significantly block all wavelengths except that of a laser used, for example, as a laser alignment tool. For example, when constant spectral behavior in two dimensions is desired, the coating thickness is made to increase with radial position on the optic, with radial position determined from the nominal ‘forward-look’ direction of the optical system. 
     The present coating could be beneficially employed as a coating on the lenses of a pair of spectacles—particularly sunglasses. This application is illustrated in  FIG. 2 . Here, the substrate  40  is a curved spectacle (ophthalmic) lens, to which a tailored optical interference coating  42  is applied. Prescribed angles of acceptance  45  are defined for the optical collection system  44 —here, a human eye—thereby defining a spatial area on substrate  40  through which light can be perceived. The coating is tailored such that incoming light  46  that reaches eye  44  is subjected to known reflectance and transmittance spectra for each point within the defined spatial area. If the coating is tailored so as to stabilize the spectrum, the optical performance of the spectacle lens—as perceived by the wearer—is consistent regardless of wearer&#39;s angular look direction (as long as it is within the prescribed angles of acceptance). Alternatively, the coating on a spectacle lens could be tailored to preferentially transmit those wavelengths for which the eye has greater visual acuity. 
     The coating could also be tailored to achieve a desired spectral response vs. angle when the optical system is fixed in the “look ahead” orientation. Consider a uniform optical coating on a sunglass that in the look ahead direction creates a circular color gradient to the wearer. A tailored optical coating can achieve a uniform perceived color here as well. 
     One possible display application, which employs a tailored optical interference coating in accordance with the present invention, is shown in  FIG. 3 . A substrate  50  is placed within the field of vision of a viewer  52 , such that it serves as a head- or helmet-mounted screen or visor. The viewer perceives light on and through substrate  50  over prescribed angles of acceptance that define a spatial area on surface  50 . A projector  54 , such as a laser scanner, projects visual content to be viewed—such as images and/or text—onto substrate  50 . The light from projector  54  that is intended for viewing by viewer  52  includes a wavelength λ s . 
     A tailored optical interference coating  56 , characterized by being highly reflective for one or more light wavelengths and transmissive for all other wavelengths, is applied to substrate  50 . As noted above, the wavelength at which such a coating is highly reflective varies depending on the AOI of light impinging on it; as such, some of the projected content may not be visible to viewer  52 . To overcome this problem, the thickness of coating  56  is varied over substrate  50 . The coating could be tailored such that viewer  52  sees a spectral content that is unchanged, or is changed in a prescribed manner, over the defined spatial area. 
     In a typical application, coating  56  is tailored such that it is highly reflective in a narrow band about λ s  (or λ 1 , . . . , λ n  if applicable) when the substrate is viewed over the prescribed angles of acceptance, such that the perceived reflectance spectrum is stabilized. For example, in  FIG. 3 , light from projector  54  might have a first AOI θ 1  when viewed from a first angular look direction θ 2 , and have a second AOI θ 3  when viewed from a second angular look direction θ 4  (all measured with respect to a line normal to the display surface). If optical coating  56  has an even thickness and is designed to be highly reflective at λ s  for an AOI of 0, it will be less reflective to wavelength λ s  for non-zero AOI&#39;s such as θ 1  and θ 3 . However, by properly varying the thickness of coating  56 , substrate  50  can be made highly reflective at wavelength λ s  over the prescribed angles of acceptance and corresponding AOIs. In this way, viewer  52  can see information and images projected onto the visor at wavelength λ s  over the defined spatial area, as well as seeing through the visor. 
     The center wavelength λ θ  of the reflectance spectrum for a given optical interference coating  56  at a given AOI θ is approximately given by: 
                     λ   θ     =       λ   0     ⁢       1   -         n   0   2     ⁢     sin   2     ⁢           ⁢   θ       n   a   2                     (     Eq   .           ⁢   1     )               
where λ 0 =the center wavelength of the reflectance spectrum at normal incidence (AOI=0), n 0  is the coating&#39;s incident index of refraction, and n a  is the coating&#39;s average index of refraction. To stabilize the reflectance spectrum as described above, the thickness of coating  56  is varied across substrate  50 . The thickness of the coating for a given position (x,y) on substrate  50  is varied such that:
   t ( x,y )=(λ 0 /λ θ )* t (0,0)  (Eq. 2) 
where t( 0 , 0 ) is the thickness of coating  56  required to make λ 0 =λ s , and t(x,y) is the thickness of the coating at position (x,y). Calculating t(x,y) for all points of interest on substrate  50  produces a thickness profile for coating  56 . When coating  56  is applied to surface  50  in accordance with the calculated profile, the reflectance spectrum is stabilized at all points of interest.
 
     An optical interference coating  56  as described herein preferably comprises a stack of dielectric material layers, which are applied to a substrate. Such coatings are well known, and are described, for example, in U.S. Pat. Nos. 4,756,602 and 4,666,250, cited above. A coating well-suited for use with the present application is arranged to form a rugate filter, as described, for example, in U.S. Pat. No. 4,952,025 to Gunning, III, and in U.S. Pat. No. 5,004,308 to Hall et al. 
     In some applications, the light to be displayed includes n discrete wavelengths λ 1 , . . . , λ n , to accommodate different text and/or image colors, such as red, green and blue, for example. In this case, an optical interference coating—applied, for example, to a substrate  50  as shown in FIG.  3 —is arranged such that it is highly reflective in narrow bands around each of λ 1 , . . . , λ n , with its thickness varied such that the substrate is reflective at wavelengths λ 1 , . . . , λ n  when viewed over the prescribed angles of acceptance. In this way, all of the content at λ 1 , . . . , λ n  can be perceived over the full viewing angle range. Coatings suitable for this purpose are described, for example, in U.S. Pat. No. 5,410,431 to Southwell. 
     The present invention can also be employed to tailor the perceived reflectance spectrum on the surface of a beam splitter or beam combiner; this is illustrated in  FIG. 4 . Here, the substrate comprises a beam splitter  60 , which is placed between a projector  62 , such as a laser scanner, and a curved mirror  64 . An optical interference coating  66  is applied to a surface  68  of beam splitter  60  such that the surface is highly reflective for one or more light wavelengths, and transmissive for all other wavelengths. The scanner, mirror, and beam splitter are arranged such that an optical collection system  70  can perceive content which is projected onto surface  68  at the reflected wavelengths over a prescribed range of acceptance angles. 
     As in the arrangement shown in  FIG. 3 , the wavelengths at which a conventional optical coating is reflective vary depending on the AOI of light impinging on it. To overcome this, the thickness of coating  66  is varied over display surface  68 , such that it is highly reflective in a narrow band about the wavelengths of interest when the surface is viewed over the prescribed angles of acceptance, such that the perceived reflectance spectrum is stabilized. 
     For example, in  FIG. 4 , light having a wavelength λ s  is reflected from mirror  64  onto beam splitter  60  with a first AOI θ 1  when viewed from a first angular look direction θ 2 , and with a second AOI θ 3  when viewed from a second angular look direction θ 4 . If optical coating  66  has an even thickness and is designed to be highly reflective at λ s  for an AOI of 0, it will be less reflective to wavelength λ s  for non-zero AOI&#39;s such as θ 1  and θ 3 . However, by properly varying the thickness of coating  66 , display surface  68  is made highly reflective at wavelength λ s  over the prescribed range of acceptance angles and corresponding AOIs, including θ 1  and θ 3 . 
     Note that it is not essential that the present coating be tailored to stabilize the perceived reflectance spectrum. As noted above, the present coating can be tailored such that spectral content is unchanged, or is changed in a prescribed manner, over the defined spatial area. 
     The applications illustrated in  FIGS. 1–4  are merely exemplary; the present invention is useful on a wide range of display surfaces. 
     One method of applying a variable-thickness optical interference coating onto a substrate involves the use of planetary deposition; one possible implementation of this method is shown in  FIG. 5 . A substrate to be coated is placed on a holder capable of spinning about a first axis ( 80 ). The holder is placed near the edge of a turntable capable of spinning about a second axis parallel to the first axis ( 82 ). A deposition mask is affixed between the substrate ( 84 ) and a deposition material source, and the turntable is placed into a chamber ( 86 ). The holder and turntable are spun about their respective axes ( 88 ), and a constituent material of the desired optical interference coating is deposited, such as by vaporization or sputtering, for example, on a surface of the substrate by means of planetary deposition. The mask is designed such that the material deposited on the substrate is thicker near the substrate&#39;s edges than near its center, thereby achieving the tailored variable coating thickness required to stabilize the substrate&#39;s perceived reflectance spectrum. 
     While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.