Abstract:
A flash system for underwater photography includes a housing for containing a camera which may be a digital camera, a flash unit externally attached to this housing for emitting flash light, a light sensor on the housing for sensing reflected light and outputting detection signals according to the sensed reflected light, and a control signal generator inside the housing. The control signal generator includes an input device for allowing a user to specify a selected light quantity to be emitted from the flash unit and also serves to generate and output signals to the flash unit according to the detection signals from the light sensor and also the light quantity selected through the input device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to an improved flash system for use when a digital camera is input inside a waterproof housing (hereinafter referred to simply as “housing”) for underwater photography and in particular for the control of flash and illumination lamps.  
           [0002]    When a digital camera is used for underwater photography, it is usually put inside a commercially available housing. Digital cameras of both the single-lens reflex type and the compact non-single-lens reflex type are available and will be hereinafter together referred to as digital cameras, or simply as cameras. Although cameras of the latter type are currently more popularly used, those of the former type may be gaining popularity in the future.  
           [0003]    Prior art apparatus for underwater photography using a digital camera had problems in the control of exposure by a flash lamp. For controlling the flash light for a non-digital single-lens reflex camera using a conventional film a method that is generally referred to as the TTL automatic control (or the TTL direct control) is employed whereby reflected flash light from the target object is passed through the lens, reflected by the film and received by a sensor disposed in front of the film so as to be converted into an electrical signal. The converted electrical signal is analyzed and a signal to stop the emission of the flash light is outputted when it is determined that a specified quantity of flash light has been emitted.  
           [0004]    This method was practical because the reflectivity of the film surface is about 15-25% and is just right for determining the exposure and usable even in underwater photography. In the case of a film this method is usable even with some deviation in exposure in view in part of the allowable latitude.  
           [0005]    Digital cameras, however, do not use a film. Instead, elements such as CMOS and CCD are used, and since these elements are black, their reflectivity is low and not appropriate for determining exposure and these elements cannot be used for a TTL automatic control.  
           [0006]    For this reason, it has been a common practice to cause flash light of weak intensity to be emitted preliminarily and immediately before the main exposure, to measure the reflected light from the target object simultaneously through the lens and by means of a sensor and to determine the level of the main exposure to follow, depending on the measured level of the reflected light.  
           [0007]    Since an adjustment is made such that the exposure will be proper above the ground, this method is not adequate in the case of underwater photography because light is attenuated differently in water from above water and also because the refractivity of light is also different in water. For example, reddish light does not reach as far inside water as bluish light. Thus, the calculated exposure based on the data received by the preliminary emission may not be right for the main exposure. As a result, overexposure at shorter distances and underexposure at larger distances are likely to result.  
           [0008]    Differences in operation and spectroscopic characteristics of the light sensor must also be taken into consideration in the case of a digital camera. In view of the problems as described above, automatic flash lamps to be externally attached are used, but such flash lamps must be used farther away from the camera lens (say, at a distance of 30 cm-50 cm) in order to prevent the so-called marine-snow phenomenon which results when small particles floating near the lens reflect the flash light and appear white on the picture taken. If the flash lamps are set thus far away, their light-measurement sensitivity is adversely affected and a proper exposure may not be made. Another problem to consider is that such automatic flash lamps cannot be manually adjusted.  
           [0009]    There are additional problems regarding automatic focusing. In underwater photography, there are usually more dark spots than above the ground. As a result, the automatic focusing function may operate slowly or may even fail to operate. In view of these problems, it has been known to provide a light source for illumination (not for exposure). Such a light source may comprise a high-intensity LED or a laser, and will be hereinafter referred to as “illumination lamp”. Such an illumination lamp may be provided not only to the camera housing but also to the externally attached flash unit (such as shown at  3  and  10  in FIG. 1). It is not good enough, however, to turn off these illumination lamps simultaneously as the flash light is emitted because the light from the illumination lamp may enter through the lens and be photographed.  
           [0010]    As illustrated in lines A and B of FIG. 8, it takes a finite length of time from the moment when the camera shutter begins to open (“ 70 ”) until it is completely opened (“ 71 ”). With some variations taken into account, it is still some time later (“ 72 ”) that the so-called X-contact of the camera is switched on and an electric signal is outputted to indicate the opening of the shutter. This delay may be about 3 milliseconds, although it depends on the type of the camera.  
           [0011]    Even if the illumination lamps are extinguished simultaneously as the X-contact is switched on (“ 73 ”), the light from the illumination lamp will be entering the camera for the length of time indicated by numeral  74  in FIG. 8. The length of time of this unwanted exposure is actually longer because the emission of light from the illumination lamp does not stop instantly as the current therethrough is shut off. It depends on various factors such as the brightness of the target object, the lens opening and the shutter speed. It happens more significantly when the target object is dark and the shutter speed is slow.  
         SUMMARY OF THE INVENTION  
         [0012]    It is therefore an object of this invention to provide an apparatus with which good underwater photographs can be taken with right exposures by means of a digital camera inside a housing and by using flash units.  
           [0013]    It is another object of the invention to prevent light of the illumination lamp from entering the camera when the target object is dark.  
           [0014]    A flash system embodying this invention for underwater photography by a digital camera, with which the above and other objects can be accomplished may be generally characterized as comprising a housing for the camera, a flash unit for emitting flash light externally attached to the housing, a light sensor on the housing for sensing reflected light and outputting detection signals according to the reflected light sensed thereby, and a control signal generator which is inside the housing, includes an input device for allowing a user to specify a selected light quantity to be emitted from the flash unit, and generates and outputs signals to the flash unit according to the detection signals from the light sensor and the selected light quantity. The housing may be divided into a main housing body and a box that is disposed adjacent and electrically connected to the main housing body such that at least a portion of the control signal generator is contained inside the box. The input device may be set either on the box or on an outer surface of the main housing body.  
           [0015]    An flash system of this invention may be characterized alternatively as comprising a housing for containing the camera, a flash unit externally attached to the housing for emitting flash light, an illumination lamp, a control signal generator inside the housing for generating and outputting signals to the flash unit according to the detection signals from the light sensor and the selected light quantity, and a lamp controller for switching off the illumination lamp by using as trigger a fully-pressed signal that is outputted when the shutter button of the camera is fully pressed.  
           [0016]    As another alternative, it may be characterized as comprising a housing for containing the camera, a flash unit externally attached to the housing for emitting flash light, an illumination lamp on the flash unit, and a lamp controller for switching off the illumination lamp by using as trigger a short pulse signal received from the housing, the short pulse being about 5 microseconds in duration and significantly shorter than the duration of the flash light.  
           [0017]    As a further alternative, it may be characterized as comprising a housing for containing the camera, a flash unit externally attached to said housing for emitting flash light, an illumination lamp, and a lamp controller for switching off the illumination lamp, a portion of the lamp controller inside the housing generating a significantly shorter control pulse signal than the duration of the flash light for the illumination lamp. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a front view of an example of flash system embodying this invention.  
         [0019]    [0019]FIG. 2 is a front view of the housing of the flash system shown in FIG. 1.  
         [0020]    [0020]FIG. 3 is a partially sectional view of the housing of FIG. 1.  
         [0021]    [0021]FIG. 4 is a back view of the housing of FIG. 1.  
         [0022]    [0022]FIG. 5 is a circuit diagram of a portion of the control signal generator on the side of the housing.  
         [0023]    [0023]FIG. 6 is a circuit diagram of the housing shown in part as a block diagram.  
         [0024]    [0024]FIG. 7 is a circuit diagram of another housing.  
         [0025]    [0025]FIG. 8 is a timing chart for showing the operations of parts of the flash system of this invention inclusive of timing charts of a prior art operation for comparison.  
         [0026]    [0026]FIG. 9 is circuit diagram of an externally attached flash unit embodying this invention shown in part as a block diagram.  
         [0027]    [0027]FIG. 10 is a circuit diagram of a portion of the externally attached flash unit of FIG. 9.  
         [0028]    [0028]FIG. 11 is a back view of another housing embodying this invention for a flash system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    The invention is applicable to digital cameras of both the single-lens reflex type and the compact non-single-lens reflex type, although it will be described below only for the case of the former type.  
         [0030]    [0030]FIG. 1 is a front view of a flash system embodying this invention, including a housing  9  with a camera inside, and FIG. 2 is a front view of the housing  9 . FIG. 3 is a sectional view of the housing  9  with a camera  15  inside. The housing  9  is of a waterproof structure and contains not only the camera  15  but also a battery  16  as a power source and a device unit  18  including various devices. On the exterior of the housing  9  are a shutter button  33  of the camera  15 , a front plate  4  made of a glass or acryl material, an illumination lamp  3 , an operation button  6  and two light signal outputting terminals  2  and  7 .  
         [0031]    [0031]FIG. 1 shows a situation wherein only one of these light signal outputting terminals ( 7 ) is connected to a light sensor  12   a  of an externally attached flash unit  11  through an optical fiber cable  8 . The optical fiber cable  8  of FIG. 1 may be replaced by a more commonly used type of lead line. Such connections are well known and hence no further detailed explanation will be presented. If the connection is by means of such a lead line, there is no need to convert the electrical signal from the camera into an optical signal and optical signal back to another electrical signal but the connector and the lead line must be waterproofed.  
         [0032]    The flash unit  11  is provided with a light bulb (“illumination lamp”)  10  and discharge tubes  13  in front. As one of the features of this invention, there is a flash light receiving sensor  5   a  attached to the aforementioned front plate  4 . This sensor  5   a  itself may be formed by inserting a photoelectric converter element such as a phototransistor into a waterproof light-screening tube. The attachment may be made without forming any hole through the front plate  4  from the backside towards the front or forming a hole and attaching the sensor  5   a  in the forward direction. If the attachment to the glass or acryl material of the front plate  4 , the supporting member therefor, usually made of aluminum, may be provided with a broadened edge portion for attaching the sensor  5   a.    
         [0033]    [0033]FIG. 2 shows an example, for the purpose of illustration, provided with another attachment  14  for the sensor  5   a . As the direction of the sensor may be adjusted to the direction towards the center of the lens, it is convenient for accurately measuring the quantity of exposed light when a target object at an extremely short distance is being photographed.  
         [0034]    The position of attachment is not limited to the places indicated in FIG. 2. When the connecting wires are externally exposed, however, care must be taken to waterproof such wiring. As another example, an optical fiber may be used as a part of the sensor  5   a . In such a case, the main part of the sensor  5   a  may be at any position. One end of the optical fiber is connected to the main body and the other end is oriented in the same direction as the axis of the lens, such that the direction of incident light will be the same as that into the lens. In the case of a sensor of this type, the position of the light-receiving end of the optical fiber will be regarded as the position of the sensor  5   a . Although both symbols  5   a  and  14  in FIG. 2 indicate an optical sensor, it is not intended to mean that there should be two sensors.  
         [0035]    The conventional type of TTL automatic control is disadvantageous because it is not the reflected light from the target object that is directly measured. Since the reflectivity of the film surface is not uniform but varies according to the type and the maker of the film, the conventional TTL control introduces variations in exposure. The present invention has the merit of reducing such variations in exposure because reflected light from the target object to be photographed is directly measured.  
         [0036]    The device unit  18  placed below the camera  15  below the battery  16  as shown in FIG. 3 includes a control signal generator which is a part of what is herein referred to as the light quantity controller for the externally attached flash unit  11 . The device unit  18  is connected to the shutter remote (cable release) terminal  21  of the camera  15  through line  22  to a connector (common referred to as the “hot shoe”  17 ) through line  20  and to the aforementioned flash light receiving sensor Sas through line  19 .  
         [0037]    As shown in FIG. 4, the back side of the housing  9  contains a liquid crystal display device  23 , a finder  24 , an operating button  25 , light quantity setting switches  26  and  27  respectively for adjusting the quantity of flash light emitted from the left-hand and right-hand side flash unit. It is to be reminded that only one flash unit  11  is shown in FIG. 1 but it is preferable to use two flash units respectively connected through the two light signal outputting terminals  2  and  7 . By independently controlling the two light quantity setting switches  26  and  27 , three-dimensional (that is, not flat) photographs can be taken by varying the balance of the light quantities from the two flash units on both sides of the camera  15 . These switches  26  and  27  need not necessarily be provided on the backside of the housing  9 . If space is available, they may be positioned on a side surface or on the top surface.  
         [0038]    The aforementioned control signal generator need not be inside the housing  9 . Since it has many components, a small box (sometimes referred to as the “grip”)  131  as shown in FIG. 11 may be provided adjacent to the housing  9  to contain some or all of the components of the control signal generator. The housing  9  and the grip  131  are connected by a lead line  132  which must be water proofed. Alternatively, the housing  9  and the grip  131  may be connected by a tubular connector and may be formed integrally with the connecting lead line disposed inside. The flash unit  11  may be directly attached to the grip  131 .  
         [0039]    The electrical system includes an illuminating lamp control circuit and the aforementioned light quantity controller for the flash unit (or units). The light quantity controller includes signal transmitting means on the side of the housing and signal receiving means on the flash unit. FIG. 6 shows the circuit structure of the device unit  18  on the side of the housing  9  (or of the interior of the grip  131 ), including the control signal generator as a part of the light quantity controller, a light-off signal generator  120  for the illumination lamp  3  and a lamp control device for the illumination lamp  3 . In FIG. 6, numerals  50 ,  51 ,  52  and  53  are common to both the light-off signal generator and the lamp control device for the lamp  3 , numeral  57  is a part of the lamp control device and all the others are the control signal generator. The LED  45  and the battery  16  are used in common.  
         [0040]    The illuminating lamp control circuit serves to extinguish the illuminating lamps  3  and  10  when the camera shutter is fully pressed. Camera shutters are usually pressed in two stages. When a camera shutter is half pressed, or pressed to a half-way position, a “half pressed signal” is outputted. When it is pressed all the way down to its full extent, a “fully pressed signal” is outputted. According to the present invention, the illuminating lamps  3  and  10  are extinguished by using the fully pressed signal as its trigger signal because, as explained above, it is not early enough to extinguish the lamps  3  and  10  as the flash unit begins to emit flash light.  
         [0041]    As shown at  77  in FIG. 8, a fully pressed signal is generated when the camera shutter is fully pressed. As explained above, the shutter begins to open somewhat later at  70  and becomes completely open at a still later time (at  71 ) and the X-contact is switched on at a further later time (at  72 ). According to this invention, the fully pressed signal  77  triggers the extinction of the illumination lamps  3  and  10  as shown at  85 . Although it takes a finite length of time for the illumination lamps  3  and  10  to become completely dark, this finite length of time is much shorter than the time for completely opening the shutter (at  71 ). Thus, the light from the illumination lamp  3  is reliably prevented from entering the camera to be photographed.  
         [0042]    The aforementioned fully pressed signal  77  may be obtained from the shutter remote terminal  21  of the camera  15 . Almost every single-lens reflex camera has a shutter remote terminal, to be used with a release cable having a long cord to be attached thereto for opening the shutter from a distant position. The shutter remote terminal  21  is connected in parallel with the shutter button  33  of the camera  15  such that the aforementioned fully pressed signal  77  is outputted also from the shutter remote terminal  21  as the shutter button  33  of the camera  15  is fully pressed.  
         [0043]    The illumination lamps  3  and  10  are switched on by pressing down the shutter button  33  half-way so as to switch on the camera circuit and too detect a signal indicative thereof. Such a signal may be detected at a terminal in the hot shoe  17 , at a detection terminal  52  shown in FIG. 6 as the “system voltage” or at the shutter remote terminal  21 .  
         [0044]    With reference to FIG. 6, as a signal is inputted from the system voltage detection terminal  52  to a lamp circuit driver  53 , transistor  57  is switched on and the illumination lamp  3  becomes lit. As the shutter  33  is fully pressed and a full-press contact  50  is switched on, this signal is used as a trigger by an inhibit circuit  51  to switch off the lamp circuit driver  53 . This causes the transistor  57  to be switched off, and the illumination lamp  3  is extinguished. Thus, the light from the illumination lamp  3  does not enter the camera.  
         [0045]    Next, the operation of the light-off signal generator  120  is explained. As the lamp circuit driver  53  is switched off, as explained above, an edge detector circuit  54  of a known type detects the sudden change (or edge) of the fully pressed signal  77  and transmits a detection signal to a pulse generator circuit  55  of also a known type. The pulse generator circuit  55  generates in response a short pulse of duration about 5 microseconds as shown at  82  in FIG. 8 and transmits it to a transistor  56 , thereby causing a signal light to be emitted from a signal-light emitting LED  45  and transmitted through the optical fiber cable  8  to the light sensor  12   a  of the flash unit  11  (as shown in FIG. 1) so as to be converted into various control signals by the circuits inside the flash unit  11  to control its illumination lamp  10 . If the output of the edge detector circuit  54  can be adjusted to about 5 microseconds, the pulse generator circuit  55  may be dispensed with.  
         [0046]    Since this signal light from the signal-light emitting LED  45  is only for the purpose of being controllable by the flash unit  11 , its width need not be exactly 5 microseconds but may be as short as 2 microseconds or as long as 8 microseconds. What is essential is that it should be significantly shorter than the duration of the flash light from the discharge tubes  13  of the flash unit II but the flash light from the discharge tubes  13  is never as short as 5 microseconds and lasts usually longer than 10 microseconds. In what follows, the expression “short pulse” is used in this limited sense.  
         [0047]    It is one of the distinguishing characteristics of the present invention to control the illuminating lamp of an externally attached flash unit by means of a pulse signal of such a short duration. According to the conventional technology, a weaker signal as shown at  78  of FIG. 8 was used and hence an amplifier circuit was required. Thus, one of the advantages of the present invention is that such an amplifier circuit can be dispensed with and another advantage is that energy is not wasted because the signal light emitting LED  45  is not caused to continuously emit light. It is to be noted, however, that the signal for emitting illumination light from the flash unit is not outputted from the camera. It is only the signal to stop its emission. In order to switch on the illumination light from the flash unit, a timer switch on the flash unit must be switched on.  
         [0048]    Although this method of the invention can be applied not only to digital cameras of the single-lens reflex type but also to non-single-lens reflex type cameras, there are many non-single-lens reflex type cameras that are not capable of detecting the fully pressed condition of the shutter. In such a situation, use may be made, as a trigger for the extinction of the illumination lamp, of the light signal from an LED which switches on when the automatic focusing has been successfully completed after the shutter button has been half pressed, or of the light signal emitted when the internally set flash lamp undergoes a preliminary emission. Since such detections can be made by conventionally known methods with conventionally available devices, explanations thereof will be herein omitted.  
         [0049]    [0049]FIG. 7 shows another circuit structure in part incorporating prior art technology, that is, it is different from the structure shown in and explained above with reference to FIG. 6 in that the edge detector circuit  54  and the pulse generator circuit  55  are removed and that the signals from the system voltage detection terminal  52  and the fully pressed contact  50  are used in combination. As shown in FIG. 7, a higher resistor  60  with higher resistance is connected to the transistor  56  than the lower resistor  59  with lower resistance such that the brightness of the signal light from the LED  45  is reduced and hence becomes distinguishable from that of the stronger flash light.  
         [0050]    In FIG. 8, these signals are indicated by numerals  78  and  79 . In other words, the flash unit  11  distinguishes between the weaker and stronger emissions to control the illumination lamp  10  and the discharge tubes  13 . This can be carried out by means of a voltage detector of a known type.  
         [0051]    Next, the flash operating mechanism, that functions after the illumination lamps  3  and  10  are switched off, will be explained with reference to FIG. 6. This mechanism includes not only the externally attached flash unit  11  but also the aforementioned light quantity controller. FIG. 6 shows the device unit  18  but its components may be contained either in the housing  9  or the grip  131  and includes the aforementioned control signal generator. It is to be noted that FIG. 6 shows only important components. Less important and/or commonly known portions are largely omitted.  
         [0052]    Operations of the flash operating mechanism are explained next sequentially. Broadly explained, the functions of this mechanism on the side of the housing are to transmit signals for starting and stopping emission of the flash light and those on the side of the flash unit are to receive these signals and too accordingly start and stop the emission of flash light.  
         [0053]    After the shutter button of the camera is fully pressed, the X-contact  41  of the camera is switched on at the timing indicated by numeral  72  as shown in FIG. 8 and a pulse signal of about 10 milliseconds is generated by a fate voltage generator circuit  40  and applied to a circuit driver  43 , a flash light sensor  5   b  and a comparator  49 . As the circuit driver  43  causes transistor  44  to be switched on, the signal-light emitting LED  45  begins to emit light as shown by numeral  80  or  83  in FIG. 8. The discharge tubes  13  of the flash unit  11  begin to emit light as shown at  75  by receiving this signal although the optical fiber cable  8 .  
         [0054]    The operations of the control signal generator are as follows after the emission of flash light is started. As the flash light emitted from the flash unit  11  is reflected by a target object, the reflected light received b y the flash light sensor  5   a  is converted into electrical signals and integrated by an integration circuit  46  with a resistor  47  for discharge. The resultant voltage of the integration is compared by means of a comparator  49 . When the voltage defined by a comparator resistor  48  and the light quantity setting switch  26  reaches a specified level, a stop signal (usually a Lo signal) is outputted from the comparator  49 . The timing of the emission of this signal is determined according to the desired level of exposure set on the camera. In other words, the stop signal is outputted such that the desired level of exposure is accomplished if the emission of the flash light is stopped in response to this signal. Although an example with an integration circuit was illustrated, this may be replaced, for example, by a calculating device comprising a microcomputer (CPU) with a digital circuit.  
         [0055]    For the sake of simplicity, description was given above only for one flash unit. Where two flash units are connected to the camera to be controlled by two independently operable light quantity setting switches  26  and  27 , as shown in FIG. 4, two comparators  49  and  32  are provided as shown in FIG. 5 corresponding thereto. In FIG. 5, terminal  28  is connected to the gate voltage generator circuit  40 , terminal  29  is connected to an inhibit circuit  42  and terminal  30  is connected to the negative terminal of the battery  16 . The output terminal  31  of the other comparator  32  is connected to another inhibit circuit (not shown). In other words, another set of circuits equivalent to that including inhibit, circuit driver, transistor and LED  42 ,  43 ,  44  and  45  is required, although not shown in FIG. 6 or  7  for convenience. The flash light sensor  5   b  and the integration circuit  46 , however, may be shared by both flash units for reducing cost.  
         [0056]    When an end signal {usually a Hi signal) is outputted from the comparator  49 , the inhibit circuit  42  switches off the circuit driver  43  and hence the transistor  44 , causing to terminate the emission of the strong light from the signal-light emitting LED  45  as shown at  81  or  84  in FIG. 8. This represents the intended optimum exposure by the flash light.  
         [0057]    As the light from the LED  45  stops, the flash unit  11  stops the emission of flash light, as shown by numeral  76  in FIG. 8.  
         [0058]    As explained above, the externally attached flash unit  11  includes a part of what is herein referred to as the light quantity controller, functioning by receiving and in response to the signals from the control signal generator and the light-off signal generator for the illumination lamp.  
         [0059]    [0059]FIG. 9 is a block diagram of the flash unit  11  and FIG. 10 shows the circuit structure of its sensor circuit and its signal detection circuit. These circuits are connected through lead lines to the circuit shown in FIG. 9, having a plus terminal  112  connected to the positive terminal of a battery  90  and a minus terminal  115  connected to its negative terminal.  
         [0060]    The portion of the circuit shown in FIG. 10 excluding lamp controllers  121  and  122  is for the flash light and includes a portion of the light quantity controller. A portion of the circuit shown in FIG. 10 is also used for the control of the illumination lamp  10 . Some of the portions not related to this invention are omitted from FIG. 10 for clarity Of the two methods of controlling the illumination lamp described above, the one by means of a short pulse explained above by way of FIG. 6 and numeral  82  of FIG. 8 will be explained for the control of the illumination lamp  10  of the flash unit  11 . The other method explained above by way of numerals  78 ,  79 ,  80  and  81  is well known and is carried out by distinguishing between stronger and weaker light from the LED.  
         [0061]    As a detection signal from the LED  45  is received through the optical fiber cable  8  by the light sensor  12   a  of the flash unit  11 , it is converted into an electrical signal and its waveform is changed by a DC-cutting capacitor  104  which also serves as a differentiator. In FIG. 10, numeral  103  indicates the load resistance of the light sensor  12   a.    
         [0062]    As the signal, with its waveform thus converted, is amplified by transistors  105  and  106 , only signals T 1  and T 2  are outputted on the positive side as the lamp control signal through terminal  113  and a lamp control signal input terminal  94  shown in FIG. 9 to a lamp-off circuit  91 . The lamp-off circuit  91  is provided with a timer function of a known kind, serving to inactivate a timer circuit  92  for a specified length of time such as one second so as to switch off a lamp driver circuit  96  and to switch off the lamp  10  for one second.  
         [0063]    The lamp-off circuit  91  is also capable of keeping the lamp  10  switched off for a longer period of time by keeping the timer circuit  92  in an inactive condition by a circuit method of operation until a timer reset  93  is switched on.  
         [0064]    When the lamp  10  is kept switched on for an extended period of time, the timer circuit  92  is not required. In such a situation, the lamp-off circuit  91  can control the lamp driver circuit  96  directly as indicated by dotted line in FIG. 9 to switch off the lamp  10 .  
         [0065]    In any of the above situations, the lamp  10  is switched off as shown at  85  in FIG. 8 and the light therefrom is prevented from entering the camera. Each of the cycles described above is initially activated by signal T 1  but there is no problem if it was at the time of T 2  because the lamp  10  already switched off by this time.  
         [0066]    A same signal as the lamp control signal is inputted from the transistor  106  to a delay circuit  111  comprising a resistor  107  and a capacitor  108 . This functions as an integration circuit for a short period of time and may also be referred to as a low pass filter, serving not to pass short signals (of about 5 microseconds) such as TI but to pass longer signals, say, longer than  10  microseconds. Such a low pass filter characteristic can be achieved by properly selecting the values of its resistor  107  and capacitor  108 . It is not preferable to make the delay time too long because the start of the light emission is excessively delayed. The output from the transistor  106  becomes T 2  alone and the lamp control signal is cut.  
         [0067]    As the emission start signal is transmitted from the synchronization terminal  114  ( 98 ) to activate a trigger circuit  99 , the discharge tubes  12  begin to emit flash light as shown by numeral  75  in FIG. 8. The flash unit has a high-voltage charge preliminarily prepared by a DC-DC converter circuit  95  and saved in a main capacitor  97  so as to be ready for the discharge and the emission of flash light.  
         [0068]    The DC-cutting capacitor  104  is also connected to a stop signal detector circuit  109  which has a similar circuit structure as the transistor  105  and serves to detect an emission stop signal T 3 . When signal T 1  is received by the stop signal detector circuit  109 , this has no consequence because this is before the emission of light from the discharge tubes  13  is started. The emission stop signal T 3  is inputted through terminals  116  and  101  to a stop signal input circuit  100 , causing a light emission control circuit  102  to switch off the current to the discharge tubes  13  and switching them off as shown by numeral  76  in FIG. 8. This completes one cycle of operations. An IGBT element of a known type may be used as the light emission control circuit  102 . The method of using it is also well known.  
         [0069]    The invention has been described in terms of only a limited number of examples but they are not intended to limit the scope of the invention. Many modifications and variations are possible within the scope of the invention. Such modifications and variations that may be apparent to a skilled person in the art are intended to be included within the scope of the invention.