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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a television camera using two image pickup
devices with different sensitivity and, more particularly, is directed to
a television camera suitable for imaging or image-picking up an object
having a large contrast.
2. Description of the Related Arts
One of the basic functions of the television camera is to reproduce an
object faithfully which has been picked up as an image. The object imaged
by the television camera, however, is of various types, and has various
brightness (luminances). That is, the object is composed of components
with complicated brightness and components which vary in brightness. The
conventional television cameras have the technique developed to faithfully
reproduce an object image even when the object is composed of components
with complicated brightness and different brightness.
A block diagram of a television camera according to the related art is
shown in FIG. 1. The television camera shown in FIG. 1 includes an image
pickup lens system 1, an image pickup device such as CCD 3, a
pre-amplifier 5, a video signal processing circuit 15, a video signal
output terminal 16, a lens iris control circuit 6, an iris structure 17 in
the image pickup lens system 1 and a drive circuit 18 for the image pickup
device 3. FIG. 2 is a photoelectric conversion characteristic diagram
showing the characteristic of the signal output versus the incident light
quantity of the image pickup device 3.
The operation of this television camera will be explained. The light
entering from an object not shown is applied to the image pickup device 3
through the image pickup lens system 1, and after photoelectric conversion
at the image pickup device 3, provides a video signal which is applied to
the pre-amplifier 5.
The video signal is amplified up to a predetermined level at the
pre-amplifier 5 and applied to the video signal processing circuit 15 and
the lens iris control circuit 6.
The video signal applied to the video signal processing circuit 15 is
subjected to signal processing such as the addition of a sync signal from
a sync signal generator not shown, and is output from the video signal
output terminal 16 as a composite video signal.
The lens iris control circuit 6, on the other hand, detects the average
value or the peak value of the video signal input from the pre-amplifier
5, and compares the detected the average or peak value value with a
reference value to extract the change in the average value or the change
in the peak value, as the case may be. In accordance with the change in
the average value or the peak value of the video signal thus extracted,
the aperture state of the iris structure 17 in the lens system 1 is
controlled in such a manner as to maintain a predetermined value of the
output video signal level of the pre-amplifier 5, thereby controlling the
quantity of the light entering the image pickup device 3.
Explanation will be made about the photoelectric conversion characteristic
diagram showing the characteristic of the signal output versus the
incident light quantity of the image pickup device shown in FIG. 2. As
shown in FIG. 2, the image pickup device outputs a signal which increases
substantially in proportion to the incident light quantity when the
incident light quantity is below a certain level Is. When the incident
light quantity is equal to or above the certain level Is, however, the
signal output reaches a saturated level where the signal output does not
substantially increase with the increase in the incident light quantity.
This level Is at which the output signal of the image pickup device
saturates will hereinafter be called as a saturation or saturated incident
light quantity.
When the quantity of light entering the image pickup device from an object
exceeds the saturated incident light quantity, the output signal level of
the image pickup device is saturated at an image portion of the object an
incident light quantity thereof exceeds the saturated incident light
quantity. When this signal output is viewed on a video monitor, the image
portion of the object whose incident light quantity exceeds the saturated
quantity assumes a monotonically white image lacking variations and cannot
be viewed as an image proportional to the incident light quantity.
The television camera, which uses an image pickup device having the
characteristics described above, includes a lens iris control circuit for
controlling the aperture state of the lens iris structure in such a manner
that the incident light quantity entering the image pickup device from a
bright object to be imaged becomes proper value. In this manner, the
quantity of the light entering the image pickup device is maintained below
a saturated incident light quantity level to prevent a bright portion of
the object from forming a monotonically white image without any
variations.
In the conventional television cameras, however, in spite of the fact that
the aperture state of the iris structure in the lens system is controlled
to control the incident light quantity entering the image pickup device,
the incident light entering the image pickup device cannot be controlled
so as to be not larger than the saturated incident light quantity Is in
the case where an object has a very bright area when the object has a
large contrast or the object is imaged under reverse light. As a result,
the signal output is saturated, and the object as viewed on an image
monitor is monotonically white without any variations. In addition, the
lens iris control circuit, which is influenced by the video signal in the
high brightness area, cannot properly control the incident light quantity
of the object to be imaged, with the result that the object to be imaged
is blackened.
In other words, when an attempt is made to obtain an image of an object of
high illuminance at a proper level, an object of low illuminance becomes
dark in the phenomenon of what is called the solid blackening. An attempt
to produce an image of an object of low luminosity at the proper level, on
the other hand, results in a high-illuminance object being whitened
monotonically in a phenomenon of what is called the solid whitening.
FIGS. 3 and 4a are diagrams showing other example configuration of the
television camera suggested considering the disadvantages of the prior art
described above. The television camera configured this way is disclosed,
for example, in JP-A-4-354277 (reference (1)).
In FIG. 3, reference numeral 1 designates a lens, 3 a CCD, 5 a
pre-amplifier for amplifying the output of the CCD 3, 24 a CCD control
circuit for controlling the storage time, i.e., the shutter speed of the
CCD 3, 7 an analog-to-digital (hereinafter referred to as A/D) converter,
26 a high-speed image memory for writing and reading the digital video
signal obtained upon the high shutter speed operation of the CCD 3 with a
shorter charge storage time, and 27 a low-speed image memory for writing
and reading the digital video signal obtained upon a low shutter speed
operation of the CCD 3 with a longer charge storage time. Reference
numeral 28 designates a high brightness segmented area detection circuit
for receiving the digital video signal upon the low shutter speed
operation to divided an image of the digital video signal corresponding to
one screen of an object image picked-up by the CCD 3 into plural segmented
areas and for detecting a high-brightness segmented area such as a
back-light area among the plurality of segmented areas, 29 a signal
switcher for selecting a digital video signal upon the low or high shutter
speed operation, 14 a digital-to-analog (hereinafter referred to as D/A)
converter, 22 an automatic gain control (AGC) amplifier, 15 a signal
processing circuit for processing the video signal into a composite video
signal, 16 a composite video signal output terminal, 6 an iris control
circuit, and 17 an iris.
In this configuration, the CCD 3 is controlled by the CCD control circuit
24 to be alternately operated in high- and low-speed shutter modes for
each field. As a result, the CCD 3 performs the high-speed operation in
even-number fields to output a video signal not saturated even for the
high-brightness portion of the object to the high-speed image memory 26.
In odd-number fields, on the other hand, the CCD 3 performs the low-speed
shutter operation, so that the video signal saturated for the
high-brightness portion of the object is output to the low-speed image
memory 27. The high-brightness segmented area detection circuit 28 divides
an image of the digital video signal corresponding to one screen into a
number N to the horizontal scanning direction and a number M to the
vertical scanning direction (both N and M are integers of 2 or more). In
this way, each image corresponding to one screen is composed of areas
segmented into a number N.times.M of segmented areas. With each segmented
area thus segmented, the average value of the digital video signal (the
output of the low-speed image memory 27) upon the low-speed shutter
operation is determined thereby to detect whether each segmented area is a
high-brightness segmented area or not. In the case where it is decided
that a segmented area is a high-brightness segmented area, the signal
switcher 29 selects the output of the high-speed image memory 26 (the
digital video signal for high-speed shutter operation) for the entire area
of the high-brightness segmented area. In the case where decision is that
a segmented area is not a high-brightness segmented area, on the other
hand, the signal switcher 29 selects the output of the low-speed image
memory 27 (the digital video signal for low-speed shutter operation).
FIG. 4B is a model diagram showing an image of an object corresponding to
one screen 80 imaged by the CCD 3. As shown in FIG. 4B, an image of the
screen 80 is segmented into nine areas 80A to 80I, for example. An image
of the object, for example, is assumed to include a high-brightness area
81 and a low-brightness area 82. As a result, in this case, since the
segmented area 80E is entirely covered by the high-brightness area 81 and
almost of the segmented area 80E is covered by the high-brightness area
81, the segmented areas 80E and 80F, for example, are determined to be a
high-brightness segmented area, so that the switcher 29 selects the
digital video signal for the high-speed shutter operation supplied from
the image memory 26 as the digital video signal for these segmented areas
80E and 80F. For the other segmented areas, decision is made to be not a
high-brightness area and hence the digital video signal of the low-speed
shutter operation supplied from the image memory 27 is selected as the
digital video signal for these other segmented areas.
FIG. 4A is a circuit diagram showing the essential parts of a modification
of the circuit of FIG. 3. The essential parts shown in FIG. 4 include a
high-speed shutter operation control circuit 25A for controlling the
control circuit 24 which in turn controls the shutter speed of the CCD 3
upon a high-speed shutter operation, and a low-speed shutter operation
control circuit 25B for controlling the control circuit 24 to control the
shutter speed of the CCD 3 upon a low-speed shutter operation.
The high-speed shutter operation control circuit 25A operates in a manner
that an image of the output video signal corresponding to one screen from
the pre-amplifier 5 is divided into three sections to the horizontal and
vertical directions to obtaine nine segmented areas, for example. The
average value of the digital video signal is determined for each segmented
area, and the shutter speed of the CCD upon the high-speed shutter
operation is controlled in such a manner that the average value of the
digital video signal for a segmented area with the highest average value
attains 80% of the full scale of the output voltage of the CCD, for
example. The low-speed shutter control circuit 25B, on the other hand,
operates in a manner that an image of the output video signal
corresponding to one screen from the pre-amplifier 5 is also divided into
nine segmented areas and the shutter speed of the CCD upon the low-speed
shutter speed operation is controlled in a manner that the average value
of the digital video signal represents 30% of the full scale of the output
voltage of the CCD, for example, for the segmented area with the lowest
average value of the digital video signal.
In this way, the proper exposure characteristics of the CCD are obtained
even at the time of imaging of an object having a large contrast or in
back light, thereby preventing the white solidification or the black
solidification of the video signal.
More specifically, in the configuration of FIG. 4A, as shown in FIG. 4B, an
image of the output video signal corresponding to one screen 80 of the
object is segmented into nine segmented areas 80A to 80I, and it is
decided whether each segmented area thus segmented is a high brightness
segmented area or not on the basis of the average value of the video
signal for the segmented area. The shutter speed, i.e., the charge storage
time, of the CCD upon the high-speed shutter operation, is thus controlled
in such a manner that the video signal for the segmented area with the
highest average value assumes the proper level. Also, the shutter speed
upon the low-speed shutter operation is controlled in such a way that the
video signal for the segmented area with the lowest average value
represents the proper level.
In FIG. 4B, the segmented areas 80E and 80F are assumed to be a high
brightness segmented area, and the other segmented areas to be not high
brightness segmented areas. Further the segmented area 80E has the highest
average value of the video signal and the segmented area 80G has the
lowest average value thereof.
The configuration of still another example of the television camera
suggested taking the above-mentioned disadvantages of the prior art into
consideration is shown in FIG. 5. The television camera configured this
way is disclosed, for example, in JP-A-5-64070 (reference (2)).
The configuration shown in FIG. 5 includes an imaging lens 1, a iris 17, a
beam splitter 2 providing a spectrometer, two CCDs 3, 4, pre-amplifiers
5a, 5b, a white level compression pre-knee circuit 30, an automatic gain
control amplifier (AGC) 31, an exposure control circuit 32, an amplifier
39, delay circuits 36, 37, a low-pass filter (LPF) 33, a comparator 34, a
pulse-width detection circuit 35, a multiplexer 38 and a signal processing
circuit 15. The optical image of the object is divided in its light
intensity or quantity by the beam splitter 2 to obtain two optical images,
which are then focused on the two CCDs 3, 4. The beam splitter 2 splits
the light quantity of the optical image of the object from the lens 1 into
the ratio of five to one and applies the two optical images to the CCDs 3
and 4 respectively. The output video signal (the output of the amplifier
5a) of the CCD 3, to which the optical image with a larger quantity of
incident light is applicated, is used for exposure control by the exposure
control circuit 32 on the one hand and subjected to the knee-processing by
the pre-knee circuit 30 on the other hand. Further, the output video
signal of the amplifier 5a is applied to the comparator 34 through the LPF
to thereby be compared with a threshold voltage V.sub.TH. When the level
of the output video signal is higher than the threshold voltage V.sub.TH,
a signal is applied from the pulse-width detection circuit 35 to the
multiplexer 38, which in turn selects and delivers the output of the delay
circuit 37 (the output video signal of the CCD 4 having a smaller light
quantity) instead of the output of the delay circuit 36. In this
configuration, in the case where the brightness peak value of the video
signal output from the CCD 4 is 40% or more of the white level (in other
words, in the case where the brightness of the bright area is at least
twice that of the dark area), the fitting synthesis of the image is
selected, while the knee processing is selected when the brightness peak
value is less than 40% of the white level. In the fitting synthesis
processing, the video signal for a relatively bright area of an image
represented by the video signal obtained from the CCD 3 is replaced by the
video signal for a corresponding area of the image represented by the
video signal obtained from the CCD 4. In the knee processing, the output
video signal of the CCD 3 is compressed in its high-brightness area.
In this way, the video signal with proper image is obtained regardless of
the magnitude of the brightness difference between the bright and dark
areas.
SUMMARY OF THE INVENTION
In reference (1) described above, the single CCD 3 is alternately switched
between low-speed and high-speed shutter operations for every field. For
example, the high-speed shutter operation is performed for even-numbered
fields, and the low-speed shutter operation for odd-numbered fields. The
video signal upon the high-speed and low-speed shutter operations are
stored in a high-speed image memory 26 and a low-speed image memory 27,
respectively. As a result, only the video signal for even-numbered fields
is stored in the high-speed image memory 26, and only the video signal for
odd-numbered fields in the low-speed image memory 27. Also, a
high-brightness segmented area detection circuit 28 determines, as to each
of the segmented areas 80A to 80I, whether the segmented area is a high
brightness segmented area or not based on the average level of the digital
video signal (the output of the low-speed image memory 27) derived upon
low-speed shutter operation for the segmented area. When a segmented area
is decided to be a high-brightness segmented area, the signal switcher 29
selects the output of the high-speed image memory 26 (the digital video
signal for high-speed shutter operation) for the entire area of the
segmented area. In the case where decision is that the segmented area is
not a high brightness segmented area, on the other hand, the output of the
low-speed image memory 27 (the digital video signal for low-speed shutter
operation) is selected for the entire area of the segmented area by the
signal switcher 29. As a result, in FIG. 4B, for example, an image is
formed only with the video signal of an even-numbered field in the
segmented areas 80E and 80F determined to be of high-brightness segmented
areas, while an image is formed only with the video signal of an
odd-numbered field for the remaining segmented areas determined to be not
a high brightness segmented area. In this way, while an image of each
segmented area is intrinsically to be formed with a video signal of both
odd-number and even-numbered fields, an image of each segmented area is
formed only with one of these two types of fields. Therefore, the vertical
resolution of the image obtained is one half that of the image which
otherwise formed by both the odd and even-numbered fields.
As described above, the high-brightness segmented area detection circuit 28
detects whether each segmented area is a high brightness segmented area or
not by determining the average level of the digital video signal for the
segmented area derived upon low-speed shutter operation (the output of the
low-speed image memory 27). As a consequence, in FIG. 4B, with respect to
the segmented areas 80E and 80F which are determined to be high brightness
segmented areas and for which the output of the high-speed image memory 26
is selected, the high-brightness area or portion 81 within these segmented
areas is represented by an image of a video signal of the proper level not
saturated. The low-brightness area or portion 82 in the segmented area
80F, however, is solidly blackened since a video signal of the area 82
becomes lower in level due to the selection of the output of the image
memory 26. In similar fashion, for each of the other segmented areas which
is determined to be not a high brightness segmented area and for which the
output of the low-speed image memory 27 is selected, on the other hand,
the low-brightness portion 82 of, for example, the segmented area 80B is
produced by a video signal of the proper level. The high-brightness
portion 81 of the segmented area 80B, however, represents a video signal
of a saturated level and is solidly whitened. In other words, for the
entire area of each segmented area, selection is uniquely made between the
output of the high-speed image memory 26 which will be a proper lever for
a high-brightness portion of the video signal and the output of the
low-speed image memory 27 which will be a proper level for a
low-brightness portion of the video signal. Accordingly, the segmented
areas where a low-brightness portion and a high-brightness portion coexist
are solidly blackened or whitened respectively like the prior art.
This problem may be eliminated by increasing the number of segmented areas
of an image corresponding to one screen. In this case, however, when a
segmented area includes portion of very high brightness (or very low
brightness), even if the position is very small, the image for the small
portion of the segmented area is likely to be solidly whitened (or
blackened). In this way, the image for an object of a high contrast having
a portion of a very high (or low) brightness becomes solidly whitened or
blackened.
With the above-mentioned reference (2), on the other hand, the output video
signals of the two CCDs, in which two optical images whose light quantity
is split in the ratio of 5 to 1 by a beam splitter are entered, are
selectively switched and output. However, this split ratio of the light
quantity is constant, and therefore brightness level of an image of the
video signal of the object does not become proper one in the entire area
thereof when the object has a very large brightness difference, thereby
appearing a solid whitened or blackened portion in the image of the
object.
Accordingly, an object of the present invention is to provide a television
camera which is capable of obviating the above-mentioned problems of the
related arts.
Another object of the present invention is to provide a television camera
in which even when an object having has a portion of very high brightness
in the case of imaging the object having a large contrast or imaging in
back light, a proper video signal high in resolution and in color
reproducibility is produced without being solidly whitened or blackened
due to the ability attached to arbitrarily enlarge the dynamic range
against the incident light quantity limited by the characteristics of the
image pickup devices.
According to one aspect of the invention, there is provided a television
camera which includes a lens system having a iris structure for receiving
the incident light from an object, a spectrometer for dispersing an
optical image of the object from the lens system and producing first and
second optical images, the spectrometer splitting the light quantity of
the original optical image of the object in a predetermined ratio and
assigning the respective portions of the light quantity between the first
and second optical images, first and second image pickup devices for
forming first and second optical images from the spectrometer and
converting the first and second optical images thus formed into a video
signal, first and second amplifiers for amplifying the video signals from
the first and second image pickup devices, a high-brightness signal (area)
detection circuit supplied with the video signal from the first amplifier
for producing a detection signal indicating a high-brightness signal
(area) upon detection that the input video signal is a high-brightness
signal (area) higher than a predetermined level lower than the saturated
output signal level of the first image pickup device, a video signal
switcher for selecting and producing one of the video signals from the
first and second amplifiers in accordance with the detection signal, a
signal processing circuit for converting the video signal from the video
signal switcher into a composite video signal and producing the composite
video signal as an output composite video signal of the television camera,
a iris control circuit for controlling the iris structure of the lens
system in accordance with the video signal from the first amplifier, a
high-brightness video signal extraction circuit for gating the video
signal from the second amplifier in accordance with the detection signal
and extracting a video signal (or a portion of a video signal)
corresponding to the high-brightness signal (area), and an image pickup
device control circuit for controlling the charge storage time of the
second image pickup device in accordance with the level of the video
signal extracted by the high-brightness video signal extraction circuit.
Preferably, the video signal switcher selects and produces the video signal
from the second amplifier during the period when the detection signal
indicates that the video signal from the first amplifier is a
high-brightness signal (area).
In this configuration, in response to a fact that the video signal from the
first image pickup device represents a high-brightness area, the output
video signal of the second image pickup device corresponding to the
high-brightness area of the output video signal of the first image pickup
device is extracted, and the charges storage time (that is, sensitivity)
of the second image pickup device is controlled in accordance with the
brightness of the video signal thus extracted. Further, the video signal
from the second image pickup device is selected as a television camera
output during the period when the video signal represents a
high-brightness area. The second image pickup device, therefore, can
output a video signal corresponding to the incident light quantity without
being saturated even for a high-brightness area of an image of an object,
with the result that the solid whitening or solid blackening can be
prevented even for an object having a very large contrast, while at the
same time achieving good color reproducibility.
More specifically, this invention is different from the reference (1)
described above in which a single CCD is operated alternatly between
low-speed and high-speed shutter operations for every field, an image of
the video signal corresponding to one screen is split into a plurality of
segmented areas, the average value of the video signal upon low-speed
shutter operation is used to decide whether or not each segmented area is
a high brightness area, and on the basis of the decision, the image of the
entire area of each segmented area is uniquely composed of the video
signal upon the high-speed shutter operation (even-numbered field) or the
video signal upon the low-speed shutter operation (odd-numbered field). In
other words, according to the present invention, one of two video signals
from two image pickup devices, each producing a video signal of odd- and
even-numbered fields, is selected in accordance with a detection signal
indicating that the video signal from a image pickup device represents a
high-brightness video signal area. As a result, video signals of the
entire image area corresponding to one screen are composed of video
signals of even- and odd-numbered fields, and therefore the vertical
resolution of the image is not deteriorated.
Also, this invention is different from the reference (1) described above in
which an image corresponding to one screen is split into a plurality of
segmented areas, the average value of the video signal for low-speed
shutter operation for each segmented area is used to decide whether the
segmented area is a high brightness segmented area or not, and the image
of the entire area of each segmented area is uniquely composed of only one
of the video signals derived upon high-speed shutter operation or
low-speed shutter operation. In other words, according to this invention,
one of the video signals from two image pickup devices is selected in
accordance with a detection signal indicating that the video signal of an
image pickup device represents a high-brightness video signal area. As a
result, the video signal with a proper brightness level is attained for
all the image areas correspond to one screen in accordance with the level
of the video signal from the image pickup devices. Therefore, no area is
solidly blackened or whitened unlike in reference (1), and good color
reproducibility is obtained.
Preferably, the spectrometer assigns the light quantity of the optical
image of an object from the lens system to the second and first optical
images in a predetermined ratio ranging from 1:2 to 1:9.
As a consequence, the second image pickup device is capable of producing a
non-saturated video signal corresponding to the brightness of a
higher-brightness portion of an object, thereby broadening the dynamic
range of the television camera.
Preferably, the image pickup device control circuit controls the charge
storage time of the second image pickup device to reduce the sensitivity
thereof to a greater degree the higher the level of the extracted video
signal.
As a consequence, the second image pickup device is capable of producing a
non-saturated video signal corresponding to a higher-brightness portion of
the object, thereby broadening the dynamic range of the television camera
accordingly.
According to another aspect of the invention, there is provided a
television camera comprising a lens system having a iris structure for
receiving the incident light from an object, a spectrometer for dispersing
the optical image of the object from the lens system and producing first
and second optical images, the spectrometer dividing the light quantity of
the optical image of the object from the lens system in a predetermined
ratio and assigning the respective portions of the light quantity between
the first and second optical images, first and second image pickup devices
for forming first and second optical images from the spectrometer and
converting the first and second optical images thus formed into a video
signal, first and second amplifiers for amplifying the video signals from
the first and second image pickup devices, a high-brightness signal (area)
detection circuit supplied with a video signal from the first amplifier
and producing a detection signal indicating a high-brightness signal
(area) upon detection that the input video signal is a high-brightness
signal (area) of more than a predetermined level lower than the saturated
output signal level of the first image pickup device, first and second
multipliers for multiplying the video signal from the first and second
amplifiers by a predetermined coefficient respectively and outputting the
product thereof, an adder for adding the video signals from the first and
second multipliers to each other and producing the sum thereof, a signal
processing circuit for converting the video signal from the adder into a
composite video signal and producing the composite video signal as an
output composite video signal of the television camera, a iris control
circuit for controlling the iris structure of the lens system in
accordance with the video signal from the first amplifier, a
high-brightness video signal extraction circuit for gating the video
signal from the second amplifier in accordance with the detection signal
and extracting a video signal (or a portion of a video signal)
corresponding to the high-brightness signal (area), and an image pickup
device control circuit for controlling the charge storage time of the
second image pickup device in accordance with the level of the video
signal extracted from the high-brightness video signal extraction circuit.
Preferably, the coefficients of the first and second multipliers assume a
positive value less than 1.0 respectively.
In the above-mentioned configuration, the charge storage time of the second
image pickup device is controlled in accordance with the brightness of a
high-brightness area of the extracted video signal, and therefore an image
of good color reproducibility is obtained without any blackening or
whitening even for an object having a high contrast. Further, the output
video signals of the first a | | |
