Abstract:
An apparatus and method is disclosed for regulating the cold spot temperature of a light emitting enclosure. A cold spot regulation system defines and controls the temperature of the cold spot. The cold spot regulation system includes an interface housing secured to the light emitting enclosure and two ducts extending from the interface housing. The cold spot regulation system uses a coolant fluid to lower the operating temperature of the light emitting enclosure. The coolant fluid is diverted into one of the ducts. The coolant fluid is passed by the cold spot of the light emitting enclosure and released out the other duct.

Description:
CROSS REFERENCE TO RELATED DOCUMENTS 
     This application is a divisional application U.S. patent application Ser. No. 09/476,398, filed Dec. 29, 1999, now U.S. Pat. No. 6,411,042 and entitled “DISPLAY COLD SPOT TEMPERATURE REGULATOR”. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to displays and, more particularly, to backlighting systems for displays. 
     BACKGROUND OF THE INVENTION 
     Backlighting an electronic display is a common need for many industries. For example, in the aviation and space industry, the backlit liquid crystal display (LCD) offers display luminance efficiency, contrast ratio and display viewing angles comparable to the once commonly used cathode ray tube (CRT). In addition, unlike CRTs, backlit LCDs provide a compact design with low power requirements, thus making the backlit LCD particularly suited for avionics displays. 
     Typically, the LCD is backlit using a fluorescent discharge lamp in which light is generated by an electric discharge in a gaseous medium. A conventional fluorescent lamp configured for backlighting a display includes a serpentine fluorescent lamp tube positioned within an interior region of a lamp housing called the backlight cavity. Filaments are mounted within free end portions of the lamp tube. Alternating current (AC) power is provided to the filaments through leads from a power supply. The lamp tube is charged with a mixture of mercury vapor and noble gas and the inner surface of the lamp tube is coated with phosphor. 
     When the fluorescent lamp is turned on, an electric field inside the lamp tube is produced which ionizes the noble gas. Free electrons become accelerated by the electric field and collide with the mercury atoms. As a result, some mercury atoms become excited to a higher energy state without being ionized. As the excited mercury atoms fall back from the higher energy state, they emit photons, predominately ultraviolet (UV) photons. These UV photons interact with the phosphor on the inner surface of the lamp tube to generate visible light. 
     The intensity of the visible light generated by the fluorescent lamp depends on the mercury vapor partial pressure in the lamp tube. At a mercury pressure less than the optimum mercury pressure, the light intensity of the fluorescent lamp is less than maximum because the mercury atoms produce fewer UV photons. At a mercury pressure greater than the optimum mercury pressure, the light intensity of the lamp is also less than maximum because so many mercury atoms tend to collide with the UV photons generated by other mercury atoms. Some of these UV photons fail to reach the phosphor coated inner surface and therefore do not generate visible light. 
     Nonetheless, many manufacturers fill the lamp tube with excess mercury so as to extend the light-output life for several years. As the lamp is burning, the mercury inside the lamp tends to be absorbed into the phosphor lining. The lost mercury is replenished from the excess mercury vapor stored in the lamp. If surplus mercury vapor is released into the lamp, however, the lamp performance diminishes. Therefore, it is desirable to maintain a reservoir within the lamp tube that holds the excess mercury until it is needed. 
     The mercury vapor pressure increases with the temperature of the coldest location (commonly known as “the cold spot”) inside the lamp tube. The cold spot serves as a point for the excess mercury to coagulate (i.e., the cooler the spot, the greater the attraction of mercury). For many avionics applications, the optimal cold spot temperature for the most favorable mercury pressure within the lamp tube is approximately 55° C. To insure that the visible light output of the fluorescent lamp is at a maximum with the least amount of power consumption, it is desirable to regulate the cold spot temperature of the lamp tube to maintain the optimal cold spot temperature. 
     One known method of regulating the cold spot temperature of the lamp tube is by a thermoelectric cooler (TEC). The typical TEC combines a metal heat sink, a resistive heater, and a diode array. A piece of copper or similar metal is fitted against the foot of the lamp body to form a “cold shoe.” The metal extends to the resistive heater and the diode array consisting of a number of individual diodes. A direct current is applied to the TEC, which causes one side to heat up, and the side near the lamp to cool down. This method is an effective way of accelerating the natural heat sinking process. 
     The TEC usually adequately regulates the cold spot temperature. Nonetheless, the diode arrays tend to be extremely fragile. The diode array should be rugged enough to avoid cracking and fracturing under vibrational loads to which aircraft and spacecraft are commonly subjected, which increases the cost of such arrays. Further, the display should be configured to avoid forces applied to the rigid metal of the cold shoe that is attached to the fragile lamp which could damage the TEC and the lamp. In addition, the TEC design requires additional electronics that tend to occupy display space and increase costs. Further, a significant amount of power may be needed to drive the TEC cooling element. 
     U.S. Pat. No. 5,808,418, issued Sep. 15, 1998 to Pitman et al., discloses replacing the TEC with a cylindrical glass tube connected to the lamp body. Referring to FIG. 1, a first portion  100  of the tube is exposed to the internal gas pressure of a lamp body  160 . A second portion  110  extends outside the housing  120  (backplate) and has a closed end  130 . A heating wire  140  is wrapped around the second portion  110  of the tube and controlled by a power supply (not shown). A temperature sensor  150  is mounted on the first portion  100  of the glass tube and coupled to the power supply (not shown). 
     In operation, the cylindrical tube cools the lamp body  160  by positioning the second portion  130  in cooler air outside the interior of the display. Beyond the backplate  120 , outside air circulates, typically from small holes in the airplane fuselage. The extended portion  110  of the tube is cooled by the outside air and thus defines a cold spot for the lamp. The temperature sensor  150  monitors the temperature of the tube near the lamp. If the temperature is below the optimal cold spot temperature range, the sensor  150  energizes the power supply (not shown) so as to deliver power to the heater wire  140 . The sensor  150  continually monitors the temperature of the tube  100  and controls, in a feedback loop, the operation of the power supply (not shown) to the heater wire  140 . 
     In another embodiment of the Pitman system, illustrated in FIG. 2, the cylindrical glass tube is replaced by a tin plated copper post  200  having cooling fins  210  attached to the extended portion  220 . The post  200  is attached to the fragile lamp body  160  by a thermally conductive silicone adhesive  230 . In operation, the copper post behaves substantially identical to the glass cylindrical tube of FIG.  1 . 
     The Pitman system alleviates the need for a TEC, but remains prone to some of the disadvantages associated with the TEC. In particular, the glass cylindrical tube is extremely fragile. Unlike the TEC, the glass tube is open to the internal gases within the fluorescent lamp. Damage to the glass tube necessarily damages the lamp because the glass tube is an integral part of the original lamp body complete with internal lamp gases. If the glass tube breaks while in operation (i.e., in aircraft flight), the entire lamp and the whole display system would be rendered inoperable. While the copper post embodiment may be more resistant to breakage, the rigidity of the post could break the lamp if enough force is applied to the post. Accordingly, the display systems are typically subject to design constraints to minimize potential breakage. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems outlined above and provides for an improved backlighting system for displays and method for regulating the cold spot temperature of a fluorescent lamp. The system comprises a light emitting enclosure having a defined cold spot. A duct disposed through a backplate is connected to a coolant fluid source at one end and exposed to the cold spot at a second end. Coolant fluid may be allowed to pass by the cold spot. 
     In an exemplary embodiment, a cold spot regulation system includes an interface housing positioned adjacent to the cold spot and two ducts connected to the interface housing. An intake duct includes an intake end configured to receive a coolant fluid flow and an exhaust duct configured to release the coolant fluid flow. The system may include an inexpensive shock-resilient material that can withstand the vibrations that are common to the aircraft cockpit. 
     In one embodiment, the system comprises a heating mechanism contiguous with the cold spot mechanism. The heating mechanism may be controlled by a power supply that receives commands from a control circuit. The system may further include a temperature sensor suitably located to monitor the temperature of the cold spot. Temperature readings are supplied to the control circuit. The control circuit energizes the power supply to the heating mechanism as needed to reach an optimum operating temperature. 
     In yet another embodiment, the heating mechanism comprises a conductive wire wrapped around the light emitting enclosure near the cold spot. In still another embodiment, the heating mechanism comprises a thin film resistive heater that is adhered to the enclosure. The heating mechanism may further comprise an airflow regulation device. The device is designed to open and close the end of the intake duct suitably configured to receive a coolant fluid flow. In operation, the temperature sensor monitors the cold spot temperature and through a control circuit increases or reduces the amount of coolant flow reaching the cold spot. 
     In still another embodiment, the end of the intake duct that is suitably configured to receive a fluid flow is constricted to form a Venturi tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
     FIG. 1 illustrates an enlarged cross sectional view of the cooling system of the prior art incorporating a cylindrical glass tube; 
     FIG. 2 illustrates an enlarged cross sectional view of an embodiment of the cooling system of the prior art incorporating a tin plated copper post; 
     FIG. 3 illustrates an enlarged cross sectional view of the cooling system of the present invention; 
     FIG. 4 illustrates a plane view of a serpentine fluorescent lamp tube and the cooling system of FIG. 3; 
     FIG. 5 illustrates an enlarged cross sectional view of an alternative embodiment of the cooling system in accordance with the present invention; 
     FIG. 6 illustrates a sectional view of a fluorescent discharge lamp incorporating the cooling system of FIG. 5; 
     FIG. 7 illustrates an enlarged cross sectional view of another embodiment of the cooling system in accordance with the present invention; and 
     FIG. 8 illustrates an enlarged cross sectional view of yet another embodiment of the cooling system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention relates to an improved cooling system and method for regulating the cold spot temperature of a fluorescent lamp. Although the cooling mechanism may be suitable for fluorescent lamps in many different industries, the present invention is conveniently described with reference to the avionics and spacecraft industry, and more particularly to the electronic display systems in the cockpit. 
     Referring now to FIGS. 3 and 4, a display backlight system according to various aspects of the present invention comprises: a light emitting enclosure  300  having a desired cold spot; a backplate  350  positioned near the light emitting enclosure  300 ; and a cold spot regulation system  302 . The cold spot regulation system  302  suitably comprises a cold spot cooling system  360  adjacent the lamp body  300 . The cold spot cooling system  360  is suitably disposed at the point of the lamp body  300  at which the cold spot is desired within the fluorescent lamp. 
     In an exemplary embodiment, the cold spot cooling system  360  comprises: a source of coolant fluid  325 ; an intake duct  330  disposed through the backplate  350  and having a first end connected to the source of coolant fluid  325  and a second end exposed to the light emitting enclosure  300 ; and an exhaust duct  340  disposed through the backplate  350  and having a first end exposed to the light emitting disclosure  300  and a second end connected to an exhaust area  326 . The ends of the ducts  330 ,  340  exposed to the light emitting disclosure  300  may be coincident such that the coolant fluid is delivered to the light emitting enclosure  300  by the intake duct  330 , circulates near the desired cold spot of the light emitting enclosure  300 , and is removed by the exhaust duct  340 . 
     The light emitting enclosure  300  selectably emits light and includes a desired cold spot. In the present embodiment, the light emitting enclosure  300  comprises a fluorescent lamp, such as a conventional serpentine fluorescent lamp used in avionics displays. The light emitting enclosure  300  may comprise, however, any suitable fluorescent lamp or other light emitting enclosure, such as a flat fluorescent lamp, described in U.S. Pat. No. 5,343,116, issued Aug. 30, 1994, to Winsor. The light emitting enclosure suitably includes an interior for containing the light emitting materials and an exterior surface, a portion of which is designated for the desired cold spot. The cold spot may be designated according to any appropriate criteria, such as accessibility or visibility. 
     The backplate  350  separates the light emitting enclosure  300  from various other components, such as the source of coolant fluid  325 . In the present embodiment, the backplate  350  comprises a backing behind the light emitting enclosure  300  and opposite a display for reflecting light through the display towards the viewer. The backplate  350  may serve various other functions, such as a mounting surface for supporting the light emitting enclosure  300  and the ducts  330 ,  340 . 
     The source of coolant fluid  325  comprises a source of relatively cool fluid, gas or liquid, for cooling the cold spot on the light emitting enclosure  300 . Any appropriate source of coolant fluid, and any suitable coolant fluid, may be used. In the present avionics application, the source of coolant fluid suitably comprises an airflow from outside the aircraft. For example, perforations in the nose of the aircraft allow air to circulate within selected portions of the fuselage. The forced air provides an efficient source of cooling for the cockpit electronics, including the electronic display systems. The source of coolant fluid may comprise, however, any suitable supply of coolant. 
     The intake duct  330  is disposed through the backplate  350  to supply coolant fluid from the source of the coolant fluid  325  to the cold spot associated with the light emitting enclosure  300 . The intake duct  330  suitably comprises a hollow tube having two open ends. The first end is open to the source of the coolant fluid  325  and the second end is open and adjacent to the cold spot associated with the light emitting enclosure  300 . Similarly, the exhaust duct  340  is disposed through the backplate  350  to draw spent coolant fluid from the cold spot associated with the light emitting enclosure  300  and transfer it to the exhaust area  326 , such as back into the airflow from the perforations in the fuselage. Exhaust duct  340  suitably comprises a hollow tube having two open ends. The first end is open and adjacent to the cold spot associated with the light emitting enclosure  300 , and the second end is open to the exhaust area  326 . 
     In the present embodiment, the cold spot cooling system  360  combines the intake duct  330  and the exhaust duct  340  into a single unit having a substantially continuous flow of coolant fluid from the intake duct  330 , across the cold spot, and out the exhaust duct  340 . The cold spot mechanism  360  includes the intake duct  330  and the exhaust duct  340  extending from an interface housing  310 . The interface housing  310  abuts the light emitting enclosure  300  to allow the coolant fluid to contact the light emitting enclosure  300  or an interface between the coolant fluid and the light emitting enclosure  300 . In the present embodiment, the interface housing  310  has an opening formed in the surface adjacent the cold spot of the light emitting enclosure  300  to allow the coolant fluid to directly contact the exterior surface of the light emitting enclosure  300 . The interface housing  310  of the cold spot mechanism  360  is preferably shaped to fit snugly around the lamp body  300 . A sealant  320  may be applied joining the light emitting enclosure  300  to the interface housing  310 . The sealant  320  also inhibits coolant fluid flow from penetrating into other parts of the enclosure. A commercially available two-part thermally conductive epoxy, for example ECCOSIL™, may be used as an adhesive for sealant  320 . 
     The duct  330 ,  340  are suitably integrally formed into the interface housing  310  so that the interface housing  310  and the ducts comprise a single unit. Both ducts  330 ,  340  extend through the housing or backplate  350 , and a sealant (not shown) may be applied between the backplate  350  and the exterior surfaces of the ducts  330 ,  340 . The intake duct  330  is suitably configured to receive airflow through an intake end  335 . The exhaust duct  340  is suitably substantially contiguous with the intake duct  330  and is suitably designed to release the fluid flow through an exhaust end  345 . For a conventional avionics display, intake end  335  and exhaust end  345  are suitably one to three centimeters in diameter. In the present embodiment, the intake end  335  is slightly smaller in diameter than the exhaust end  345 . By widening the exhaust end  345  in relation to the intake end  335 , back pressure caused by the warm released air and the effects from eddy currents may be decreased. 
     The cold spot cooling system  360  is suitably constructed of an appropriate material for the application. For example, in the present embodiment, the cold spot cooling system  360  comprises a thermally conductive and temperature resilient material. To achieve maximum thermal efficiency, it is desirable to form the cold spot cooling system  360  from a material that effectively transfers heat from the lamp body  300  and more particularly from the cold spot on the lamp body. Further, the material preferably withstands a wide range of temperatures, for example from −40° C. to 120° C. Various kinds of metals, plastics, resins, hard rubbers, synthetic rubbers, or other flexible yet durable materials may be used to form the cold spot cooling system  360 . In the present embodiment, the cold spot cooling system  360  is suitably configured to support the light emitting enclosure  300  on the backplate  350 . Accordingly, the cold spot cooling system  360  preferably comprises a durable and resilient material which tends to absorb shocks and vibrations attendant to flight. 
     To provide greater temperature control, the cold spot regulation system  302  may further include a heating system in addition to the cold spot cooling system  360 . Additional heating capability allows a particular desired temperature to be maintained. For example, in many applications, the optimal operating temperature of an avionics display lamp is around 55° C. Thus the optimal temperature of the cold spot is slightly below 55° C. Accordingly, a heating mechanism may be implemented to offset the cooling performed by the cold spot cooling system  360 . 
     Referring now to FIGS. 5 and 6, the cold spot regulation system  302  suitably further comprises a lamp heating system  505  adjacent the lamp body  300 . The lamp heating system  505  is suitably disposed at the point of the lamp body  300  at which the cold spot is desired within the light emitting enclosure  300 , or may alternatively be disposed around other parts of the light emitting enclosure  300 . For example, a lamp heating system  505  may comprise a resistive heater  510 , such as a copper nickel wire wrapped around the light emitting enclosure  300  near the cold spot area. In another embodiment, a thin film resistive heater may be adhered to the surface of the light emitting enclosure  300  near the cold spot area. 
     The cold spot regulation system  302  further suitably comprises a temperature sensor  520 . The temperature sensor  520  is suitably located near the cold spot of the lamp to monitor the cold spot temperature. In the present embodiment, temperature sensor  520  effectively and accurately monitors a range of temperatures from −40° C. to 120° C. For example, temperature sensor  520  may be, but is not limited to, a solid state current modulating sensor or a thermistor. A control circuit (not shown) receives the temperature readings from the temperature sensor  520  via lead wires  620  and energizes a direct current (DC) power supply  600  shown in FIG.  6 . Resistive heater  510  is supplied power via lead wires  610 . Through a feedback loop, the temperature sensor  520 , the lamp heating system  505 , and the cold spot cooling system  360  may effectively maintain the optimal cold spot temperature. 
     Referring to FIG. 7, an alternative embodiment of the cold spot regulation system  302  includes an airflow regulation device  700 . The airflow regulation device  700  may be used in addition to or instead of the lamp heating system  505 . The airflow regulation device  700  is suitably configured to regulate the amount of airflow into the intake end  335  of the intake duct  330 . FIG. 7 discloses an example of one such form. The airflow regulation device  700  may be secured to backplate  350  and extend parallel to the intake duct  330 . As shown, a sliding attachment  710  is perpendicular to intake end  335 , though the airflow through the ducts  330 ,  340  may be regulated at any point in either duct, such as at the exhaust end  345 . A control circuit (not shown) receives temperature readings from temperature sensor  520  and energizes a power supply (not shown) to the airflow regulation device  700 . If the temperature reading is below the desired operating temperature, the control circuit moves the sliding attachment  710  across the opening of intake end  335 , thereby closing the opening of intake duct  330  to effectively reduce the amount of airflow circulating. Similarly, if the temperature reading is above the desired temperature, the control circuit moves the sliding attachment  710  to open the intake duct  330  and increase the amount of circulating cool air. 
     The configuration of the ducts  330 ,  340  may also be modified to affect the circulation of the coolant fluid. For example, referring to FIG. 8, another embodiment of the cold spot regulation system  302  in accordance with various aspects of the present invention includes the intake duct  330  suitably configured to increase the airflow speed. In the present embodiment, the intake duct includes a Venturi tube to increase the speed of the coolant fluid. The Venturi tube is formed by constricting the airflow through the intake duct  330 . As shown in FIG. 8, the intake duct  330  includes a constricting member  800  on the inside wall of tube portion  330 . The force of the air stream into the cold spot regulation system  302  may be effectively increased, thereby accelerating the cooling of lamp body  300 . 
     FIG. 8 further illustrates airflow regulation device  700  in conjunction with constricting member  800 . The airflow regulation device  700  may operate in substantially the same manner as described above. However, with the Venturi tube, cooling the lamp body  300  may require less airflow through the intake end  335 . This may be particularly useful during aircraft or spacecraft start up when air speeds are at a minimum and at higher altitudes as air pressure begins to decrease. As the aircraft ascends and gains speeds, device  700  may be closed as needed to reach the optimal operating temperature. 
     The present invention has been described above with reference to preferred embodiments. However, changes and modifications may be made to the preferred embodiments without departing from the scope of the present invention. These and other changes or modifications are intended to be included within the scope of the present invention, as expressed in the following claims.