Abstract:
A detector facilitates visualization of electromagnetic radiation, including radio frequency radiation. The electromagnetic radiation is converted to thermal energy by partial absorption of the radiation, and is made visually observable by coupling the pattern of thermal energy to a thermally-responsive imaging surface, such as a liquid crystal display.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to methods and apparatus for visualizing electromagnetic radiation; and, more particularly, relates to methods and apparatus for providing spatial properties of electromagnetic radiation. 
     Many applications are known wherein it is useful to detect the presence of electromagnetic forces (EMF) such as radio frequency (RF) waves. Devices are known to electrically detect the presence of electromagnetic waves. Such devices typically include electric circuits which are tuned to be sensitive to the presence of electromagnetic waves within selected frequency ranges. Such devices typically suffer from the potential disadvantage that the devices are sensitive to electromagnetic radiation only within a specific frequency range. In many cases, these ranges of sensitivity are extremely narrow. Additionally, conventional devices typically do not allow a user the opportunity to readily determine or visualize the propagation patterns of detected electromagnetic radiation. This is true notwithstanding the fact that such visualization would be extremely useful in many applications, such as detecting leaks from microwave ovens, or in servicing devices such as cellular telephones or other types of communication equipment, including transmitting antennas. 
     Accordingly, the present invention provides a new method and apparatus whereby electromagnetic radiation, and in particular radio frequency emissions, can be rendered visually detectable; and which can, in selected implementations, be constructed to be sensitive to radiation over a wide range of wavelengths and frequencies. Additionally, such devices may be constructed to detect emissions without regard to waveform or modulation. 
     SUMMARY OF THE INVENTION 
     The present invention facilitates imaging of electromagnetic wave propagation. In accordance with the present invention, electromagnetic waves are partially absorbed by a plurality of generally discrete particles forming a generally resistive medium. In one preferred embodiment, the resistive medium will have a sheet resistance 1,000 to 5,000 ohms per square. In a particularly preferred embodiment, the medium will comprises a plurality of carbon particles within a sheet or matrix, placed to yield a sheet resistance of approximately 2,000-3,000 ohms per square. When placed proximate a source of radiation, the carbon particles will absorb a very small portion of the RF radiation and will generate heat in response to such absorption. The energy absorbing medium is placed in thermal communication with a heat responsive imaging surface, for example, a liquid crystal sheet. In one particularly preferred embodiment, the energy absorbing medium is adhesively coupled through a heat conducting medium to the rear surface of a liquid crystal sheet. Heat generated within the medium is transferred to form a visual pattern in the liquid crystal sheet, facilitating detection and observation of radiation patterns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts an exemplary embodiment of a electromagnetic radiation detector in accordance with the present invention, illustrated from an oblique perspective. 
     FIG. 2 depicts a portion of the radiation detector of FIG. 1, illustrated in lateral vertical cross-section. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings in more detail, and particularly to FIG. 1, therein is depicted an exemplary embodiment of an electromagnetic radiation detector 10 in accordance with the present invention. As will be apparent from the discussion to follow, a large variety of sizes and configurations of detectors may be constructed in accordance with the present invention. Electromagnetic radiation detector 10 is described herein in the context of a detector configured to detect radio frequency waves. Radio frequency (or &#34;RF&#34;) propagations are generally considered to encompass electromagnetic waves at frequencies between 10 kHz and 100 GHz. Such frequency range is generally considered to be that in which EMF radiation may be detected and amplified as electric current at the wave frequency. 
     Depicted detector 10 may be constructed in any convenient size. For example, a detecting area of approximately four inches by four inches may be satisfactory for many applications, such as evaluating RF shielding in consumer products. Detector 10 is depicted as having a handle 12 which is coupled to a detecting assembly, indicated generally at 14, to facilitate support and movement of detecting assembly 14 to a desired location and in a desired manner. 
     Referring now also to FIG. 2, therein is depicted detecting assembly 14 from a lateral view and in vertical section. Detecting assembly 14 preferably includes three primary components. The first component is an energy absorbing material or layer 16, which, for most applications, will be partially transparent to radio frequencies. Energy absorbing material 16 is coupled, such as through a heat conductive adhesive 18 to a thermally responsive visual imaging surface 20. In a preferred implementation, visual imaging surface 20 will comprise a liquid crystal sheet. Typically, liquid crystal sheets are very transparent to radio frequencies. The heat conductive adhesive may be a conventional spray adhesive, such as an aerosol photo adhesive with thermal conductivity. 
     In a particularly preferred implementation, energy absorbing material 16 includes a matrix of &#34;KAPTON.&#34; KAPTON is a polyimide material manufactured by DuPont Chemical Co. The KAPTON is impregnated with carbon particles to a degree sufficient to yield a desired sheet resistance. Typically, the sheet resistance will be between 1,000 and 5,000 ohms per square, and a sheet resistance of 2,000-3,500 ohms per square will be preferred for many applications. In one preferred implementation, a resistivity level of approximately 3,000 ohms per square is desired. As will be appreciated by those skilled in the art, the carbon level within the KAPTON may be adjusted to vary the sheet resistance. The greater the desired sheet resistance, the lower the volume of carbon which will be provided within the KAPTON matrix. The sheet resistance will be selected in response to the power of RF to be detected. For example, the greater the power of the radiation source to be examined, the greater resistivity that can be utilized, and the lower the volume of carbon which is needed. 
     For many applications it will be preferable that the detector provide minimal interference to the radio waves being detected, so as to allow the waves to propagate substantially unimpeded by detector 10. Accordingly, a material such as carbon impregnated KAPTON is selected because it is relatively transparent to radio frequency waves. Such carbon impregnated KAPTON may be utilized in accordance with the present invention to absorb less than 0.5% of RF energy, and preferably to absorb a range of 0.125-0.25% of RF energy. Materials other than KAPTON may also be utilized within such an absorbency range. For example, a sheet polyester material such as that sold under the tradename &#34;MYLAR,&#34; also manufactured by DuPont Chemical Co. may be coated with carbon particles (in a manner similar to that of carbon paper) to provide an alternative energy absorbent layer. Additionally, materials other than carbon may be utilized. Carbon is a desirable RF absorbent for this application because it has a relatively low resistivity, and also will heat generally linearly in a response to absorption of RF radiation. Materials such as nitrides, however, may also be utilized as a RF absorbent medium. 
     The RF absorbent medium will be selected to yield a thermal imprint which is resolvable by the thermal imaging surfaces. In the case of a liquid crystal imaging surface, a temperature differential of 4-5 degrees of above ambient will typically be satisfactory. 
     In operation, radio frequency emissions will contact the carbon particles in the KAPTON sheet or other surface, and will generate an incremental amount of heat in the particles. This heat is then transferred through adhesive 18 to the rearward surface 22 of liquid crystal sheet 20. Liquid crystal sheet 20 is responsive to the heat differential, and will transfer the pattern of the heat generated within the energy absorbing layer, thereby allowing the pattern of the impinging radio waves to viewed. 
     Liquid crystal sheet 20 may have an operating temperature range between approximately 77° F. and 86° F. However, as is known to those skilled in the art, liquid crystal is manufactured in a variety of operating temperature ranges and other operating ranges may be used, as desired. If, for example, a detector was intended to be used outside, or in another potentially warm environment, then liquid crystal having range of approximately 88° F.-100° F. might be selected. Liquid crystal media will typically be able to resolve a thermal image in an ambient below the designated operating range. 
     Many modifications and variation may be made in the techniques and structures described and illustrated herein without departing from the spirit and scope of the present invention. Accordingly, it should be readily understood that the embodiments described herein are illustrative only and are not to be as limitations upon the scope of the present invention.