Abstract:
A system for mounting an infrared (IR) remote control receiver in a rear projection television apparatus features a mirror having a coating wherein visible light is reflected to a viewing device, and light in the infrared regions is passed through the mirror and detected by an IR receiver mounted behind the mirror.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of projection televisions (PTV), and more particularly to a system for mounting infrared receivers in a PTV. 
     BACKGROUND OF THE INVENTION 
     When an infrared (IR) remote control device is used with a rear projection television (PTV) set, a receiving IR device is typically mounted either on an area of a front panel near the edge of a focusing lens assembly, or at the bottom of a reflecting mirror. The selection of a specific location is made to provide a line-of-sight path from the IR receiver to a viewer&#39;s controlling device. However, to preserve controller battery life, the transmission path is minimized, thereby providing a sufficiently strong received signal for a given transmitted power. 
     Disadvantageously, such an arrangement occupies front panel surface area. This forces a chassis size to be larger than necessary to accommodate the IR receiver sensing element in a typically esthetically-pleasing arrangement within the overall PTV enclosure. To compensate for this impact on chassis size, the target area of the receiving sensor is typically reduced, thereby reducing the received signal power. 
     SUMMARY 
     A rear projection television (PTV) system employing a reflecting mirror for redirecting an image to be projected, wherein the mirror is fabricated with a coating material having dual light properties according to a preferred embodiment of the present invention. With the dual properties comprising a high reflectivity property to incident visible light beams (wavelengths&lt;790 nm) and a transparency property to IR light beams (wavelengths&gt;790 nm), an IR receiver element can be positioned behind the mirror rather than on a front panel of the PTV. In addition, a fresnel lens used for focusing the created image can also concentrate the incident IR beam to provide for a stronger received IR signal than would otherwise be possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a conventional projection television (PTV) system indicating exemplary locations for a remote control infrared (IR) receiver. 
     FIG. 2 shows a PTV system indicating an exemplary location for a remote control infrared (IR) receiver according to a preferred embodiment of the present invention. 
     FIG. 3 shows a plot of the reflectivity vs. wavelength for an incident light beam for a conventional mirror. 
     FIG. 4 shows a plot of the reflectivity vs. wavelength for an incident light beam for a mirror coated according to a preferred embodiment of the present invention. 
     FIG. 5 shows a plot of the transmissivity of the mirror of FIG. 3 vs. wavelength for the incident beam of light. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a conventional rear projection television (PTV) system  10  having a lighted image source  12 , such as a cathode ray tube (CRT), that directs a visible light beam to a planar mirror  14 , where the beam is redirected to a target plane either on a focusing fresnel lens  16  or onto a display screen (not shown). Typically, such a system  10  has an infrared (IR) remote control apparatus consisting of a remote user interface device  18  having an IR transmitter in communication with an IR receiving device  20 . IR receiving device  20  is typically located on a front surface of a chassis of a rear PVT apparatus  22 , to ensure a maximum strength line-of-sight signal at a sensor of IR receiving device  20 . 
     Alternative mounting locations for IR receiving device  20  in a rear PTV apparatus  22  can also be inside the chassis at a location  24  where the IR signal can be reflected to an IR receiving device  20   a  by planar mirror  14 . While such a redirected IR configuration presents a longer signal path, and thus a weaker IR signal, than a front panel mounting arrangement, it has an added advantage in that the chassis does not require external esthetic modification to accept IR receiving device  20 . A primary design consideration is the ability to provide a strong enough IR signal at IR receiving device  20  when remote interface device  18  is not directly in front of the PTV apparatus  22 . Since an angle of incidence on planar mirror  14  will be the angle of reflection, the reflected beam will not typically align the center of an IR beam onto IR receiving device  20   a , further degrading the ultimate received IR signal. 
     FIG. 2 shows a PTV system  26  indicating an exemplary location for a infrared (IR) receiver  28  according to a preferred embodiment of the present invention. By modifying the coating of a planar mirror  30 , an IR wavelength beam  32  from remote interface device  18  can pass through the planar mirror  30 , while a visible wavelength beam  34  from the CRT  12  can be reflected to a target plane or fresnel lens  16 . Such a semi-reflecting planar mirror  30  allows IR receiving device  28  to be positioned and mounted behind planar mirror  30 . Thus, an IR beam received at IR receiving device  28  would be independent of an incident transmission angle, since all incoming IR beams will have a direct line-of-sight path to IR receiving device  28 . 
     IR receiving device  28  can be mounted in a variety of ways behind planar mirror  30 , and, since no particular esthetic considerations are required, a significant reduction in parts count, assembly time, and cost can be realized. An exemplary mounting arrangement could be a molded plastic frame secured to chassis frame of the PTV apparatus  22 . 
     The reflective coating of planar mirror  30  would preferably have a spectral transmission/absorption characteristic that provides 1) a high reflectivity surface to light wavelengths below an exemplary wavelength λ of 790 nanometers (nm) and 2) a transparent medium for infrared wavelengths above that exemplary 790 nm wavelength. For example, below 790 nm the reflectivity could be approximately 90% and the transmission approximately 10%, whereas above 790 nm the reflectivity could be approximately 15% and the transmission approximately 85%. Such a mirror is known in the art as a “cold mirror.” Alternative wavelengths can be used as the partition between the visible light and the communications IR link, for example, 600 nm, 900 nm, or 1200 nm wavelengths. 
     Fabrication of such a cold mirror consists of coating a generally transparent material, such as glass or plastic, said coating being deposited on at least one of the major planar surfaces, said coating further consisting of a material which is one or more compounds from the exemplary group consisting of titanium oxide, silicon dioxide and tantulum oxide. The finished semi-reflective planar element would be positioned in the light path between the lighted image source  12  and the target plane as shown in FIG.  2 . 
     An added benefit of having fresnel lens  16  serving as the target plane for visible light beam  34  in FIG.  2  and in the radiation path for IR signal  32 , is that fresnel lens  16  would also concentrate incoming IR beam  32 . This concentration increases the energy density at IR receiving element  28 , thus, allowing for lower energy usage. Further, since visible light both from the CRT and other external light sources includes some IR radiation, baffles may be used to minimize any degradation effects of such unwanted beams. 
     FIG. 3 shows a plot  36  of the reflectivity vs. a wavelength of an incident light beam for a conventional mirror. Dotted line  38  represents a wavelength of 790 nm, which will be an exemplary partition point for FIGS. 4 and 5. A natural attenuation of the plot occurs above 1200 nm. 
     FIG. 4 shows a plot  40  of the reflectivity vs. a wavelength of an incident light beam for a mirror coated according to a preferred embodiment of the present invention. A clear demarcation begins a roll-off at point  42 , so that the reflectivity of the planar element of the present invention is essentially eliminated for wavelengths above 790 nm at point  44 . 
     FIG. 5 shows a plot of the transmissivity of the mirror of FIG. 2 vs. wavelength for an incident beam of light. The practical effect is to merge the plot shown in FIG. 3 with the plot shown in FIG. 4, yielding a generally gaussian curve centered on a wavelength of 1000 nm. 
     Numerous modifications to and alternative embodiments of the present invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. Details of the embodiments may be varied without departing from the spirit of the invention, and the exclusive use of all modifications which come within the scope of the appended claims is reserved.