Patent Publication Number: US-2003230725-A1

Title: Sensor arrangement having a capacitive light sensing circuit

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] This invention relates to a system having a light sensing circuit and more particularly to a light intensity sensor having a light sensitive film for detecting and measuring the intensity of ultraviolet radiation.  
       [0003] 2. Background Art  
       [0004] The hazards of ultraviolet (UV) radiation are well known. One of the most common sources of ultraviolet radiation is UV rays generated by sun light or artificial light sources such as lights in a tanning salon bed. Ultraviolet rays are generally classified as either ultraviolet A (UVA) or ultraviolet B (UVB) rays. Excessive exposure to these ultraviolet lights, especially UVB rays, can lead to skin tissue damage. In recent years, scientists have also recognized the harmful effects of UVA rays on skin tissue.  
       [0005] Although ultraviolet radiation plays a vital role in the human body&#39;s ability to produce vitamin D, overexposure to such rays commonly results in a sunburn condition. Sunburn not only causes a great deal of immediate discomfort, it can lead to other skin problems, such as skin cancer or photo-aging. Skin cancer and photo-aging are not mutually exclusive. Rather, the conditions often coincide and coexist as a result of years of excessive sun exposure. Generally, those having severe sunburns or numerous sunburns are subject to a high risk of skin cancer.  
       [0006] Photo-aging, on the other hand, is a process of skin changes resulting from overexposure to sun over a number of years. These skin changes include color, wrinkles, freckles, dryness, skin growths, easy bruising, and liver spots. As with skin cancer, photo-aging is more prevalent in humans who are susceptible to sunburn, especially those with fair skin.  
       [0007] Human susceptibility to ultraviolet radiation is dependent upon many factors, including time of day, weather, altitude, and proximity to reflective surfaces. Humans are unable to detect the amount of ultraviolet radiation they are exposed to until the radiation&#39;s effects show up in the form of a sunburn. Recently, several health equipment systems have appeared on the market to measure the intensity of ultraviolet light. These sunburn alarms provide information to prevent sunburn, or additional sun related skin disorders. For example, ultraviolet light intensity information can be used to select the appropriate type of sun screen protection, or to determine how long to remain outdoors on days with extremely strong ultraviolet light.  
       [0008] The light sensors of existing sunburn alarm systems typically utilize solar powered batteries and photo diodes to detect ultraviolet radiation conditions. The photo diodes typically include a thin semiconductor wafer formed of silicon, germanium or gallium that converts incident light photos into electron-hole pairs. The semiconductive materials form a thin disk crystalline lattice structure of about 20-30 cm in diameter having about 15,700 individual sensors fabricated on a single 20 cm disk.  
       [0009] A typical fabrication plant considers 25 disks as a single production unit forming 392,500 photo diode sensors. Therefore, the production of only a few thousand photo diodes is difficult and expensive. Further, both silicon, germanium in crystal form are essentially insulators and conduct little electricity because they exhibit a high degree of chemical purity and do not provide a photo-electronic response. For these elements to provide a photo-electronic response, a process known as doping must be used to create a lattice defect, or an electron hole in the crystallized matter. Accordingly, trace amounts of impurities need to be added to the crystalized matter creating the lattice defect to produce much greater conductivity. Especially with crystal-based light sensors, materials added to the crystallization such as arsenic or potassium are generally hazardous to humans.  
       [0010] In photo sensors using semi-conductor crystallization technology, the atoms that constitute the crystal and its alignment are already predetermined. Therefore, with the crystallization structure included, the light wavelength and sensitivity characteristics are established. There are no other choices for wavelength selection.  
       [0011] In lower priced light sensors, cadmium sulfide is often used as the resistive element. However, cadmium sulfide is poisonous, and therefore, not desirable for assembly or use. In addition, cadmium sulfide is only sensitive to specific light wavelengths associated with visible light, thus limiting its usefulness in detecting harmful ultraviolet radiation or rays.  
       [0012] Crystal based light sensors suffer additional limitations which reduce the effectiveness of sensors. The photo electronic effect of crystal-based light sensors converts the incoming light into electrical current. However, analog electronic circuits are required to amplify these minute electrical currents, raising them to levels where other circuits can process them. Thus, electrical consumption becomes problematic and consistent operation becomes difficult in battery driven instruments.  
       [0013] It would be advantageous to provide a system having a light sensing circuit using a capacitive light sensing film for detecting and measuring the intensity of ultraviolet rays which solves the problems referenced above. Further, it would be advantageous to provide an inexpensive and safe way of producing a light sensing film for measuring the intensity of ultraviolet radiation. It would also be advantageous to provide a system for detecting and measuring light intensity which allows for greater freedom in selecting the particular light wave lengths to be sensed and which uses low-power electronic circuitry to generate the light intensity signal.  
       SUMMARY OF THE INVENTION  
       [0014] Accordingly, a system having a sensor detecting light intensity using a light sensing film is disclosed. The system includes a light sensor for sensing light intensity having a capacitance which varies based on light intensity. The light sensor includes first and second layers forming first and second electrodes and a photosensitive dielectric layer disposed between the first and second electrodes.  
       [0015] The photosensitive dielectric layer has a dielectric constant that varies with light intensity such that the sensor has a capacitance representative of light intensity. A controller in communication with the sensor measures the capacitance of the sensor, compares the measured capacitance values to stored capacitance values and generates an output signal based on the comparison. The output signal is configured for use in providing an indication of light intensity.  
       [0016] The above aspects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017]FIG. 1 is a top plan view of a timepiece having a system for detecting light intensity in accordance with the present invention;  
     [0018]FIG. 2 is a cross-sectional view of the timepiece along line  2 - 2  of FIG. 1 illustrating the light intensity sensor of the present invention;  
     [0019]FIG. 3 is a cross-sectional view of the timepiece along line  3 - 3  of FIG. 1 illustrating the sensor housing of the present invention;  
     [0020]FIG. 4 is a plan view of the timepiece display panel of the present invention;  
     [0021]FIG. 5 is a cross-sectional view of the light sensing film of the system of the present invention;  
     [0022]FIG. 6 is a cross-sectional view of the photosensitive material of the light sensing film of the present invention;  
     [0023]FIG. 7 is a cross-sectional view illustrating an alternative aspect of the light sensing film of the present invention; and  
     [0024]FIG. 8 is a schematic view illustrating an example of the system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0025] Referring now to the Figures, a system having a sensor for detecting light intensity is disclosed. FIG. 1 illustrates a preferred aspect of the invention. A timepiece or watch  10  including an optical sensor  12  having a light sensing film is provided proximate a face portion  14  of the watch housing  16 . Face portion  14  is generally formed of a transparent polymeric or glass material disposed above a display  26  of watch  10 . Watch housing  16  includes a plurality of buttons  18 ,  20  provided on exterior surfaces of the housing  16  to allow an operator to control a variety of functions of the watch  10 .  
     [0026] For example, buttons  18  are provided on a front surface  22  of housing  16  to control a stop watch or timer function incorporated in watch  10 . Buttons  20  provided on side surfaces  24  of watch housing  16  may control watch functions such as activating a backlighting panel or light emitting diode (LED) which illuminates the display  26  of the disposed below the face  14  of watch housing  16 . It is understood that ultraviolet sensor  12  can be incorporated with a variety of watch styles, including a digital timepiece, analog timepiece, or a combination of both.  
     [0027] Display  26  includes a liquid crystal display (LCD) panel  28  provided within watch housing  16  below protective lens or face  14 . LCD panel  28  is operably connected to a control circuit  30  on printed circuit board  32  to provide a graphic display of relevant information to the user. For example, as illustrated in FIG. 4, the watch may display the current time in hours, minutes and seconds as well as date information. Alternatively, the display panel  28  may illustrate countdown timer, stopwatch and alarm functions activated when the user controls these functions using buttons  18 ,  20 . LCD display panel  28  further includes a bar line display  34  which illustrates an indication of ultraviolet light intensity based on an output signal. A further description of the method of detecting ultraviolet light intensity will be discussed in great detail below.  
     [0028] Referring now to FIGS.  2  or  3 , sensor  12  is disposed proximate the front surface  22  of the watch housing  16  above the face portion  14  of LCD panel  28  to detect ultraviolet light intensity. A protective layer or cover  36  extends above ultraviolet sensor  12  to protect the sensor  12  from physical damage. Cover  36  generally comprises a transparent polymeric or glass material and may include a filter to assist sensor  12  in the detection of ultraviolet rays. Ultraviolet sensor  12  is operably connected to printed circuit board  32  via a sensor spring  38 . Temperature sensor  40  provided adjacent ultraviolet sensor  12  below cover  36  is connected to printed circuit board  32  by sensor spring  42  to detect the ambient temperature adjacent cover  36 .  
     [0029] Referring now to FIGS.  5 - 8 , a description of the system of the present invention is described in greater detail. Sensor  12  includes a light sensing film  44  for sensing light intensity. Light sensing film  44  includes a first layer having a first electrode  46 , a second layer including a second electrode  50  and a photosensitive dielectric layer  48  disposed therebetween. In one aspect of the invention, film  44  is disposed proximate protective layer  36 . In an alternative aspect of the invention, protective layer  36  comprises a transparent plastic layer integrated into the light sensing film  44 .  
     [0030] In one aspect of the invention illustrated in FIG. 5, protective layer or cover  36  comprises a transparent plastic layer which forms a physical protection barrier for light sensing film  44  which allows light to pass therethrough. Cover  36  includes a top surface  51  and a lower surface  52  disposed proximate the first electrode  46 . In a preferred aspect of the invention, protective layer  36  is printed or evaporation coated as the backing material for the first electrode  46 .  
     [0031] First electrode  46  is formed of a transparent conductive material which allows light to pass through to the photosensitive dielectric layer  48 . First electrode  46  is preferably formed as an indium tin oxide (ITO) film comprising indium oxide (In 2 O 3 ) doped with tin oxide (SnO 2 ). It is understood that ITO coatings provide outstanding electronic conductors while having optically transparent characteristics. In one aspect of the present invention, the ITO film of the first electrode  46  is vacuum deposited in a thin film on the bottom surface  52  of transparent protective layer or cover  36  using a process known as magnetron sputtering to create a high molecular compound film.  
     [0032] Photosensitive dielectric layer  48  is disposed between the first electrode  46  and the second electrode  50 . Photosensitive dielectric layer  48  includes a photosensitive dielectric material, such as common flourescent material, cadmium sulfide (CdS), or zinc sulfide (ZnS) for detecting the wavelength of light  54  passing through layer  36  and first electrode  46 . Photosensitive dielectric layer  48  is preferably screen printed to transparent first electrode  46 . However, it is fully contemplated that photosensitive dielectric layer  48  may be formed proximate the first layer by other means to a variety of thicknesses. The electron orbit of the particles of the photosensitive dielectric material  48  will change according to light wavelengths and strengths. The change in the electron orbit will appear as a change in the dielectric constant of the photosensitive material.  
     [0033] Referring additionally now to FIG. 6, photosensitive dielectric layer  48  is illustrated with an excited portion  56  and an unexcited portion  58 . Excited portion  56  of dielectric layer  48  is activated and excited by light  54  passing through cover  36  and first electrode  46 , while unexcited portion  58  of dielectric layer  48  remains inert. The dielectric layer  48  is sensitive to inherent light wavelengths and is excited by a reaction to specific wavelengths of light. Changes in light intensity alters the dielectric layer&#39;s  48  ability to support an electric field between the first electrode  46  and second electrode  50 , thereby varying the capacitance of light sensing film  44 .  
     [0034] In one aspect of the present invention, photosensitive dielectric layer  48  is formed of zinc sulfide which may display a light sensitivity down to 350 angstroms (or 35 nm), in accordance with the absorption wavelength of zinc sulfide. Further, in using zinc sulfide, there is minimal sensitivity to visible light, having wavelengths beginning near 400 nm. Ultraviolet radiation lies between wavelengths of about 100 nm on the x-ray side of the electromagnetic spectrum and about 400 nm on the side of visible light. In this manner, dielectric layer  48  of capacitive light sensing film  44  is suitable for sensing ultraviolet radiation in light  54 .  
     [0035] Alternatively, selection of various photosensitive dielectric materials makes selection of various light wavelengths for sensing possible, thus measuring a wide spectrum of light wavelengths can be freely established. The photosensitive dielectric material  48  will display sensitivity to each inherent light wavelength. For example, if red flourescent material is used as the photosensitive dielectric layer, capacitive light sensing film  44  is more sensitive to red wavelengths. On the other hand, if white flourescent material (a mix of a variety of wavelengths) is used as photosensitive dielectric layer  48 , capacitive light sensing film  44  is more sensitive to general visible light. In another aspect of the invention, a printable powder may be used as the photosensitive dielectric layer  48 . By altering the photosensitive material of photosensitive dielectric layer  48 , a particular light wavelength can be selected for measurement, thereby providing a degree of freedom in selecting wavelengths to monitor with watch  10 . Additionally, the sensitivity and maximum intensity value of the light can be regulated by adjusting the thickness of photosensitive dielectric layer  48 .  
     [0036] A lower protective material  60  is disposed proximate second electrode  50 . Lower protective material  60  is preferably formed as a coating or plate for protecting the second electrode  50  and second dielectric layer  64 . Although lower protective material  60  is included in a preferred embodiment of the present invention, it is fully contemplated that lower protective material  60  is not always necessary in this or alternative embodiments. Preferably, lower protective material  60  can be screen printed to a bottom surface of the second electrode  50 . It is also understood that lower protective material  60  may be formed by other means, such as bonding or the like.  
     [0037] Transparent first electrode  46 , second electrode  50  and dielectric layers  48  and  64  combine to form capacitor arrangement  62  of sensor  12 . Capacitor  62  is a parallel plate capacitor having capacitance, C, approximated by:  
             C   =         ɛ   o          ɛ   r        A     d             (     Eq   .              1     )                       
 
     [0038] where:  
     [0039] ε o =permittivity of free space;  
     [0040] ε r =dielectric constant;  
     [0041] A=surface area of each electrode plate; and  
     [0042] d=distance between electrode plates.  
     [0043] In a preferred aspect of the invention, light  54  enters light sensing film  44  through the transparent protective layer  36  through the transparent first electrode  46 , into the photosensitive dielectric layer  48 . When the light  54  reaches the dielectric layer  48 , the dielectric layer  48  senses and reacts to certain wavelengths or energy levels in light  54 , increasing the dielectric constant, ε r . The stronger the light intensity, the farther the light  54  will penetrate into photosensitive dielectric layer  48 , and subsequently, the higher the dielectric constant, ε r  will rise. As a result, stronger light increases the capacitance, C, of capacitor  62 .  
     [0044] In another aspect of the invention illustrated in FIGS.  6 - 7 , lower dielectric layer  64  is disposed between photosensitive dielectric layer  48  and the second electrode  50 . Lower dielectric layer  64  is provided to adjust the base capacitance, C, of capacitor  62 . Referring to equation 1 above, capacitance, C, is generally proportional to the areas of the upper and second electrodes  46  and  50 , as well as the dielectric constant of photosensitive dielectric layer  48 . In this embodiment, the dielectric constant is determined by the combination of photosensitive dielectric layer  48  and lower dielectric layer  64 .  
     [0045] Preferably, lower dielectric layer  64  is formed from a material such as barium titanate or the like. It is also understood that lower dielectric layer  64  may be omitted to adjust the capacitance, C, according to the required capacitance of capacitor  62  or to make the entire capacitive light sensing film  44  transparent relying only on photosensitive dielectric layer  48 . If crystallizing compounds, such as zinc sulfide, are used to form the photosensitive dielectric layer  48 , formation by evaporation is possible. In this event, by omitting lower dielectric layer  64  and by forming the second electrode  50  from ITO, capacitive light sensing film  44  becomes entirely transparent.  
     [0046] Referring now to FIG. 8, the system having a capacitive light sensing circuit  66  incorporating light sensing film  44  is described and shown. It is difficult to directly read the capacitance, C, of light sensing film  44 . By integrating light sensing film  44  into light sensing circuit  66 , an appropriate signal for measuring light intensity and ultraviolet radiation can be generated. In this embodiment, capacitive light sensing circuit  66  comprises a resistor  68 , oscillator  70 , frequency counter  72 , controller  74  and power source  76  in communication with light sensing film  44 . The oscillating frequency of oscillator  70  is determined by the series combination of the resistor  68  and light sensing film  44 . Accordingly, changes in the capacitance, C, of light sensing film  44  result in changes in the oscillating frequency of oscillator  70 . The following equation generally describes this relationship.  
             f   =     k   ·     1     2      π                 RC                 (     Eq   .              2     )                       
 
     [0047] In other words, the oscillating frequency, f, is proportional to the inverse value of capacitance, C, of light sensing film  44 . According to normal conditions of light  54  entering the ultraviolet sensor  12 , capacitance, C, will generally increase and the peripheral frequency, f, will generally decrease. Therefore, the strength of the light is distinguished by the degree of decrease of frequency, f. The oscillating frequency, f, of oscillator  70  is counted by frequency counter  72  and calculated by controller  74 . Controller  74  indirectly detects the capacitance, C, of light sensing film  44 , thereby detecting the intensity of light  54  detected by sensor  12 .  
     [0048] Controller  74  can be programmed to send the various measured values of frequency counter  72  to external equipment. In a preferred aspect of the invention, the light intensity signal calculated by controller  74  can be utilized to generate a visual and/or audible sunburn alarm. In this embodiment, capacitive light sensing circuit  66  quantifies the amount of ultraviolet radiation present, compares the detected values against stored values and produces a desired preventive alarm if optimal ranges are exceeded.  
     [0049] In an alternative aspect of the invention, capacitive light sensing circuit  66  may be used in actinometers embedded in cameras for measuring visible light intensity in order to adjust aperture diaphragms. Actinometers typically use cadmium sulfide in their light sensors. A light sensing film  44  containing zinc sulfide may be substituted to perform the same functions with better results. If light sensing film  44  is configured as shown in FIG. 8, the actinometer will measure and display the intensity of light. However, in the case of ordinary silver film or digital camera, the wavelength sensitivity of the photosensitive dielectric material must match CCD over CMOS sensors. Ordinarily, flourescent materials can be used for this process.  
     [0050] In another alternative aspect of the invention, capacitive light sensing circuit  66  is utilized to detect visible sunlight for integration with an automatic street light switching device. Capacitive light sensing circuit  66  may be configured for sunlight quantity detection. As such, with a decrease in the amount of visible light, the capacitive light sensing circuit  66  determines whether an evening or dusk condition exists, which would automatically switch on streetlights. Conversely, with an increase in the amount of visible light, capacitive light sensing circuit  66  could detect a morning or dawn condition and automatically switch off the streetlights.  
     [0051] In yet another alternative aspect of the invention, light sensing film  44  integrated in capacitive light sensing circuit  66  may be used in the plastic processing industry as a sensor in an ultraviolet curing process. By exposing soft synthetic resins to ultraviolet light, resins can be hardened or softened. Reaction times of these polymers depend on the intensity of the ultraviolet light. By measuring the intensity of ultraviolet light with light sensing film  44 , the optimal processing reaction time can be determined according to the measured value.  
     [0052] While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.