Photography – With object illumination for exposure – Having artificial illumination termination control
Reexamination Certificate
2001-01-29
2003-05-27
Adams, Russell (Department: 2851)
Photography
With object illumination for exposure
Having artificial illumination termination control
C396S164000
Reexamination Certificate
active
06571062
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
The disclosure of the following priority application is herein incorporated by reference: Japanese Patent Application No. 2000-032234, filed Feb. 9, 2000.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an electronic flash device capable of performing dimming control.
2. Description of the Related Art
FIG. 8
is a diagram showing a conventional example of an electronic flash device.
Hereinafter, an overview will be given of the operation of the conventional example with reference to FIG.
8
. Initially, a photodiode PD produces electron-hole pairs according to the intensity of emission of the electronic flash device. These electron-hole pairs are separated across the depletion region inside the photodiode PD, and then efficiently led out through an imaginary short between the input terminals of an operational amplifier OP
1
to make a photocurrent Ir. This photocurrent Ir flows through a diode D before getting absorbed into the output terminal of the operational amplifier OP
1
. Here, the output terminal of the operational amplifier OP
1
carries the bias voltage V
1
dropped by a forward voltage of the diode D. This output voltage of the operational amplifier OP
1
is applied to the emitter of a transistor Tr
2
. Meanwhile, a gain control voltage V
2
is applied to the base of the transistor Tr
2
through a voltage follower circuit consisting of an operational amplifier OP
2
.
As a result, the photocurrent which has been logarithmically compressed by the forward voltage characteristic of the diode D is in turn logarithmically decompressed by the (Vbe-Ic) characteristic of the transistor Tr
2
, whereby a photo-detection current Ip corresponding to the light intensity is restored. Here, increasing/decreasing the gain control voltage V
2
allows the gain of the photo-detection current Ip over the light intensity to be adjusted to film speed or the like.
The photo-detection current Ip obtained thus is passed through the loads, or a capacitor C and a resistor Rd, so that it is converted into a photo-detection voltage Vp. This photo-detection voltage Vp is compared with a threshold voltage Vth in a comparator CMP. The comparator CMP, when this photo-detection voltage exceeds the threshold Vth, outputs an emission stop signal STOP to an emission stop circuit (not shown) in the electronic flash device. Incidentally, the transistor Tr
1
is a switching circuit for resetting the storage charge in the capacitor C, and is kept short until the point of starting light emission.
In such an operation, modifications to the threshold voltage Vth allow control over the quantity of emission (the integrated quantity of light up to an emission stop) of the electronic flash device.
FIGS.
9
(A)-(C) are emission waveforms in the electronic flash device described above. Immediately after the output of the emission stop signal STOP, the emission waveforms keep their light emission with attenuation until complete light-out. The quantity of the remaining light (hereinafter, referred to as “the quantity of overrun light”) contributes a control error to dimming control.
Conventionally, such a control error has been mended by differential correction using the resistor Rd. Across this resistor Rd occurs in real time a voltage drop corresponding to the light intensity. This voltage drop is added to the storage capacitance in the capacitor c (an integrated value of light intensities, corresponding to the quantity of emission), thereby elevating the photo-detection voltage Vp. Thus the higher the instantaneous light intensities are, the greater the photo-detection voltage Vp appears to be, which leads to earlier output of the emission stop signal STOP. In general, higher emission intensities at the point of emission stop would produce greater quantities of overrun light. Therefore, such differential correction could improve the control error in the dimming control up to a certain degree.
By the way, in weak light emissions, the quantity of overrun light forms a great proportion to the target quantity of emission as shown in FIG.
9
(B), with a possible control error of the order of 30%.
Nevertheless, in the conventional differential correction, the resistor Rd could produce only an extremely small voltage drop in weak light emissions, thereby promising little correction effects.
SUMMARY OF THE INVENTION
In view of the foregoing problem, an object of the present invention is to provide an electronic flash device which can improve the dimming precision even in weak light emissions.
To achieve this object, the present invention is configured as stated below.
An electronic flash device according to the present invention comprises: an emission unit for performing flash emission; an emission monitoring unit for monitoring the quantity of emission by the emission unit; and an emission control unit for stopping the emission by the emission unit based on a comparison between the quantity of emission monitored by the emission monitoring unit and a predetermined target quantity of emission. Here, the emission control unit predicts the quantity of overrun light after stopping of the emission based on the target quantity of emission and corrects emission stop timing in accordance with the quantity of overrun light.
In the configuration described above, the emission stop timing is corrected based on the quantity of overrun light predicted from the target quantity of emission. Therefore, in contrast to the conventional differential correction, it becomes possible to reliably make a correction to cover the quantity of light overrun, independent of the magnitudes of instantaneous light intensities. This allows a sure improvement to the precision of the dimming control even in weak light emissions.
Here, it is particularly preferable for the emission monitoring unit to receive light from the emission unit directly. In this case, the emission monitoring unit is free from receiving external effects, such as to-subject distances and subject reflectance. Thus, the conditions for the light quantity monitoring remain constant almost each time. This allows predictions to be made without consideration of these external effects, thereby ensuring higher accuracy for the predictions on the quantity of overrun light. Moreover, since the conditions for the light quantity monitoring remain constant almost each time, it naturally follows that corrections when the emission stop timing is advanced improves in accuracy also. These synergistic effects bring about further improvements to the precision of the dimming control.
The emission monitoring unit in the present invention preferably includes: a photoelectric transducer for receiving light from the emission unit to generate an output according to the light intensity; a storage unit for storing the output generated by the photoelectric transducer; a discharge control unit for sequentially discharging a predetermined amount of storage out of the storage unit so that the storage in the storage unit is maintained generally constant; and a counter for counting the number of times the discharge control unit discharges the predetermined amount of storage and for outputting the count result as the result of monitoring the quantity of emission.
The predictions on the quantity of overrun light according to the present invention is generally suitably effected through digital processing, including prediction computing and making table reference. To execute these kinds of digital processing as part of the dimming control in the electronic flash device, it is preferable for the dimming control itself to be digitally controlled.
Nevertheless, digitally converting such high-speed, wide-dynamic-range phenomena as flash emission in real time inevitably requires an A/D conversion circuit with appropriate high speed and performance. On this account, simply realizing a digital dimming control would result in a negative effect that the electronic flash device complicates in configuration and increases in cost.
Thus, in the above-described confi
Adams Russell
Nikon Corporation
Oliff & Berridg,e PLC
Smith Arthur A
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