Discharge lamp and electronic flash device using the same

Details Discharge lamp and electronic flash device using the same Discharge lamp and electronic flash device using the same

2001-12-14

2003-03-11

Philogene, Haissa (Department: 2821)

Electric lamp and discharge devices: systems

Condenser in the supply circuit

Condenser in shunt to the load device and the supply

C315S134000, C315S158000, C315SDIG005, C313S594000, C313S602000, C313S633000

Reexamination Certificate

active

06531832

ABSTRACT:

TECHNICAL FIELD
The present invention relates to discharge lamp employed as an artificial light source in an electronic flash device incorporated in a photographic camera, and the electronic flash device mounted to a photographic camera. More particularly, the present invention relates to a discharge lamp emitting stable light by stabilizing a discharge current and an emitting waveform, and the electronic flash device using the same lamp.
BACKGROUND ART
FIG. 8
is a cross section of a conventional discharge lamp.
In
FIG. 8
, main electrodes
118
and
121
are sealed at both the ends of glass bulb
117
. Trigger electrode
122
made of transparent and conductive coating is provided on the entire outer surface of bulb
117
. Bulb
117
contains a necessary amount of rare gas such as xenon. Main electrode
118
comprises metallic member
119
and sintered metal member
120
mounted to the tip of metallic member
119
. Metallic member
119
is made of tungsten, Kovar or the like. Sintered metal member
120
is made by sintering tungsten powder, tantalum powder, or mixed powder of tungsten and tantalum.
The conventional discharge lamp structured above is built in, e.g. an electronic flash device as shown in FIG.
9
.
FIG. 9
illustrates an automatic electronic flash device which automatically controls the amount of light emitted from the discharge lamp by sensing the light radiated to a photographic object. Power source
123
supplies a high voltage (approx. 300V), and charges main-discharging capacitor
124
with a charging current, thus approx. 300V is applied across capacitor
124
. Trigger circuit
125
produces a high trigger voltage to energize discharge lamp
126
. Light-emitting control section
127
stops discharge lamp
126
to emit the light on its way. Photo-receptor
128
comprises photo-receiving element
129
and circuit
130
producing a light-emitting-stopping signal.
An operation of the conventional automatic electronic flash device structured above is described hereinafter.
Capacitor
124
is charged at a high voltage with the charging current from power source
123
. Trigger circuit
125
is activated to apply a high voltage to a trigger electrode of discharge lamp
126
. Then discharge lamp
126
is energized to emit light by charged energy stored in capacitor
124
, thereby radiating a photographic object. The light reflected from the object enters photo-receiving element
129
. When the amount of light entering to photo-receiving element
129
reaches a given amount, circuit
130
outputs a light-emitting-stopping signal to light-emitting control section
127
. Section
127
then conducts switching operation thereby stopping the discharge lamp
126
to emit the light.
FIG. 10
shows waveforms of discharge-current of the conventional discharge lamp.
FIG. 11
shows waveforms of the light emitted from the same bulb. Discharge lamp
126
is energized with a trigger voltage produced by trigger circuit
125
, and lamp
126
is discharged by the energy charged in capacitor
124
. The waveforms in
FIG. 10
illustrate time-varient discharge current. This discharge-current rises sharply approx. at the same time when the trigger voltage is applied, and then starts flowing. When light-emitting-stopping section
127
does not operate switching on the way of emitting the light, i.e. when section
127
is in a complete emitting mode, this flash device finishes discharging by consuming all energy charged in capacity
124
toward an photographic object away from the camera.
On the other hand, brightness of discharge lamp
126
starts increasing not simultaneously with the start of flowing the discharge current but with some time lag, as shown in FIG.
11
. When the conventional discharge lamp shown in
FIG. 8
is employed, the discharge current of this bulb
126
draws different waveforms marked with
200
and
250
in
FIG. 10
at each firing of the lamp, and no stable waveforms are obtained. Light emission of lamp
126
also draws different waveforms marked with
300
and
350
in
FIG. 11
at each firing, and no stable waveforms are obtained. In particular, the unstable light emission waveforms as shown in
FIG. 11
cause a reduction in precision in automatic light emission control.
It is necessary to detect precisely an amount of reflective light—out of the light emitted from discharge lamp
126
—from a photographic object for realizing precise control over the light emission from lamp
126
. For that purpose, photo receptor
128
should synchronizes exactly with an emission timing of lamp
126
. There are two methods for activating photo receptor
128
; (1) Trigger circuit
125
energizes discharge lamp
126
, and the discharge current shown in
FIG. 10
starts flowing. At the same time, an operable voltage is supplied to photo-receptor
128
. (2) When the discharge current reaches a given amount, this is detected and then the operable voltage is supplied to photo-receptor
128
.
When method (2) is employed for powering photo-receptor
128
, light-emitting waveform varies every time the discharge lamp fires as shown in FIG.
11
and this causes the following inconvenience: Although photo receptor
128
is ready to detect reflective light from the object, if lamp
126
would delay emitting as shown with waveform
350
, receptor
128
receives external lights other than the reflective light from the object during this delay, i.e. a period before lamp
126
starts emitting. Then, receptor
128
cannot receive the reflective light exactly from the object, and thus the light amount radiated to the object is less than an appropriate amount.
On the contrary, when photo-receptor
128
starts operating later than discharge lamp
126
, e.g. as shown with waveform
300
in
FIG. 11
, lamp
126
have already started emission before receptor
128
becomes ready to receive the reflective light from the object. The reflective light is thus not received by receptor
128
until receptor
128
is ready, and thus the light amount radiated to the object exceeds the appropriate amount.
The discussion described above proves that a slight time lag between a light emission timing and an operation start timing of the photo-receptor affects the amount of light emission only a little when the object is away from the camera. However, it affects the amount of light emission substantially when the object is close to the camera.
Independent of the prior art discussed above, Japanese Patent Application Non-Examined Publication No. S57-165948 discloses a flash discharge lamp of which noise at turning on to the peripheral systems is reduced. In this lamp, electrodes of anode and cathode are disposed closely to a line trigger electrode provided along the outer wall of a glass tube. This arrangement allows an instantaneous voltage drop in a waveform of a trigger signal to be reduced, thereby lowering the noise. The electrodes of anode and cathode are closely disposed to the line trigger electrode, so that discharge between the anode and cathode occurs along the trigger electrode. This may somewhat contribute to stabilizing discharge current and discharged light emitting comparing with the method previously discussed; however, this method still does not produce a satisfactory result because of the following reason: Indeed, the section, where the electrodes of anode and cathode are closely placed to trigger electrode
4
, forms an acute angle comparing with the center section of the glass tube; however, the anode and cathode face each other in parallel, so that the discharged current and the waveform of light emitted are not always stabilized while the waveform of a trigger signal is stabilized due to the acute angle formed by the anode and cathode is closely placed to the trigger electrode.
SUMMARY OF THE INVENTION
The present invention addresses the problem discussed above and aims to provide a discharge lamp emitting light with stable waveforms of both discharge-current and light-emission. This discharge lamp is employed in an electronic flash device which emits the light by consuming the energy cha

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