Patent Publication Number: US-6664997-B2

Title: Method of driving fluorescent print head and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of driving a fluorescent print head used as a light source for an optical printer (an image forming apparatus) such as a color printer, which forms an image on a recording medium such as a photosensitive film (e.g. an instant film) or a photographic paper (e. g. a silver salt paper), and to an image forming apparatus. 
     A fluorescent print head mounted on an optical printer such as a color printer, which uses light emitted due to electrons hitting a fluorescent substance and creates a desired image on a recording medium (e.g. a photosensitive film and a photographic paper), is well known. 
     FIG. 3 is a cross-sectional view partially illustrating a fluorescent print head of the above-mentioned type. FIG. 4 is a plan view illustrating luminous dots of a fluorescent print head. FIG.  5 ( a ) is a perspective view partially illustrating the anode substrate of a fluorescent print head. FIG.  5 ( b ) is a plan view partially illustrating the anode portion. FIG. 6 is a side view illustrating an optical printer having three fluorescent print heads for R (red), G (green) and B (blue) luminous colors. 
     As shown in FIG. 6, the optical printer  1  has as a dot array three fluorescent print heads  2  ( 2 R,  2 G,  2 B). Each fluorescent print head  2  has luminous dots emitting R (red), G (green) or B (blue) color (or a R, G, or B filter is combined with a luminous dot of a fluorescent substance with a broad wavelength (e.g. a ZnO:Zn fluorescent substance) containing R, G and B components). A recording medium such as a film is exposed to the light beams from respective print heads  2  to form a desired image. 
     The three fluorescent print heads  2 R,  2 G, and  2 B have the same structure (However, the combination of either fluorescent substances or fluorescent substances and R, G, and B filters is different). Here, the structure of a fluorescent print head  2 R emitting red color light will be described below as an example. 
     As shown in FIG. 3, the fluorescent print head  2 R has a container  6 , being a box assembled with an anode substrate  3 , side plates  4 , and a rear substrate  5  by bonding together with a sealing glass. The inside of the container  6  is evacuated in a vacuum state. 
     As shown in FIG. 4, a first luminous dot column  8  of plural luminous dots  7  and a second luminous dot column  9  of plural luminous dots  7  are arranged in parallel along the longitudinal direction of the anode substrate  3  over the inner surface of the anode substrate  8 . Each luminous dot  7  has an anode electrode  10  (made of a frame-like conductive thin film of aluminum), patterned on the anode substrate  10  using the sputtering and the photolithography, and a fluorescent substance layer  11  coated on the anode  10 . 
     The fluorescent substance layer  11 , for example, made of zinc oxide fluorescent substance (ZnO:Zn), or cadmium sulfide series fluorescent substance ((Zn,Cd)S:Ag,Cl), is formed in such a way that the layer  11  has an opening wider than the square opening  10   a  of the anode  10  and does not run off the frame. The light emitted from the surface of the fluorescent substance layer  11  radiated outside through the fluorescent substance and the anode substrate  3  from the opening of the anode  10 . Hence, the area of each luminous dot  7  corresponds to the effective luminous area of the fluorescent substance layer  11  defined by the opening  10   a  of the anode  10 . 
     In the first and second luminous dot columns  8  and  9 , respective luminous dots  7  are led out with the anode conductor  12  and are electrically connected to the control circuit  14  on the circuit substrate  13 , using, for example, TAB (tap-automated bonding, as shown in FIGS. 3 and 6. 
     Here, the shape of each luminous dot  7  and the arrangement of the first and second luminous dot columns  8  and  9  will be described. As shown in FIG. 4, each luminous dot  7  is in a square form of which one side has a length (a). In the first and second luminous dot columns  8  and  9 , a large number of luminous dots  7  are arranged at intervals of (a) in the primary scanning direction. The luminous dots in the luminous dot column  8  and the luminous dots in the luminous dot column  9  are shifted to each other by the pitch P (=a) in the scanning direction. Moreover, the luminous dots in the luminous dot column  8  and the luminous dots in the luminous dot column  9  are spaced away from each other by the pitch (b) (an integer multiple of the pitch P in the primary scanning direction) in the secondary scanning direction. The luminous dot columns  8  and  9  also are arranged in parallel and in zigzag form. 
     As shown in FIG. 3, a flat control electrode  15  is arranged as a control electrode on the upper surface of the anode substrate  3 . The flat control electrode  15 , which is made of a conductive film (e.g. aluminum), surrounds the luminous dots  7  and anode conductors  12  and is disposed so as to be flush with the luminous dots  7 . A positive voltage is always applied to the flat control electrode  15  upon drive operation to maintain the adjacent electric field at a fixed level. 
     In the container  6 , as shown in FIG. 3, the first filament cathode  16  and the second filament cathode  17 , each being a thermionic cathode, are suspended above the first and second luminous dot columns  8  and  9 . The first filament cathode  16  and the second filament cathode  17  are arranged in the primary scanning direction and are spaced substantially at equal distances from the centers of the luminous dot columns  8  and  9 . In the filament cathodes  16  and  17 , an electron emission material is coated on an ultra-fine tungsten alloy wire (e.g. tungsten or rhenium tungsten) of a diameter of 7 μm to 50 μm. The electron emission material is made of a ternary oxide containing barium oxide, calcium oxide, and strontium oxide. That oxide is uniformly coated at a thickness of 5 μm to 10 μm over a tungsten of a diameter of several μm to several tens μm. The filament voltage is adjusted to set the filament cathodes  16  and  17  to 600° C. to 700° C. Thus each of the filament cathodes  16  and  17  functions as a thermal electron source. 
     A NESA film  18   a , being an anti-static translucent conductive film, is formed on the inner surface of the rear substrate  5 . A anti-reflection layer  18   b  formed of graphite is formed on the NESA film  18   a . The anti-static layer  18   b  absorbs light from the luminous dot  7  (anode  10 ) to prevent it from being reflected back to the luminous dot  7 . With omission of the anti-static layer  18   b , the light reflected back to the light emission side leaks from the gap between the anode  10  and the flat control electrode  15 . This decreases the display contrast. 
     Inside the enclosure  6  shown in FIG. 3, a first shield electrode  19  of a stainless steel thin plate is disposed outside the luminous dot column  8  and the first filament cathode  16 . Similarly, the second shield electrode  20  of a stainless steel thin plate is disposed outside the luminous dot column  9  and the second filament cathode  17 . The shield electrodes  19  and  20  are connected together to the same potential. Each of the shield electrodes  19  and  20  is a plate having a nearly L-shaped cross section, viewed from the plane perpendicular to the primary scanning direction. The flange plates are disposed in parallel on the surface of the anode substrate  3 . The shield electrode  19  or  20  may be a flat plate. The flange plate of each of the shield electrode  19 ,  20  is disposed above the anode substrate  3  via the insulating layer  21  containing main components (e.g. a low-melting point glass) (or with the gap of about 0.5 mm or less). The shield electrodes  19  and  20  surround the filament cathodes  16  and  17  and the upper ends thereof are positioned above the filament cathodes  16  and  17 . The shield electrode  19 ,  20  prevents the surface of the insulating layer  21  from being charged up. The shield electrode  19 ,  20  covers the anode conductor  12  for the luminous dot  7  and the conductor for the flat control electrode  15  to reduce the reactive current. Moreover, the reactive current passing the flat control electrode  15  and luminous dots  7  can be reduce by restricting the aperture of the opening defined by the shied electrodes  19  and  20 . 
     In three fluorescent print heads  2 R,  2 G and  2 B shown in FIG. 6, the luminous dot columns  8  and  9  are arranged in parallel and at predetermined intervals. The longer side of the anode substrate  3  corresponds to a horizontal direction (in the vertical orientation with respect to the paper surface) and the shorter side of the anode substrate  3  corresponds to a vertical direction (in the upper orientation of the paper surface). In the fluorescent print heads  2 R,  2 G and  2 B, the dot-like light beam emitted from each luminous dots  7  passes through the translucent anode substrate  3  and irradiates horizontally and forward (in the right orientation on this paper). In each fluorescent print head  2 R,  2 G,  2 B, an imaging optical system  24  formed of a prism (or a reflecting mirror)  22  and a Selfoc lens array (an equi-magnification imaging lens array)  23  is mounted on the front side of the anode substrate  3 . 
     The imaging optical system  24  forms an erect equi-magnification image. The opening  10   a  of an anode  10  in the fluorescent print head  2  acts as a focal point. The photosensitive surface of the film  25  (a recording medium) acts as a projected image point  23 . The imaging optical system  24  bends at a right angle the optical path of the dot-like light beam irradiated from the fluorescent print head  2  to the front side of the anode substrate  3  and guides it vertically and downward. As to the relationship between the luminous dot  7  and the photosensitive surface of the film  25  (a recording medium) in horizontal state, the longer side of the anode substrate  3  corresponds to a horizontal direction (the vertical orientation of this paper and the direction perpendicular to the shorter side of the anode substrate  3  corresponds to a vertical direction (the right orientation of this paper). 
     As shown in FIG. 6, the red filter R, the green filter G and the blue filter B are disposed under the Selfoc lens arrays  23 , respectively. The filters R, G and B confront the film  25  so as to be spaced away from it a predetermined distance. 
     The three fluorescent print heads with the above-mentioned structure are mounted and modulalized as one container  27 , together with the drive circuit  26 . The drive circuit  26  includes the control circuit  14 , mounted on the circuit substrate  13 , for controlling the drive operation of various electrodes (such as anodes  10 , flat control electrodes and filament cathodes  16  and  17 ) and the power source circuit  33 . 
     In the recording operation of the optical printer  1  with the above-mentioned structure, the film  25  is relatively moved in the secondary direction with respect to light beams emitted from the fluorescent print heads  2 R,  2 G and  2 B, as shown in FIG.  6 . Image data decomposed into R, G and B colors are respectively sent to the corresponding fluorescent print heads  2 R,  2 G and  2 B. The luminous dots columns  8  and  9  of each fluorescent print head  2  glow with a predetermined timing in sync with the relative movement. 
     In this drive operation of each fluorescent print head  2 , the luminous dots  7 , which is arranged in zigzag form in the luminous dot columns  8  and  9 , continuously emit light beams onto the film  25  in parallel to the primary direction and in a straight line. Each fluorescent print head  2  repeatedly irradiates light beams onto the film  25  to create a desired full-color image. 
     However, in the optical printer  1  provided with the conventional fluorescent print heads  2  each configured of a fluorescent luminous tube, the problem is that the light amount decreases as the fluorescent luminous tube is driven and lit for a long period of time on the occasion of printing. 
     A decrease in light amount of the fluorescent luminous tube causes a lack of the density necessary for a recording medium (or a photographic paper). As a result, the print image quality is deteriorated. 
     The light amount of a fluorescent luminous tube depends on the magnitude (input: voltage×current) of a flow of electrons exciting a fluorescent substance, a luminous time period, and the luminous efficiency of a fluorescent substance. The fluorescent substance itself does not substantially change its property because the accelerating voltage is low (20 to 70 volts, 30 to 40 volts on average). 
     In the fluorescent print head  2 , as shown in FIG. 3, filament cathodes  16  and  17  heated at high temperatures (about 700° C.) are suspended above the fluorescent substances. The electron emission material formed of a ternary carbonate, particularly, barium (Ba) coated on the surface of the filament cathode  16 ,  17  is gradually evaporated during a long period of time and thus adheres to and contaminates the surface of the fluorescent substance layer  11 . For tat reason, the contaminant adhered to the surface of the fluorescent substance layer  11  limits the accelerated electrons emitted from the filament cathode  16 ,  17  and blocks the light emission of the fluorescent semiconductor layer  11 , thus decreasing the light amount. 
     Evaporation of the electron emission material of the filament cathode  16 ,  17  deteriorates the electron emission capability and decreases the electron flow, thus decreasing the light amount. 
     As described above, a decrease of the light amount (an initial light amount=brightness×time) mainly is caused by the evaporation of the electron emission material (mainly Ba) of the filament cathodes  16 ,  17 . The evaporation rate is controlled by the operational temperature of the filament cathode  16 ,  17 . That is, as shown in FIG. 8, increasing the operational temperature of the filament cathode  16 ,  17  leads to a high rate of evaporation, thus accelerating a decrease in light amount. On the other hand, decreasing the operational temperature of the filament cathode  16 ,  17  leads to a low rate of evaporation, thus delaying a decrease in light amount. 
     As shown in FIG. 9, lowering the operational temperature of the filament cathode  16 ,  17  prolongs the operational lifetime. However, as shown in FIG. 10, excessively lowering the temperatures results in the electron emission amount (emission current) of the filament cathode  16 ,  17 . Under the same drive conditions, the filament cathode  16 ,  17  moves to the temperature restriction region (the region depending on the temperature of the filament cathode  16 ,  17  itself) shown in FIG. 10 (vacuum tube characteristics of a thermionic cathode in case of the filament 5MG). This decreases the light amount, thus resulting in unstable light emission. 
     Generally, in the fluorescent print head  2  built in an image forming apparatus such as an optical printer, some output images have the spot being in luminous state at all times or the spot being in non-luminous state at all times. Particularly, the spot in non-luminous state varies its light amount because gases remaining inside the container  6  adhere to the surface of the fluorescent substance layer  11 . When the light amount varies, the light amount of the luminous dot  7  previously being in non-luminous state varies at the time of outputting a different image. As a result, the density of an output image varies partially. 
     For that reason, conventionally, when the optical printer  1  having the fluorescent print head  2  prints an image, a variation in light amount of the non-luminous portion is alleviated. Hence, a previous light emitting operation (hereinafter, referred to as pre-light emission) is performed by light-emitting all dots, for example, for several minutes before the print operation. After stabilization of the light emission, an image printing operation is performed. In this case, the pre-light emission is preliminary light emission performed in advance to stabilize the luminous condition. 
     In further explanation, in the conventional structure, as shown in FIG. 7, the filament cathode  16 ,  17  is heated and driven at 600° C. to 700° C. immediately before the pre-luminous period T 1 . During the pre-luminous period T 1 , the anode voltage and the grid voltage are driven under the rated conditions (the rated condition means the same drive condition (voltage) as that in the print luminous period T 2 ). Until the print luminous period T 2  ends, the filament cathodes  16  and  17  are heated and driven while the anode voltage and the grid voltage are driven under rating conditions. 
     However, in the conventional pre-light emission operation, because the luminous time period not contributing to printing is added, the operational lifetime of the fluorescent print head is fastened. 
     SUMMARY OF THE INVENTION 
     The present invention is made to solve the above-mentioned problems. 
     An object of the invention is to provide a fluorescent print head driving method capable of improving an operational lifetime. In this method, a filament voltage is decreased during a luminous dot glowing period except a printing period, thus decreasing the evaporation amount of Ba contained in an electron emitter material for a filament cathode heated and driven, so that deterioration of the luminous efficiency is suppressed. 
     Another object of the invention is to provide an image forming apparatus capable of improving an operational lifetime. 
     In order to achieve the above-mentioned objects, an aspect of the present invention is characterized by a method of driving a fluorescent print head, the fluorescent print head having a filament cathode, a control electrode for controlling electrons emitted from the filament cathode, a fluorescent substance layer having a plurality of luminous dots each which emits light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated. Moreover, a pre-luminous period during which the luminous dots glow before an image is formed on a recording medium and a print luminous period during which the luminous dots glow to create an image on the recording medium are provided. The method comprises the steps of, during the pre-luminous period, controlling at least one of an anode voltage to be applied to the anode and a grid voltage to be applied to the control electrode, to a rated voltage or less in a print luminous mode; and controlling a filament voltage to be applied to the filament cathode to a rated voltage or less in a print luminous mode. 
     Another aspect of the present invention is characterized by a method of driving a fluorescent print head, the fluorescent print head having a filament cathode, a control electrode for controlling electrons emitted from the filament cathode, a fluorescent substance layer having a plurality of luminous dots each which emits light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated. Moreover, a pre-luminous period during which the luminous dots glow before an image is formed on a recording medium and a print luminous period during which the luminous dots glow to create an image on the recording medium are provided. The method comprises the steps of, during the pre-luminous period, controlling an anode voltage to be applied to the anode or a grid voltage to be applied to the control electrode, with a predetermined duty ratio; and controlling a filament voltage to be applied to the filament cathode to a rated voltage or less in a print luminous mode. 
     Another aspect of the invention is characterized by a method of driving a fluorescent print head, the fluorescent print head having a filament cathode, a control electrode for controlling electrons emitted from the filament cathode, a fluorescent substance layer having a plurality of luminous dots each which emits light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated. Moreover, a pre-luminous period during which the luminous dots glow before an image is formed on a recording medium and a print luminous period during which the luminous dots glow to create an image on the recording medium are provided. The method comprises the steps of controlling an anode voltage to be applied to the anode or a grid voltage to be applied to the control electrode, to a rated voltage or less in a print luminous mode; controlling the anode voltage or the grid voltage with a predetermined duty ratio; and controlling a filament voltage applied to the filament cathode to a rated voltage or less in a print luminous mode. 
     Another aspect of the invention is characterized by a method of driving a fluorescent print head, the fluorescent print head having a filament cathode, a control electrode for controlling electrons emitted from the filament cathode, a fluorescent substance layer having a plurality of luminous dots each which emits light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated. Moreover, a blank period of a grid voltage applied to the control electrode corresponding to the blank period between glow states during which an anode voltage to be applied to the anode is written for each line of a recording medium is provided. The method comprises the steps of, during a print luminous period for which said luminous dots glow when an image is created on the recording medium, controlling the anode voltage and the grid voltage, to a rated voltage in a print luminous mode with a duty ratio corresponding to the blank period; and controlling a filament voltage to be applied to the filament cathode to a rated voltage or less in a print luminous mode. 
     The method according to the invention further comprises the steps of providing a blank period of the grid voltage corresponding to a blank period between glow states during which an anode voltage to be applied to the anode is written for each line of the recording medium; during a print luminous period for which the luminous dots glow when an image is created on the recording medium, controlling the anode voltage and the grid voltage to a rated voltage in a print luminous mode with a duty ratio corresponding to the blank period; and controlling the filament voltage to a rated voltage or less in a print luminous mode. 
     Another aspect of the invention is characterized by a method of driving a fluorescent print head, the fluorescent print head having a filament cathode, a fluorescent substance layer having a plurality of luminous dots each which emits light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated. Moreover, a pre-luminous period during which the luminous dots glow before an image is formed on a recording medium and a print luminous period during which the luminous dots glow to create an image on the recording medium are provided. The method comprises the steps of controlling an anode voltage to be applied to the anode to a rated voltage or less in a print luminous mode; and controlling a filament voltage to be applied to the filament cathode to a rated voltage or less in a print luminous mode. 
     Another aspect of the invention is characterized by a method of driving a fluorescent print head, the fluorescent print head having a filament cathode, a fluorescent substance layer having a plurality of luminous dots each which emits light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated. Moreover, a pre-luminous period during which the luminous dots glow before an image is formed on a recording medium and a print luminous period during which the luminous dots glow to create an image on the recording medium are provided. The method comprises the steps of controlling an anode voltage to be applied to the anode with a predetermined duty ratio; and controlling a filament voltage to be applied to the filament cathode to a rated voltage or less in a print luminous mode. 
     According to further another aspect of the invention, an image forming apparatus comprises a fluorescent print head having a filament cathode, a fluorescent substance layer having a plurality of luminous dots each emitting light when electrons emitted from the filament cathode impinges against the fluorescent substance layer, and an anode on which the fluorescent substance layer is coated; a controller for acquiring and outputting pre-luminous pattern data based on a pre-luminous signal and a voltage switching signal and acquiring and outputting image data and a voltage switching signal based on a print starting signal; a voltage selector for selecting the filament cathode drive voltage and the anode drive voltage to the fluorescent print head based on a voltage switching signal input from the controller; and a driver for driving and emitting the luminous dots of the fluorescent print head based on pre-luminous pattern data or image data input from the controller; whereby the luminous dots of the fluorescent print head are previously emitted based on a drive voltage input from the voltage selector and pre-luminous pattern data input from the voltage selector; whereby the luminous dots of the fluorescent print head are emitted based on a drive voltage input from the voltage selector and image data input from the driver and a desired image is created by illuminating light from the luminous dots onto a recording medium. 
     The apparatus further comprises a control electrode for controlling electrons emitted from the filament cathode. The voltage selector selects a drive voltage for the filament cathode, a drive voltage for the anode and a drive voltage for the control electrode of the fluorescent print head based on a voltage switching signal input from the controller. 
     In the apparatus according to the present invention, the voltage selector comprises plural power sources each for producing a different drive voltage and a selector circuit for selecting a power source which produces a drive voltage corresponding to a voltage switching signal input from the controller. 
     In the apparatus according to the present invention, the controller acquires and outputs pre-luminous drive data based on a pre-luminous signal and print drive data based on a print starting signal. The voltage selector includes a variable power source for variably producing a different drive voltage corresponding to a voltage switching signal input from the controller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other objects, features, and advantages of the present invention will become more apparent upon a reading of the following detailed description and drawings, in which: 
     FIG. 1 is a block diagram illustrating an image forming apparatus according to the present invention; 
     FIG. 2 illustrates a timing chart of an image forming apparatus according to the present invention; 
     FIG. 3 is a cross-sectional view partially illustrating a fluorescent print head mounted on an optical printer acting as an image forming apparatus; 
     FIG. 4 is a plan view illustrating luminous dots of the fluorescent print head of FIG. 3; 
     FIG.  5 ( a ) is a perspective view partially illustrating the anode substrate of the fluorescent print head of FIG.  3  and FIG.  5 ( b ) is a plan view partially illustrating an anode electrode; 
     FIG. 6 is a side view illustrating an optical printer including three print heads respectively for a R (red) luminous color, a G (green) luminous color, and a B (blue) luminous color; 
     FIG. 7 illustrates a timing chart of a drive circuit for a conventional fluorescent print head; 
     FIGS. 8 is a diagram illustrating the relationship between filament temperature and Ba evaporation rate; 
     FIG. 9 is illustrating the relationships between filament temperature and operational life; and 
     FIG. 10 is a diagram illustrating the relationship between filament voltages and emission current (Ik). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a block diagram illustrating the configuration of an image forming apparatus according to an embodiment of the present invention. FIGS.  2 ( a ) to  2 ( e ) are timing charts each for the drive circuit of a fluorescent print head in an image forming apparatus. 
     The image forming apparatus in this embodiment, which includes plural fluorescent print heads, is used to an optical printer such as a color printer (for example, with the structure shown in FIG. 6) that forms images on a recording medium (such as a photosensitive film or photographic paper). Each fluorescent print head has the same configuration as that shown in FIGS. 3,  5  and  6 . Like numerals are attached to the same elements as those shown in FIGS. 3,  5  and  6 . Hence, the duplicate explanation will be omitted here. 
     When the optical printer  1 , on which fluorescent print heads  2  are mounted, forms an image on a recording medium (film  25 ), the light amount necessary for light-exposing the recording medium is fixed. In the anode/grid voltage conditions at the time of printing, an anode voltage (Eb)=20 to 60 volts and grid voltage (Ec)=40 to 60 volts, preferably, Eb≦Ec. 
     The operational lifetime of the fluorescent print head  2  depends on the filament temperature, as pointed out with the conventional problems. It is necessary to prolong the operational lifetime by decreasing the filament temperature. 
     As shown in FIG. 10, the lower limit of the filament temperature in a space charge area depends on Ik (emission current). For that reason, the drive circuit  31  for a fluorescent print head  2  mounted on the image forming apparatus (the optical printer  1 ) decreases the emission current Ik, thus decreasing the filament voltage Ef to drop the filament temperature. 
     That is, the drive circuit  31  decreases the drive voltage as much as possible with the timing of no actual contribution to printing, that is, with the timing of no light emission, thus decreasing the average current. Thus, the operational lifetime of the fluorescent print head is improved by alleviating the load (Ik: lowered emission current) to the filament cathode  16 ,  17 , thus lowering the filament temperature. 
     Conventionally, because pre-light emission is performed by drive operation under the same conditions as those for the print light emission, the pre-light emission causes further reduction of the operational lifetime. For the countermeasures, the drive circuit  31  further alleviates the load on the filament cathode  16 ,  17  in pre-light emission mode, compared with the light emission mode during printing, to decrease the filament temperature. 
     In this embodiment, as shown in FIG.  2 ( a ), the drive circuit  31  controllably drives various electrodes (anodes  10 , a flat control electrode  15 , and filament cathodes  16  and  17 ) with the timings shown in FIGS.  2 ( b ) to  2 ( e ). Each timing, as shown in FIG.  2 ( a ), has three periods including (1) the pre-luminous period T 1  during which luminous dots  7  glow before an image is formed on a recording medium, (2) the print luminous period T 2  during which luminous dots  7  glow when an image is formed on a recording medium, and (3) the non-luminous period T 3  during which luminous dots  7  are lit off to form no image on a recording medium. 
     As shown in FIG. 6, the drive circuit  31  includes the control circuit  32  and the power source circuit  32 , mounted on the circuit substrate  13  inside a modulalized housing  27 . 
     As shown in FIG. 1, the control circuit  32  consists of a signal processor  32   a  and a controller (CPU)  35 . The power source circuit  33  includes a voltage selector  33   a . The drive circuit  31  controllably drives the anodes  10 , the flat control electrode  15 , and the filament cathodes  16  and  17  during three periods T 1 , T 2  and T 3 . 
     Moreover, as shown in FIG. 1, the body of the optical printer  1  has a circuit substrate on which a system controller (CPU)  34 , a controller (CPU)  35 , and an image memory  36  are mounted. A motor drive circuit  37  and a stepping motor  38  are mounted on the printer  1  to relatively move the fluorescent print head  2  and the recording medium  25  during printing. 
     The system controller  34  comprehensively controls various portions for print operation and inputs a pre-luminous signal, a print starting signal and color image data to the controller  35 . The system controller  34  also outputs a control signal to the motor drive circuit  37  to control the rotation and halt of the stepping motor  38  when the fluorescent print head  38  and the recording medium  25  are relatively moved. 
     When a pre-luminous signal is input from the system controller  34 , the controller  35  captures pre-luminous pattern data (pre-luminous drive data) from the image memory  36  and then outputs it to the signal processor  32   a.    
     When a print starting signal is input from the system controller  34 , the controller  35  captures color image data (print drive data) from the system controller  34  and then outputs it as image data for each three primary colors (R, G, B) to the signal processor. At the same time, the controller  35  captures data regarding power source drive conditions from the image memory  36  and outputs a voltage switching signal to the voltage selector  33   a  according to the captured data. 
     The image memory  36  stores data regarding power source drive conditions (such as voltage) and luminous pattern conditions (such as light emission in thick pattern). 
     The signal processor  32   a  outputs a control signal to the driver  32   b  based on pre-luminous pattern data or image data input from the controller  35 . 
     The driver  32   b  is electrically connected to the fluorescent print heads  2 R,  2 G, and  2 B, each which performs light exposure in terms of a corresponding primary color. The driver  32   b  outputs a drive control signal to the anode  10  corresponding to the fluorescent print head R, G or B in accordance with the three periods T 1 , T 2  and T 3 , based on the control signal from the signal processor  32   a.    
     The voltage selector  33   a  selects drive voltages for the filament cathodes  16  and  17  and the anodes  10  (and the flat control electrodes  15 ) of the fluorescent print head  2 , based on the voltage switching signal input from the controller  35 . 
     The voltage selector  33   a  also may include plural power sources (not shown), each which generates a different drive voltage, and a selector circuit (not shown) that selects a power source that generates a drive voltage corresponding to a voltage switching signal input from the controller  35 . The voltage selector  33   a  may be a variable power source that generates a drive voltage corresponding to a voltage switching signal input from the controller  35 . 
     Referring to FIG. 1, a combination of the controller  35  mounted on the body of the optical printer  1  and the signal processor  32   a  mounted on the fluorescent print head  2  corresponds to the controller (CPU) defined in claims. The controller  35  and the signal processor  32   a  can be integrated as one controller (CPU). In such a case, the controller may be arbitrarily mounted on the body of the optical printer or on the fluorescent print head  2 . 
     In the configuration of FIG. 1, an additional memory may store data on power source conditions and data on luminous pattern conditions, without storing in the image memory  36 . Thus, the system controller  34  reads the stored data and then outputs it to the controller  35 . Moreover, the image memory  36  may be mounted on the fluorescent print head  2 . Thus, the controller  35  reads data stored in the image memory  36 . 
     Next, various operations of the control circuit  32  in the drive circuit  31  during the pre-luminous period T 1 , the print luminous period T 2 , and the non-luminous period T 3  will be described here. In the following explanation, the rated condition during the pre-luminous period T 1  is defined as the drive condition (drive voltage) for the print luminous period T 2  during which the luminous dot  7  glows when an image is formed on a recording medium (film  25 ). Moreover, the rated condition during the print luminous period T 2  is defined as a filament voltage applied to a filament cathode when the anode voltage and the grid voltage are not controlled with the duty ratio corresponding to the blank period 
     (1) Pre-luminous Period 
     In the pre-luminous period T 1 , at least one of the anode voltage Eb applied to the anode  10  and the grid voltage Ec applied to the flat control electrode  15  is adjusted to a predetermined voltage lower than the rated condition to drive the anode  10  and the flat control electrode  15 . 
     In FIG. 2, the grid voltage Ec is controlled to a predetermined voltage lower than the rated condition. However, the anode voltage Eb may be controlled by a predetermined voltage lower than the rated condition. This can alleviate the load on the filament cathode  16 ,  17 , thus reducing the average current (Ik). Thus, the filament temperature can be lowered by controlling the filament voltage Ef to a predetermined voltage lower than a rated condition and heating and driving the filament  16 ,  17 . As a result, the evaporation amount of Ba on the filament cathode  16 ,  17  is decreased. Thus, deterioration of the luminous efficiency is suppressed and the operational lifetime can be prolonged. 
     In another drive operation during the pre-luminous period T 1 , the anode voltage applied to the anode  10  and/or the grid voltage applied to the flat control voltage  15  are set to a predetermined duty ratio, as shown in FIG.  2 ( e ). The anode  10  and/or the flat control electrode  15  are driven in time-divisional mode to blink luminous dots  7 . By doing so, the load on the filament cathode  16 ,  17  is alleviated so that the average current (Ik) can be decreased. Moreover, the filament temperature can be decreased by controlling the filament cathode Ef to a predetermined voltage lower than the rated condition and then heating and driving the filament cathode  16 ,  17 . As a result, the evaporation amount of Ba on the filament cathode  16 ,  17  decreases and a variation of light amount in the pre-luminous mode is alleviated. Finally, suppressed deterioration of the luminous efficiency contributes to an improved serviceable lifetime. 
     The anode voltage and the grid voltage are set to the same predetermined duty ratio and the anode  10  and the flat control electrode  15  are synchronously driven in time-divisional mode. 
     The above-mentioned driving methods can be combined together (not shown). Specifically, the anode voltage and/or the grid voltage are set to a predetermined voltage lower than the rated condition and to a predetermined duty ratio. The anode  10  and/or the flat control electrode  15  are driven in time-divisional mode. This driving method can more alleviate the load on the filament cathode  16 ,  17 , thus decreasing the average current (Ik). As a result, the luminous efficiency is further suppressed so that the operational lifetime can be prolonged. 
     Even when the anode voltage, the grid voltage, or both is decreased during the pre-luminous period T 1 , the above-mentioned effects can be obtained by uniformly impinging thermal electrons against a fluorescent substance and removing the residual gas on the surface of the fluorescent substance. However, since the pre-light emission is performed to remove gas adhered to a fluorescent substance and to stabilize the light emission (light amount), a higher anode voltage is better to the gas adhered to the fluorescent substance. A drive operation by a decreased grid voltage, not a decreased anode voltage, is effective to alleviate the filament load, without decreasing the gas removing effect. 
     For example, when a center-tapped filament is driven, the emission current (Ik) flows to both ends of the filament. For that reason, the temperature of both the ends of the filament rises the superimposed emission current by the Joule heat. The local temperature rise deteriorates the uniformity of the filament temperature in the longitudinal direction (or in the primary scanning direction) (this is applicable to a DC drive operation of a filament). The temperature change leads to a different evaporation rate of Ba in the longitudinal direction of a filament, thus resulting in variations of the operational lifetime. Hence, in order to improve the operational lifetime, it is necessary to decrease the anode voltage and the grid voltage and to reduce the emission current (Ik). Reducing the emission current (Ik) causes reducing the filament temperature, so that emitted electrons are reduced. Hence, since the filament temperature rises, it is required to decrease the filament temperature. 
     (2) Print Luminous Period 
     During the print luminous period T 2 , the anode voltage and the grid voltage are controlled under rated conditions when the recording medium is printed to drive the anode  10  and the flat control electrode  15 . The anode voltage and/or the grid voltage corresponding to the blank period t between light emission and light emission in a write mode for each line of each recording medium is controlled to a voltage lower than the rated condition. 
     FIG. 2 is shows the timing with which five recording media are printed during the print luminous period T 2 . Particularly, when a line light source (a linear light source) is used, the anode voltage applied to the anode  10  between light emission and light emission in the write mode for each line of a recording medium has periodic blank periods. The grid voltage corresponding to the periodic blank period is controlled to be less than a rated voltage. Compared with the conventional method, that operation can alleviate the load on the filament  16 ,  17  so as to decrease the total average current (Ik). Moreover, the filament temperature can be decreased by controlling the filament voltage Ef to a predetermined voltage lower than a rated condition and heating and driving the filament  16 ,  17 . As a result, the reduced temperature reduces the evaporation amount of Ba on the filament  16 ,  17  and suppresses deterioration of the luminous efficiency, thus resulting in an improved operational lifetime. Since the blank periods of the anode voltage and the grid voltage are very short, the filament temperature is not recognized as a temperature change but appears as a change of the total average emission current (Ik). 
     Actually, the anode voltage has blank periods which are formed with an image signal. Since the pulse width gradation control (PWM) is performed, the period (blank period) during which the anode voltage becomes off exists so long as the image signal is not in full gradation. That is, when an image is output, the average gradation component of the image undergoes an apparent Du (duty) drive operation so that the total average emission current (Ik) drops. 
     In further explanation, when a linear light source is used, the light amount is controlled with the pulse width during one-line write period. This light amount control includes the correction control of light amount variation and the light amount control in accordance with the gradation number of image data, and the correction control of individual difference of a photosensitive amount and provides non-luminous blank periods. 
     It is now assumed that the variation control results in a 20% light-amount drop at both the ends of a filament and that the minimum value correction results in a luminous time of 90% (Max 80%) on average and that the individual difference correction of each fluorescent print head results in 70% (Max 60%) on average. In such a case, even when the input data is a full gradation of 1024, an average of 40% of a blank period occurs. 
     (3) Non-luminous Period 
     During the non-luminous period T 3 , the anode voltage and the grid voltage are controlled to be 0 volts. Since current does not flow through the anode  10  and the flat control electrode  15 , the filament voltage is controlled to be 0 volts. This allows the filament temperature to be reduced. 
     During the pre-luminous period T 1  and the print luminous period T 2 , the drive circuit  31  in the fluorescent print head  2  controls the anode voltage and/or the grid voltage and drops the filament voltage, at the time of light emitting luminous dots  7  except the time of printing an image onto a recording medium (film  25 ). Thus, the evaporation amount of Ba contained in an electron emission material on the filament cathode  16 ,  17  can be reduced. This can suppress deterioration of the luminous efficiency and reduce the consumption energy and improve the serviceable lifetime of a fluorescent print head. Moreover, the alleviation of dimming due to lowered temperatures and the alleviation of changes in light amount due to pre-light emission allow the light amount to be stabilized. 
     During the pre-luminous period T 1 , because it is desirable to remove gas on adhered to a fluorescent substance, the drive voltage may be lower than the drive voltage during the print luminous period T 2 . In other words, because it is not required to impose the load on the filament during the pre-luminous period T 1 , the filament voltage can be set at a low value. 
     Table 1 shows comparisons between numerals in drive operations under conventional rated conditions and numerals in drive operations (during the pre-luminous period T 1  and the print luminous period T 2 ) according to the present invention. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Rated 
                 Invention 
                 Invention 
               
               
                   
                 Condition 
                 (pre-luminous 
                 (print luminous 
               
               
                   
                 (prior art) 
                 period: T1) 
                 period: T2) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Filament 
                 4.0 V 
                 Ef = 2.5 V 
                 Ef = 3.3 V 
               
               
                 Voltage Ef 
                 (If = 100 mA) 
                   
                 (If = 88 mA) 
               
               
                 Grid voltage 
                 40 V 
                 20 V 
                 40 V 
               
               
                 Ec 
                 Ic = 20 mA 
                 Ic = 6 mA 
                 Ic = 16 mA 
               
               
                 Anode voltage 
                 40 V 
                 40 V 
                 40 V 
               
               
                 Eb 
                 Ib = 10 mA 
                 Ib = 6.3 mA 
                 Ib = 7 mA 
               
               
                 Ik 
                 (Ib + Ic)Ik = 30 mA 
                 12.3 mA 
                 23 mA 
               
               
                 Temperature 
                 Rating 
               
               
                 (refer to 
                 (100%) 
                 (about 40%) 
                 (about 75%) 
               
               
                 FIG. 10) 
                 Filament 
                 about 550° C. 
                 about 600° C. 
               
               
                   
                 temperature 
               
               
                   
                 about 650° C. 
               
               
                 Lifetime (refer 
                 1.0E 3  to 1.0E 4   
                 1.0E 5  to 1.0E 6   
                 1.0E 4  to 1.0E 5   
               
               
                 to FIG. 9) 
               
               
                   
               
            
           
         
       
     
     In FIG. 1, the grid voltage is driven with a voltage lower than the rated condition during the pre-luminous period T 1  of the present invention. A blank period of a grid voltage to be applied to the flat control electrode  15  is provided during the print luminous period T 2 . The grid voltage blank period corresponds to the blank period between light emission and light emission at the time when an anode voltage applied to the anode  10  is written for one line on the recording medium  25 . The anode average current value and the grid average current value are set at lower value than the value under the rated condition (an average drive current value at rating). 
     As understood from Table 1, according to the present invention, the filament can be driven at a lowered filament temperature, compared with the driving operation under the conventional rated conditions. The evaluation of Ba containing the electron emission material on a filament cathode can be reduced. Compared with the drive operation under the conventional rated conditions, the operational lifetime can be improved by one digit. 
     In this embodiment, the fluorescent print head  2  including a grid electrode (the flat control electrode  15 ) for controlling electrons emitted from the filament cathode  16 ,  17  or uniformly maintaining the electric field has been explained as an example. However, the configuration with no grid electrodes may be employed for the configuration and the driving method in the present embodiment. 
     As apparent from the above explanation, the present invention can suppress deterioration of the luminous efficiency of a fluorescent substance, reduce the consumption energy, and improve the operational lifetime of a fluorescent print head. Moreover, the alleviation of dimming due to lowered temperatures and the alleviation of changes in light amount due to pre-light emission allow the light amount to be stabilized.