Electric lamp and discharge devices: systems – Condenser in the supply circuit – Condenser in shunt to the load device and the supply
Reexamination Certificate
2002-05-01
2003-10-07
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Condenser in the supply circuit
Condenser in shunt to the load device and the supply
C315S24100S, C396S159000
Reexamination Certificate
active
06630798
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electronic flash device of an automatic light adjusting type. More particularly, the present invention relates to an electronic flash device of an automatic light adjusting type in which flash light can be automatically adjusted without an error in the adjustment.
2. Description Related to the Prior Art
A lens-fitted photo film unit is known, in which a housing is pre-loaded with unexposed photo film and which includes a taking lens, a shutter device and other relevant elements. There is a flash type of lens-fitted photo film unit in which a flash device is incorporated for applying flash light to a photographic object to be taken typically at night or indoors. The flash device in the available type of the lens-fitted photo film unit applies flash light at a predetermined amount irrespective of an object distance. In a close-up photography, there arises a problem of an overexposure of a principal object due to a very near distance. The principal object is likely to be photographed in a whitish manner as viewed in a print produced from the frame. Even if the principal object is reproduced with suitable density, a background image is likely to be too dark.
An auto flash device is known as a flash device of an automatic flash emitting type. In the auto flash device, a flash discharge tube emits flash light. The photographic field reflects the flash light, which is received by a photoreceptor element. A photoelectric current is obtained by the photoreceptor element, and integrated. When the result of the integration reaches a predetermined level, a non-contact switch such as a thyristor is changed over, to discontinue the discharge of the main capacitor. The flash emission of the flash discharge tube is quenched. Thus, the light amount of the flash light is optimized.
JP-B 52-047327 (corresponding to U.S. Pat. Nos. 3,783,336, 3,809,951, 3,818,266, 3,857,064, 3,992,643 and 4,164,686) discloses a construction for preventing errors in operation of a light adjusting circuit due to influence of unrelated light, for example, light from other flash Devices. In
FIG. 10
, the flash device includes a main capacitor
80
, a light adjusting circuit
81
and a power source circuit
83
. A capacitor
82
in the light adjusting circuit
81
is charged by the power source circuit
83
together with the main capacitor
80
. When a trigger voltage is applied by a trigger circuit
84
to a flash discharge tube
85
, the main capacitor
80
is discharged in a path with the flash discharge tube
85
, which emits flash light.
At the same time as the start of flash emission, the capacitor
82
starts being discharged in a path with the flash discharge tube
85
, a Zener diode
86
and a resistor
87
. Voltage generated across the Zener diode
86
is applied to the light adjusting circuit
81
which starts operation. A photo diode
88
receives and reflected flash light from the object. An integration capacitor
89
stores charge according to the light amount of the reflected flash light at the photo diode
88
. When the charged voltage across the integration capacitor
89
reaches a predetermined level, a UJT (unijunction transistor)
90
is turned on. A turn-on capacitor
91
is responsively discharged in a path with a resistor
92
. Voltage generated across the resistor
92
is applied to a gate of a thyristor
93
. The thyristor
93
is turned on, to discharge the main capacitor
80
in a path with the thyristor
93
. The flash emission is quenched.
Should another flash device happen to exist beside the camera, the light adjusting circuit
81
is likely to receive influence of the external flash device, to turn on the thyristor
93
. The main capacitor
80
may be discharged accidentally without any purpose. However, the construction according to the prior art can prevent such a problem, because the light adjusting circuit
81
operates at the same time as the start of flash emission.
However, there are several problems in the construction in the above document. Failure is likely to occur in the automatic light adjustment specifically when the object distance is a very near distance, or when a photographic object has a very high reflectance. Flash light is emitted in full emission without restriction, to result in an overexposure. The turn-on capacitor
91
, which starts being charged simultaneously with the flash emission, has the voltage coming up to a level enough for turning on the thyristor
93
. The failure occurs when the UJT
90
is turned on by an increase of the integration capacitor
89
before the voltage across the turn-on capacitor
91
comes up to the level enough for turning on the thyristor
93
. This is because a light amount of reflected flash light is remarkably high.
A photo transistor is used as photoreceptor element. When the voltage is applied to the photo transistor as effective voltage for light detection, the photo transistor outputs a current in amplification of a current input to the photo transistor because of junction capacitance. The integration capacitor
89
is charged by the current after the amplification. A problem arises, as it is likely that the flash emission is quenched earlier than it should be. The precision in the light adjustment is lower.
SUMMARY OF THE INVENTION
In view of the foregoing problems, an object of the present invention is to provide an electronic flash device of an automatic light adjusting type in which flash light can be automatically adjusted without an error in the adjustment.
In order to achieve the above and other objects and advantages of this invention, an electronic flash device of an automatic light adjusting type includes a main capacitor for storing charge at a high voltage, a flash discharge tube for generating flash light upon discharging the main capacitor therewith, and a light adjusting circuit for adjusting a light amount of the flash light by feedback. In the flash device, the light adjusting circuit includes a reflected light measuring unit for outputting an integration voltage for representing a light amount of reflected light from a photographic field illuminated with the flash light. A non-contact switch short-circuits terminals of the main capacitor upon being turned on, to quench emission of the flash light in the flash discharge tube. A turn-on capacitor is charged by a current of discharge in response to the emission of the flash light, and is discharged when the integration voltage comes up to a prescribed level, to turn on the non-contact switch responsively. A control switching element keeps the reflected light measuring unit disabled while the turn-on capacitor is initially charged, and enables the reflected light measuring unit after voltage across the turn-on capacitor comes up to a predetermined level.
Consequently, full emission of flash light can be prevented specifically for photography at a very near distance.
Furthermore, a discharge switching unit is connected in parallel with the turn-on capacitor, for being turned on when the integration voltage from the reflected light measuring unit comes up to the prescribed level, to discharge the turn-on capacitor.
The reflected light measuring unit includes a photo transistor, supplied with electric power by the control switching element, for outputting a photoelectric current proportional to intensity of the reflected light incident thereon. There is an integration capacitor for integration of the photoelectric current to output the integration voltage.
Furthermore, a bypass switching element is turned on in response to becoming conductive of the control switching element, and short-circuits terminals of the integration capacitor until lapse of a predetermined time, to inhibit charging with the photoelectric current.
Consequently, flash light can be automatically adjusted without an error in the adjustment.
The bypass switching element is a bypass transistor, having a collector and emitter connected with respectively electrodes of the integration capacitor, and a base co
Kubota Takeshi
Motomura Katsumi
Fuji Photo Film Co. , Ltd.
Vu David
Young & Thompson
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