Patent Publication Number: US-2013250403-A1

Title: High infrared transmission window with self cleaning hydrophilic surface

Description:
SUMMARY 
     Various embodiments described herein are generally directed to methods, systems, and apparatuses that facilitate high infrared transmission through a window having a hydrophilic surface. In one embodiment, an optical transmission window includes a dielectric substrate that is transparent at an infrared wavelength. A titanium dioxide coating is disposed on an external surface of the dielectric substrate. The titanium dioxide coating has an optical thickness of m plus one-half of the infrared wavelength, where m comprises a whole number greater than or equal to zero. 
     These and other features and aspects of various embodiments may be understood in view of the following detailed discussion and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The discussion below makes reference to the following figures, wherein the same reference number may be used to identify the similar/same component in multiple figures. 
         FIGS. 1A-1C  are block diagrams of window structures according to example embodiments; 
         FIGS. 2A-2B  are graphs illustrating analytic results of reflectivity versus wavelength for window structures according to example embodiments; and 
         FIG. 3  is a flowchart illustrating a procedure according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates generally to a window usable for optical devices that operate over a predefined range of wavelengths. In addition to providing isolation from the physical environment, the window is self-cleaning, anti-fogging, and anti-spotting. Such a window can be used, for example, to enclose an optical device such as an infrared (IR) camera that operates over a relatively small range of wavelengths. In such a case, the window can be formed of materials and dimensions that optimize self-cleaning properties, even if it results in optical performance that might be sub-optimal for wider-band optics uses (e.g., a visible light camera). 
     There are at least two different technical approaches for self-cleaning coatings: hydrophilic and hydrophobic. Both types of coatings clean themselves through the action of water. In the case of the hydrophobic surface, rolling droplets take away dirt and dust. In the case of the hydrophilic surface, sheeting water carries away dirt. In the present embodiments, a titanium oxide (e.g., titanium dioxide, TiO 2 ) coating is described as being used as a hydrophilic self-cleaning surface. Although alternate metal oxides may be used, TiO 2  is described in the examples illustrated herein because it has highly efficient photoactivity, is quite stable, and is available at low cost. 
     A TiO 2  coating material has photocatalytic and photo-induced hydrophilic properties when combined with ultraviolet (UV) light. The UV light can be from ambient sunlight or other UV light sources. The hydrophilic property of a TiO 2  coating prevents fogging, water spotting, and promotes a washing flow of rain water instead of beading. The photocatalytic properties of a TiO 2  coating prevents the buildup of dirt, dust, and various organic materials. A photochemical reaction proceeds on a TiO 2  surface when irradiated with ultraviolet light. This causes photo adsorption which results in decomposition of organic substances. The decomposition is effective when the number of incident photons is much greater than that of filming molecules arriving on the surface per unit time. 
     A TiO 2  layer may be used as a durable thin film dielectric material for optical coatings, with some restrictions. A TiO 2  coating has a relatively high refractive index (approximately 2.6) which produces a single surface Fresnel reflection of approximately 20% at an air interface. So arbitrarily applying the material over a window or lens can significantly reduce the optical transmission of the window or lens. As a result, for general-purpose glass windows and lenses, a TiO 2  coating may be unsuitable due to the high refractive index causing significant reflection. Also, thick coatings of TiO 2 , while maximizing self-cleaning properties, may provide unacceptable attenuation at some wavelengths. 
     The proposed embodiments utilize a coating with an external TiO 2 /air interface that achieves a high optical transmission over a particular range of wavelengths while providing the self-cleaning features described above. The range of wavelengths may include portions of the IR spectrum, such as near infrared (NIR) spectral bands. A TiO 2  coating with such properties may be useful, for example, in applications such as NIR surveillance cameras. This type of camera may use NIR LED illuminators with center wavelengths in the 780 nm to 1000 nm range. An NIR surveillance system may require light collection optical systems that are optically efficient over a relatively small range of wavelengths, and that can withstand exposure to the elements for long periods of time without maintenance (e.g., manual cleaning of viewing windows). 
     In reference now to  FIG. 1A , a block diagram shows a window  100  according to one embodiment. The window  100  is formed from a sheet  102  of dielectric material (e.g., glass) that is transparent at least at a light wavelength of interest (e.g., NIR), and may be transparent over other wavelengths as well. The glass is used as a substrate for forming a externally facing coating  104  (not shown to scale) of a titanium dioxide, e.g., titanium dioxide (TiO 2 ). The surfaces of the glass  102  can be uncoated or anti-reflection (AR) coated prior to applying the TiO 2  coating  104 . 
     It has been found that if only a small, predetermined, band of wavelengths is to be transmitted without significant attenuation through the window  100 , a thicker coating  104  of TiO 2  tuned to those wavelengths can be applied, thus exhibiting the desired physical characteristics (e.g., self-cleaning) while permitting any desired treatment to the remainder of the optical assembly. In some applications of TiO 2  coatings, it may be permissible or even desirable to have a visible effect (e.g., lower reflection, greater transmissibility) on the transmitted light. However, this may require a thinner, less hardy and harder-to-apply coating. 
     The coating  104  has photocatalytic and photo-induced hydrophilic properties described above when combined with UV light. The TiO 2  coating  104  may have an optical thickness of approximately one half wavelength of light at a wavelength of interest, which can be extended to include m plus half the wavelength, where m=0, 1, 2, 3, . . . . This maximizes transmissibility of the coating  104  around that wavelength, and makes the window  100  substantially transparent at the wavelengths of interest. For NIR applications, the optical thickness may range from 390 nm to 500 nm. 
     The optical thickness of the coating  104  is proportional to a physical thickness  106  of the coating  104  based the refractive index of the coating  104  at the wavelength of interest. The optical thickness is equal to the physical thickness  106  multiplied by the refractive index of the layer material. So the optical thickness of the TiO 2  layer  104  for 850 nm light is 850 nm/2=425 nm, which corresponds to a physical thickness  106  of 425 nm/2.6=163 nm, where 2.6 is the refractive index of TiO 2  at 850 nm wavelength. The NIR optical thickness range from 390-500 nm noted above corresponds to a physical thickness  106  of 150-192 nm. 
     As shown in  FIG. 1A , the window  100  may be used with an enclosure  108  to protect an optical device  110 . The optical device is configured to emit and/or receive a narrowband spectrum of infrared light centered at a target wavelength, such as 850 nm which is in the NIR portion of the spectrum. The optical device  110  may include, but is not limited to, an infrared detector, camera, illuminator, etc. The window  100  is optimized to produce minimal attenuation for the light sent and/or received by the optical device  110 . The window  100 , together with the enclosure  108 , provides a sealed environment that allows the device  110  to be used in harsh conditions. Due to the self-cleaning properties of the coating  104 , the device  110  is provided with good visibility through the window  100 , and this visibility can be maintained with minimum intervention even under harsh environmental conditions. 
     As mentioned above, a window according to example embodiments may include an AR coating. One type of AR coating is formed from a substance with a refractive index that is matched to the refractive index of the glass  102  to reduce reflections from the window  100 , thereby improving light transmission efficiency. For example, a single layer AR coating may be chosen such that an index of refraction of the coating is the square root of the refractive index of the glass  102 . Magnesium fluoride (MgF 2 ) has a refractive index of about 1.38, and is therefore often used as an AR coating for optical glass, which has an index of refraction of about 1.52. Other AR coatings may absorptive or include nanostructures that reduce reflections. More complex, higher performance multilayer AR coatings may also be used. 
     Example configurations of windows  120 ,  130  with an AR coating are shown in  FIGS. 1B and 1C . For convenience, the same reference numbers are used to refer to like elements described in  FIG. 1A , although it will be appreciated that the thicknesses, composition, etc., of these components may vary between different embodiment depending on the desired characteristics and interactions with the AR layers and coatings. In  FIG. 1B , window  120  includes an AR coating  122  on a surface of the glass  102  opposite the TiO 2  coating  104 . In  FIG. 1C , window  130  includes an AR layer  132  between the TiO 2  coating  104  and glass  102 . This window  130  also includes inside AR coating  122 , although this coating layer  122  may be optional. 
     In  FIGS. 2A and 2B , graphs  200 ,  210  show results of analyses performed on windows according to example embodiments. In  FIG. 2A , curve  202  represents intensity reflection versus wavelength for a window arrangement  102  as shown in  FIG. 1 , with a TiO 2  coating  104  directly on glass  102  substrate. In this example, the optical thickness of the TiO 2  coating is 425 nm (which is equal to the refractive index of TiO 2  at 850 nm multiplied by the physical thickness  106  of the coating), corresponding to a half wavelength of 850 nm NIR light. Similar properties should hold for an optical thickness equal to m+½ times the infrared wavelength for m=0, 1, 2, 3, . . . . Curve  204  represents the same analysis for uncoated glass. As graph  200  shows, reflection of the TiO 2  coated surface (represented by curve  202 ) is nearly as low as uncoated glass (represented by curve  204 ) for wavelengths proximate 850 nm. The half-wavelength optically thick TiO 2  layer is not an AR coating, but instead behaves like a null coating at and near the center wavelength of the NIR. 
     In  FIG. 2B , the graph  200  shows a similar analysis, but in this case curve  312  represents results for a TiO 2  coating with an optical thickness of 425 nm  104  is formed on an AR layer  132  as shown in  FIG. 1C  (without opposite facing AR layer  122 ). For this analysis, the AR layer  132  is formed of MgF 2  with 212.5 nm optical thickness (which is equal to the physical thickness of the layer multiplied by the refractive index 1.38 of MgF 2  at 850 nm). Curve  214  represents the same analysis for AR coated glass without a TiO 2  layer. Again, reflection of the TiO 2  coated surface (represented by curve  212 ) is nearly as low as the AR-only surface (represented by curve  212 ) for wavelengths proximate 850 nm. Also of note is that the minimum reflectance of curve  212  is lower than that of curve  202  in  FIG. 2A . This shows that the AR coating is effective at the wavelength of interest, even with the addition of the TiO 2  outer coating. 
     As these results show, coating with a high refractive index (relative to glass) at an air interface can achieve high transmission performance in a dielectric (e.g., glass, plastic, etc.) window or lens spectral band or narrow spectral band. Optical coating designs that utilize a half-wave optically thick TiO2 layer can achieve high transmission in a dielectric (e.g., glass, plastic, etc.) window or lens within an LED emission spectral band or narrow spectral band. This technique can achieve a self-cleaning high transmission window or lens within an LED emission spectral band or narrow spectral band. 
     In reference now to  FIG. 3 , a flowchart illustrates a procedure according to an example embodiment. A dielectric substrate (e.g., glass, plastic) is provided  302 , the substrate being is transparent at an infrared wavelength. A titanium dioxide coating is formed  304  on an external surface of the dielectric substrate. The titanium dioxide coating has an optical thickness m plus one-half of the infrared wavelength, where m is a whole number greater than or equal to zero. Optionally, an anti-reflective coating is formed  306  on the dielectric substrate. 
     The foregoing description of the example embodiments has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Any or all features of the disclosed embodiments can be applied individually or in any combination are not meant to be limiting, but purely illustrative. It is intended that the scope of the invention be limited not with this detailed description, but rather determined by the claims appended hereto.