Television – Camera – system and detail – Optics
Reexamination Certificate
1997-12-19
2001-05-29
Garber, Wendy R. (Department: 2612)
Television
Camera, system and detail
Optics
C348S363000, C348S297000
Reexamination Certificate
active
06239840
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an exposure controller, more particularly to an exposure controller for a camera incorporating an electronic iris.
Instead of mechanical exposure control, electronic exposure control has recently been used to meet the needs for miniaturization of electronic cameras.
Conventional electronic exposure control has been disclosed in Japanese Laid-open Patent Application No. 5-48975, for example.
A conventional exposure controller is described below.
FIG. 8
is a block diagram of this conventional exposure controller. Referring to
FIG. 8
, numeral
81
represents a rectifier circuit for receiving an image signal from an image pickup device at an input terminal and for rectifying the image signal, numerals
82
and
83
represent comparator circuits, numerals
84
and
85
represent reference voltage setting circuits, numeral
86
represents a counter circuit, and numeral
87
represents an exposure time control circuit for controlling exposure time.
The operation of the exposure controller having the above-mentioned configuration is described below. First, an image signal from the image pickup device is rectified by the rectifier circuit
81
. A reference voltage, that is, a potential difference corresponding to the amount of change in the output level of the image signal when the amount of input light entering the image pickup device is doubled, is generated by the reference voltage setting circuit
84
. Another reference voltage, that is, a potential difference not less than the above-mentioned potential difference, is generated by the reference voltage setting circuit
85
. The two reference voltages generated by the reference voltage setting circuits
84
,
85
are compared with the output level of the rectified image signal by the comparator circuits
82
,
83
, respectively. The count of the counter circuit
86
is incremented, decremented or stopped depending on the outputs of the comparator circuits
82
,
83
, and the exposure time of the image pickup device is controlled by the exposure time control circuit
87
depending on the output of the counter circuit
86
.
However, in the above-mentioned conventional configuration, the level of the picked-up image signal at each exposure time control process changes larger as the exposure time becomes shorter, and the problem of hunting occurs at the convergent point of the image signal level.
Accordingly, an object of the present invention is to provide an exposure controller free from hunting by using a coring value adapted to the discharge pulse count of a solid-state image pickup device.
SUMMARY OF THE INVENTION
A first embodiment of the present invention comprises a lens, a solid-state image pickup device for picking up the image of light having passed through the lens, an AD converter for converting the image picked up by the solid-state image pickup device into a digital signal, a luminance level detector circuit for detecting the luminance level of the image signal digitized by the AD converter, a register having stored the target value of the luminance level, a subtracter for calculating the difference between the luminance level detected by the luminance level detector circuit and the target value of the luminance level, a discharge pulse calculation circuit for calculating a discharge pulse count (hereinafter referred to as sub) to be output to the solid-state image pickup device within one field period on the basis of the polarity of the difference and a discharge pulse hold signal to be output from a coring circuit described later without changing the discharge pulse count when the discharge pulse hold signal is H, or depending on the polarity of the difference between the luminance level and the target value of the luminance level to be output from the subtracter when the discharge pulse hold signal is L, and for outputting sub, an absolute value circuit for calculating the absolute value (hereinafter referred to as a luminance level error) of the difference between the luminance level and the target value of the luminance level to be output from the subtracter, a coring circuit for defining the quotient obtained from sub divided by a predetermined setting value and plus 1 as a coring value, for setting the discharge pulse hold signal at H when the coring value is larger than the luminance level error, or at L in other cases, and for outputting the discharge pulse hold signal, and a drive pulse generator circuit for converting sub into a discharge pulse signal and for outputting the discharge pulse signal to the solid-state image pickup device.
According to this embodiment, when light enters the solid-state image pickup device, the device performs photoelectric conversion, stores charges in a period during which no discharge pulse signal is input, and outputs the charges as an image signal. The luminance level detector circuit assigns weights to the screen center portion of the digitized image signal so as to average luminance levels on the screen, and outputs the average level as the luminance level of the image signal. The subtracter circuit calculates the difference between this luminance level and the target value of the luminance level, which has been stored in a register. The coring circuit defines the value of sub divided by a predetermined value and plus 1 as a coring value, and when
luminance level error>coring value
the discharge pulse hold signal is set at L, or when
luminance level error≦coring value
the discharge pulse hold signal is set at H, and the signal is output to the discharge pulse calculation circuit.
The discharge pulse calculation circuit determines the magnitude relationship between the luminance level and the target value depending on the polarity of the luminance input signal, and when
luminance level>target value
the discharge pulse calculation circuit increments sub so as to shorten the charge time of the solid-state image pickup device and to lower the luminance level, and then outputs sub, or when
luminance level≦target value
the discharge pulse calculation circuit decrements sub so as to lengthen the charge time of the solid-state image pickup device and to raise the luminance level, and then outputs sub. In case the discharge pulse hold signal is H at this time, sub remains unchanged and is output. The drive pulse generator circuit outputs the discharge pulse signal having the same number of pulses as the value of sub to the solid-state image pickup device.
Since the exposure controller is provided with the coring circuit as described above, this exposure controller can be embodied as an exposure controller free from hunting by using a coring value adapted to the value of the discharge pulse count.
A second embodiment of the present invention, having a coring circuit different from that of the first embodiment, comprises a lens, a solid-state image pickup device for picking up the image of light having passed through the lens, an AD converter for converting the image picked up by the solid-state image pickup device into a digital signal, a luminance level detector circuit for detecting the luminance level of the image signal digitized by the AD converter, a register having stored the target value of the luminance level, a subtracter for calculating the difference between the luminance level detected by the luminance level detector circuit and the target value of the luminance level, a discharge pulse calculation circuit for calculating a discharge pulse count (hereinafter referred to as sub) to be output to the solid-state image pickup device within one field period on the basis of the polarity of the difference and a discharge pulse hold signal to be output from a coring circuit described later without changing sub when the discharge pulse hold signal is H, or depending on the polarity of the difference between the luminance level and the target value of the luminance level to be output by the subtracter when the discharge pulse hold signal is L, and for outputting sub, an absolute value circuit for calcul
Ishiguro Keizo
Shibuya Fuminori
Garber Wendy R.
Matsushita Electric - Industrial Co., Ltd.
Stevens Davis Miller & Mosher L.L.P.
White Mitchell
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