Abstract:
A camera flash apparatus having a flash tube enclosing a pair of electrodes in an ultraviolet light transparent envelope containing gas molecules normally presenting a high impedance between the electrodes. An ultraviolet light source directed at the tube is actuated at the start of a flash picture-taking event to ionize at least some of the gas in the envelope lowering the internal resistance of the tube to allow a flash energy source coupled to the electrodes to discharge through the flash tube. A flash unit incorporating the ultraviolet source is also disclosed

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method and apparatus for discharging a flash tube and, more particularly, to such a method and apparatus wherein ultraviolet light is used in triggering the flash tube to emit flash light.  
       BACKGROUND OF THE INVENTION  
       [0002]     A plasma can be generated in a gas by ionizing the gas molecules. In a xenon flash tube typically used for flash photography, a filamentary plasma is generated to create an energy discharge from the anode to the cathode of the flash tube. This energy discharge, in turn, creates a brilliant flash of white light used to illuminate the scene or subject being photographed. The method used to create the filamentary plasma is accomplished by initially ionizing the xenon gas by use of a high voltage source. To generate this high voltage, a component known as a trigger transformer is used to convert a lesser voltage pulse on its primary winding to a much higher voltage on its secondary winding, on the order of 4500 volts, which is then applied to the tube.  
         [0003]      FIG. 1  shows a basic flash circuit  10  for operating a flash tube  12  with a conventional external trigger wire  13 . A storage capacitor  14  and a trigger capacitor  16  are charged by a voltage source  18 . A high valued resistor  20  serves as a current limiter that allows trigger capacitor  16  to be charged to its proper potential while limiting current flow to trigger capacitor  16  during flashing. When a trigger switch  22  is closed, trigger capacitor  16  discharges through the primary winding  24   p  of a trigger transformer  24 , thus producing a high voltage pulse of 4500 v in the secondary winding  24   s  which is applied to the external trigger wire  13 . The high voltage trigger pulse excites and/or ionizes molecules of gas in the flash tube  12 , which greatly lowers the impedance between the flash tube electrodes  12   a  and  12   b . Once this impedance is lowered, the flash voltage stored on capacitor  14  discharges through flash tube  12 . This voltage discharge causes electron flow through flash tube  12  which excites some of the electrons of the gas molecules to a higher energy state while other electrons are removed completely (ionized) from the gas molecules. When these electrons return to the ground state, energy is released in the form of light which produces the brilliant flash of light for photography.  
         [0004]     External triggering of the flash tube  12  with a trigger wire suffers from certain practical disadvantages. The trigger wire coming from the trigger transformer needs to be placed in close proximity to the flash tube glass envelope and spurious arcing from the external trigger wire  13  to adjacent components of the flash tube apparatus can occur. Also the glass envelope of the flash tube needs to have a conductive coating on it to distribute the high voltage from the trigger transformer across the flash tube and discoloration of the envelope of the flash tube  12  can be created due to the high voltage applied to the tube envelope. Also, relatively poor triggering reliability can result, especially at lower trigger voltages. Another drawback is that arcing of the high voltage from the trigger transformer has to be suppressed otherwise no flash from the flash tube will occur.  
         [0005]     To solve problems of this sort, it is known that the flash tube  12  can be triggered in a series-injection mode of operation as illustrated by the circuit of  FIG. 2 . The circuit  10 ′ of  FIG. 2  is similar to that of  FIG. 1  except that the flash tube  12 ′ does not utilize an external trigger wire and does not have an external trigger coating, the output of trigger transformer secondary winding  24   s  being connected directly across the electrode terminals  12   a ′ and  12   b ′ of tube  12 ′. When the trigger switch  22  is closed, the high voltage pulse from secondary winding  12   s  is applied directly across the electrode terminals  12   a ′ and  12   b ′ and produces a strong arc across the electrodes, thereby lowering the impedance between the tube electrodes to allow the flash charge on storage capacitor  14  to discharge through the flash tube  12 .  
         [0006]     Applying the high voltage trigger pulse directly to the flash tube electrodes places extreme stress on these electrodes which can lead to early tube failure. Moreover, in actual practice, injection triggering adds inductance in the discharge path, thereby increasing the time duration of the flash and reducing peak intensity of light output.  
         [0007]     It is known in industry and educational institutions that plasmas can also be created in the gas of interest by exposure to light. For example, exposure to Vacuum Ultraviolet (VUV) light will cause xenon gas to ionize. Such uses for ionized xenon gas are for inducing continuum structures, high resolution spectroscopy, or for secondary ionization of another gas such as used in the purification of flue gases.  
         [0008]     In Patent Application Publication No. U.S. 2002/0074559 A1, published Jun. 20, 2002, ultraviolet light emitting diodes (UVLEDs) are described as being used in a pulsing or flashing mode on a camera to create different effects or for power supply conservation. Direct exposure of the UVLED to the subject as the primary flash light or for special effects would not be appropriate for general photography use.  
       SUMMARY OF THE INVENTION  
       [0009]     In accordance with the invention, therefore, camera flash apparatus is provided which comprises a flash tube having a pair of electrodes in an ultraviolet light transparent envelope containing gas molecules normally presenting a high impedance between the electrodes. The apparatus further comprises a trigger circuit which includes an ultraviolet light source adjacent the envelope and a switch for actuating the ultraviolet light source to ionize the gas molecules in the tube so as to lower the impedance between the electrodes. The apparatus also comprises a flash energy storage circuit coupled to the flash tube electrodes to fire the tube by discharging through the lowered impedance between the electrodes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic diagram of a prior art circuit illustrating a commonly used form of a camera flash tube trigger circuit;  
         [0011]      FIG. 2  is a schematic diagram of a prior art circuit illustrating a series injection form of flash tube trigger circuit;  
         [0012]      FIG. 3  is a schematic diagram of a camera flash circuit embodying an ultraviolet light flash tube trigger in accordance with the present invention;  
         [0013]      FIG. 4  is a schematic diagram of a prior art circuit utilizing a simmer circuit conjunction with a series injection trigger circuit;  
         [0014]      FIG. 5  is a schematic diagram of a camera flash circuit utilizing a simmer circuit in conjunction with an ultraviolet light trigger in accordance with another form of the present invention;  
         [0015]      FIG. 6  is a schematic illustration of a flash apparatus utilizing an ultraviolet in accordance with the invention; and  
         [0016]      FIG. 7  is a schematic illustration of an alternative embodiment of the invention utilizing an ultraviolet light flash tube trigger. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     Turning now to  FIG. 3 , a camera flash circuit  30  according to one aspect of the invention includes a flash tube  12 ′ having a pair of electrodes  12   a ′ and  12   b ′ in an ultraviolet light transparent envelope  12   c ′, the envelope containing gas molecules, e.g. xenon gas, normally presenting a high electrical impedance between the electrodes. Flash tube  12 ′ of circuit  30  is similar to flash tube of  12 ′ of  FIG. 2  in that it does not have an external trigger coating or any trigger wire associated therewith. Quartz is an ultraviolet transparent material suitable for use as the envelope  12   c ′ and, in fact, is commonly used for conventional flash tubes. A flash energy storage circuit, comprises an energy storage capacitor  14  which is charged by an energy source  18  to a suitable flash voltage level of approximately 330 v. Storage capacitor  14  is coupled to the flash electrodes  12   a ′ and  12   b ′ to fire the tube  12 ′ by discharging stored energy from capacitor  14  when the electrical impedance of the gas in the tube is lowered. A trigger circuit  32  used to lower the gas impedance in accordance with the invention includes an ultraviolet light source  34  positioned adjacent the tube envelope  12   c ′, an energy source  38 , such as a battery, and a trigger switch  36  for actuating the ultraviolet light source from the battery  38 . A resistor  39  serves as a current limiter in the trigger circuit. Battery  38  may be the same battery as that normally used in energy source  18  to charge storage capacitor  14 . In a camera application, switch  36  is preferably coupled to a shutter actuating button or to the shutter itself to be closed when the shutter is opened to take a flash picture. A suitable ultraviolet light source may be a light emitting diode (UVLED) of the type described in aforementioned Patent Application U.S. 2002/0074559 A1. There is a range of wavelengths which produce optimum ionization results which depends, in part, on the gas purity and temperature. The ultraviolet wavelength range can fall between 100 nm and 400 nm. Optimum ultraviolet wavelengths for gas ionization are closest to the 100 nm range.  
         [0018]     In operation, voltage source  18  charges storage capacitor  14  to a typical voltage level of approximately 330 v. Capacitor  14  does not initially discharge through the flash tube  12 ′ because the enclosed xenon gas has near infinite electrical impedance. When trigger switch  36  is closed upon opening of the camera shutter, ultraviolet light source  34  is activated and the ultraviolet light thus produced is directed at flash tube  12 ′. The applied ultraviolet light excites and ionizes gas molecules within the envelope of the flash tube thereby lowering the impedance between the flash tube electrodes  12   a ′ and  12   b ′ to the point of conduction. A burst of voltage from the storage capacitor  14  then discharges through flash tube  12 ′ causing the tube to fire.  
         [0019]      FIG. 4  shows a circuit  40  for firing a flash tube in a simmer mode of operation known to be useful in high speed flash photography. Circuit  40  is similar to that of  FIG. 2  and corresponding components bear the same reference numerals. The differences are that a simmer supply  42  has been added in parallel with flash tube  12 ′, and a flash enable switch  44  is inserted between flash capacitor  14  and the flash tube  12 ′. It will be appreciated that in high speed flash operation, switches  36  and  44  would most likely take the form of SCR devices in known manner. Trigger switch  22  is arranged to be closed separately from and in advance of the closure of flash enable switch  44 . Initially, trigger capacitor  16  and flash capacitor  14  are charged by voltage source  18  to the flash voltage of 330 v. When trigger switch  22  is closed, trigger capacitor  16  discharges through the primary winding  24   p  of the trigger transformer  24 , and produces a high voltage pulse of about 4500 v across the secondary winding  24   s . This high voltage trigger pulse is applied directly across the electrodes  12   a ′ and  12   b ; of the flash tube  12 ; and lowers the impedance between the tube electrodes, as previously described for the circuit of  FIG. 2 . Because of the lowered impedance between the electrodes, the simmer supply  42  is able to establish a low current dc arc between the electrodes. Typically, the current of such a simmer arc is on the order of 20-100 ma. When it is desired to fire the tube  12 ′, flash enable switch  44  is closed and a flash voltage from the storage capacitor  14  discharges through tube  12 ′ producing the desired flash of light. Under ideal operating conditions, the simmer arc will not be extinguished upon firing of the flash tube and the tube can be repetitively fired by merely closing the switch  44  as the storage capacitor  14  is charged.  
         [0020]     To completely eliminate the use of a high voltage trigger pulse and the attendant adverse affects upon tube life, the present invention provides a circuit  50 , shown in  FIG. 5 , for operating a flash tube in the simmer mode. In this circuit, the voltage injection trigger circuit of  FIG. 4  is replaced with the ultraviolet light trigger circuit  32  of  FIG. 3 . A source of ultraviolet light  20  is activated by trigger switch  36  and the ultraviolet light thus produced is directed at flash tube  12 ′. As previously described, the applied ultraviolet light excites and ionizes gas molecules within the envelope of the flash tube thereby lowering the impedance between the flash tube electrodes  12   a ′ and  12   b ′ to the point of conduction. Simmer supply  42  then establishes a simmer arc between the tube electrodes  12   a ′ and  12   b ′. Firing of the flash tube is accomplished by closing the flash enable switch  44  which allows the storage capacitor  14  to discharge through flash tube  12 ′. In the event that the simmer arc is extinguished, either due to the firing of the flash tube or because of excessive heat build up, it is a simple matter to retrigger the simmer arc by applying another pulse of ultraviolet light from trigger circuit  32 . Thus, the high voltage trigger pulse which adversely effects tube life is eliminated entirely.  
         [0021]     Because the present invention provides a method of initiating the simmer arc without the use of a high voltage trigger pulse, the flash tube can now be operated much with less power and produces substantially less heat. In accordance with the invention, the simmer power supply is purposefully operated so that the simmer arc will be extinguished upon firing of the flash tube. This can be done by reducing the current of the simmer arc to a marginal value such that the arc is “blown out” upon the main discharge. Alternatively, a simmer arc switch  52  (shown in dotted line form) may be substituted in the simmer supply circuit  42  leading to flash tube electrode  12   a ′ and coupled in tandem with switch  44  to open the simmer supply as flash tube  12 ′ is discharged and to then close when switch  44  is reopened.  
         [0022]     In operation, therefore, when a pulse of ultraviolet light from the ultraviolet light source  34  is directed at flash tube  12 , the impedance of flash tube  12 ′ is lowered and a simmer arc is then established between the flash tube electrode  12   a ′ and  12   b ′. When switch  44  is closed and the simmer arc switch  52  is opened, the flash voltage on the storage capacitor  14  is discharged through the flash tube causing the tube to fire and the simmer arc to be extinguished. The flash enable switch  44  is then opened and the simmer arc switch  52  is closed while the capacitor  14  recharges. Even though the simmer switch is now closed, the simmer arc is not re-established because of the high internal impedance of the tube  12 ′. When it is desired to re-fire the flash tube, trigger switch  36  is closed and another pulse of ultraviolet light from the ultraviolet light source  34  is applied to flash tube  12 ′ to lower the impedance between the electrodes  12   a ′ and  12   b ′. The simmer arc is thus re-established and the switch  44  can again be closed to cause the storage capacitor  14  to discharge through the flash tube.  
         [0023]     In this embodiment, switches  36  and  44  may be actuated by a two stage shutter release button in which initial pressing of the button during the first stage closes trigger switch  36  to initiate the simmer arc. Pressing the button further into the second stage then closes flash enable switch  44  allowing the flash storage capacitor  14  to discharge through the flash tube  12 ′ simmer arc. Because the simmer arc is not maintained between flashes, less power is used by the simmer supply and less heat is produced by the flash tube. Importantly, however, because the simmer arc is re-established by means of a pulse of ultraviolet light rather than a high voltage trigger pulse, tube life is extended.  
         [0024]     In  FIG. 6 , one possible placement of the ultraviolet light source is shown for a camera flash unit. In this arrangement, the ultraviolet light source, such as the aforementioned UVLED, is positioned behind the flash reflector  60  and in alignment with an aperture  62  formed in the rear of the reflector so that the UV light rays  64  shine through the aperture  62  directly onto the flash tube  12 ′. Flash light rays  66  are then emitted directly forward of the reflector without being impeded by the UVLED  34 .  
         [0025]     In  FIG. 7 , another placement for the UVLED is shown in which the UVLED  34  is placed in front of the flash reflector in an off-axis position in which the UVLED has minimal obstruction to the emission of flash light  64  when flash tube  12 ′ is triggered and the white light is reflected off of reflector  60 .  
         [0026]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.  
                                         PARTS LIST                                10.   prior art camera flash circuit       12.   flash tube       13.   external trigger wire       14.   flash storage capacitor       16.   trigger capacitor       18.   voltage source       20.   current limiting resistor       22.   trigger switch       24.   trigger transformer       30.   camera flash apparatus ( FIG. 3 )       32.   UV flash trigger circuit       34.   UV light source       36.   trigger switch       38.   battery       39.   current limiting resistor       40.   camera flash apparatus ( FIG. 4 )       42.   simmer supply circuit       44.   flash enable switch       50.   camera flash apparatus ( FIG. 5 )       52.   simmer arc switch       60.   flash reflector       62.   reflector aperture       64.   ultraviolet rays       66.   flash light rays