JP2714248B2 - Imaging device having gradation control function - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to, for example, an image pickup device for picking up an image signal, and particularly to an image pickup device having a gradation control function suitable for backlight correction and the like.

[Prior Art] When capturing an image of a subject with a video camera or the like, the gradation in one screen becomes extremely wide, such as in the case of backlit shooting with high illuminance.

An imaging device using a solid-state imaging device or the like used in a video camera or the like does not have a dynamic range width enough to convert all of such wide gradations into a video signal, and thus the brightness level may be increased or decreased. Causes clipping. This is a phenomenon called “overexposure” or “underexposure”, in which the gradation of extremely bright or dark portions cannot be expressed in one screen.

Especially in backlit shooting, the main subject is shaded and extremely dark compared to the brightness of the background,
"Black crush" will occur.

As a countermeasure against this, conventionally, there has been used a method called BLC (Backlight Control) for performing backlight correction by increasing the exposure amount. Hereinafter, with reference to the drawings, backlight correction by a conventional method will be described in detail.

First, the BLC in the exposure adjustment mechanism using iris
Will be described with reference to the backlight correction circuit configuration diagram shown in FIG.

The light rays from the object incident from the optical system 10 are iris 11
As a result, the light amount is limited, and an image is formed on the image sensor 12. A photoelectric conversion signal corresponding to the imaging state is output from the image sensor 12, converted into a video signal by the signal processing circuit 13, and output as a video signal.

On the other hand, the above-described video signal is also supplied to the AE circuit 14, and the AE circuit 14 generates an iris control signal according to the video signal. The driver 16 receives an iris control signal from the AE circuit 14 via the adder 15, generates a drive signal for driving the ig meter 17, and realizes automatic adjustment of the light amount by the iris.

In the concept of the automatic adjustment of the light amount described above, the light amount is generally adjusted in accordance with the integral value of one screen of the luminance level of the video signal, so that attention is paid only to the average brightness of the entire screen. You are doing. for that reason,
In the case of backlight photography, for example, the amount of light incident on the image sensor 12 is adjusted in accordance with a very bright background, so that the main subject becomes extremely dark and “blackout” occurs.

In such a case, the photographer needs to turn on the BLC and issue a backlight correction instruction. In the circuit diagram of FIG. 18,
By turning on the BLC switch 19, the correction signal generator 1
From (8), the signal for turning the iris to the “open” side is
Is supplied to the driver 16. Driver 16
The meter 17 is driven in the opening direction, the amount of light incident on the image sensor 12 is increased, and backlight correction is performed.

By turning on the BLC switch 19 and performing the backlight correction according to the conventional method, the image becomes as shown in FIG. 19 (a).
The state changes from BLC-OFF (before backlight correction) shown in (b) to BLC-ON (after backlight correction) shown in (b). As can be seen from the figure, with BLC-OFF, the background is well captured, but the main subject is "black out" and not well captured. On the other hand, in the BLC-ON, the gradation of the main subject is expressed and well captured.

[Problem to be Solved by the Invention] However, in the above-described conventional imaging apparatus,
As can be seen from FIG. 19 (b), performing backlight correction increases the amount of light received by the image sensor, so that the main subject looks good, but the background causes "whiteout", and I can't copy it.

That is, in the backlight correction by the conventional method, clipping occurs at one of the upper and lower levels due to the restriction of the dynamic range of the image sensor, and both the main subject and the background cannot be imaged well. There was.

Therefore, the imaging apparatus of the present invention has been made in view of the above-described problems, and has as its object to realize good exposure to both the main subject and the background, and to have an apparently wide gradation. An object of the present invention is to provide an imaging device having a tone control function capable of expression.

[Means for Solving the Problems] In order to solve the above-mentioned problems and achieve the objects, an imaging apparatus having a gradation control function according to the present invention employs a sensing device which receives an optical signal, performs photoelectric conversion, and stores information. Means for controlling the amount of storable charge of the sensing means, the control means changing the amount of storable charge during a unit light accumulation period for forming one screen. An imaging apparatus having a function, comprising an amplification factor changing unit that changes an amplification factor of a color signal in association with the change in the storable charge amount.

Further, in the image pickup apparatus having a gradation control function according to the present invention, the control unit may set the storable charge amount to:
In the unit light accumulation period, the amount of light is changed so that the light amount is small at the front part and is large at the rear part.

[Operation] In the above configuration, the image pickup apparatus having the gradation control function of the present invention is configured such that the color signal of high luminance overflows beyond the storable charge amount due to the change in the storable charge amount. By lowering the amplification factor, it works to prevent the generation of a false color signal.

Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of one embodiment. In the figure, the lower half from the center is the same as the circuit configuration of the conventional example shown in FIG. The concept of the operation of the present embodiment will be described with reference to FIG.

Light rays from the subject incident from the optical system 10 are
The light amount is adjusted by 11 to form a subject image on the image sensor 1. The signal photoelectrically converted and output by the image sensor 1 is converted into a video signal by the signal processing circuit 13 and output to the outside as a video signal, while being supplied to the AE circuit 14 and the image analysis circuit 2. You.

The image analysis circuit 2 analyzes the luminance information and the like of the video signal, and supplies, for example, a luminance histogram and the like to the controller 3. By switching the mode switching switch 5 in accordance with the condition of the subject, one of the shooting modes, NORMAL-MODE (normal shooting mode) and BLC-ON-MODE (backlight correction mode), is selected. The controller 3 receives a photographing mode selection signal from the mode switching switch 5 and information such as a luminance histogram from the image analysis circuit 2,
The drive control of the image sensor 1 is performed according to the information.

On the other hand, an iris control signal corresponding to the video signal is generated from the AE circuit 14 and is applied to the driver 16 via the adder 15. The driver 16 receives the iris control signal, generates a drive signal for driving the ig meter 17, and performs automatic light amount adjustment by the iris.

A correction signal used in a state such as backlight is generated by the correction signal generator 4. The BLC switch 19 is interlocked with the mode switching switch 5 described above. When the subject is in a backlight state, that is, when the mode switching switch 5
ON when C-ON-MODE (backlight compensation mode) is selected
State. When the BLC switch 19 is turned on, the correction signal from the correction signal generator 4 is applied to the adder 15 as in the case of the conventional backlight correction, and the AE circuit is opened in the direction in which the iris 11 opens.
Shift the output of 14.

Therefore, in the present embodiment, in the backlight state, the iris control similar to the conventional backlight correction is performed, and the amount of light incident on the image sensor 1 is increased (the BLC switch 19 is ON).
The controller 3 applies a predetermined drive control to the image pickup device 1 so as to obtain a good exposure state for both the main subject and the background as shown in FIG. 2B.

By the way, in the case of BLC-OFF, it was explained in the prior art.
As in the case of the BLC-OFF, the “blackout” shown in FIG. 2A has occurred.

Next, the imaging device 1 controlled by the controller 3 will be described below.

There are many types of imaging devices based on the principle of photoelectric conversion, such as CCD, MOS, and BASIS. The explanation is given with the example of a CCD that is currently most commonly used for video cameras.

Among CCDs, there are differences in charge readout methods such as FT type (frame transfer type), IT type (interline transfer type), IT type (interline transfer type), and FIT type (frame interline transfer type). , VOD type, etc. are classified according to the difference in the unnecessary charge processing in the semiconductor device structure. Here, VOD is an abbreviation for Vertical Overflow Drain.

First, a basic structure of an interline transfer type (IT type) CCD will be described, and a high-speed shutter operation which is one of the basic operations of the CCD will be described.

Fig. 4 (a) shows an interline transfer type (IT
(Type) Conceptual diagram of CCD, 41 is a sensor unit (sensing means) for performing photoelectric conversion, 42 is a vertical transfer register, 44 is a horizontal transfer register, and 45 is an output amplifier. FIG. 4B is a cross-sectional view and a potential diagram along the line AA ′ in the figure.

In FIG. 4 (b), reference numeral 46 denotes a channel stop (CS) for separating pixels, 47 denotes a readout gate (ROG) for transferring the electric charge accumulated in the sensor section 41 to the vertical transfer register 42, 48
Denotes a substrate, and 49 denotes an oxide film.

The operation at the time of the high-speed shutter will be described with reference to FIGS. FIG. 5 is a diagram of one field (for example, about 1/60 second in NTSC) of a standard television signal,
φROG is a pulse applied to the readout gate 47,
When the logic level is “H”, the potential of the readout gate 47 decreases, and the electric charge of the sensor unit 41 is transferred to the vertical transfer register 42. The removal pulse φSUB is a pulse applied to the substrate 48, and sweeps out (removes) the electric charge accumulated in the sensor unit 41 at the time of “H” through the φSUB terminal.

In this example, in FIG. 5, φROG is during the vertical retrace interval, and φSUB is during the horizontal retrace interval. After reading the charge of the sensor unit 41 at time t0, the next period starts,
During the horizontal flyback period at time t1, φSUB = “H”, so that the charge from t0 to t1 does not remain in the sensor unit 41. Since φSUB = “L” during the period from time t1 to t2, the electric charge during this period is accumulated in the sensor unit 41, and φROG = at time t2.
The signal is transferred to the vertical transfer register 2 by the “H” pulse. After all,
The exposure time in this case is (t2−t1).

In this way, a high-speed shutter operation of the IT type CCD is realized.

Next, the frame interline transfer type (FIT
Type) The operation of the CCD will be described. FIG. 6 is a conceptual diagram of a frame interline transfer type (FIT type) CCD. The difference from the interline transfer CCD shown in FIG. 4A is that a storage unit 63 is provided. The number of storage cells in the storage unit 63 is determined by the sensor unit (sensing means) 61.
Is the same as the number of cells. After the charge from the sensor unit 61 is transferred to the vertical transfer register 62, the charge is stored in the storage unit during the vertical retrace period.
The data is transferred to the horizontal transfer register 64 at a predetermined timing, and is read out through the output amplifier 65.

The cross-sectional view and the potential diagram along the line AA 'in FIG. 6 are the same as those in FIG. 4B, and the mechanism for discharging the electric charge and reading out the electric charge from the sensor unit 61 to the vertical transfer register 62. Is the same.

 The above is the operation of the FIT type CCD.

Next, the operation of the VOD type CCD image sensor shown in FIG. 3 will be described. The subject light indicated by the upper arrow is received by the photodiode (PD) of the sensor unit 33 via the oxide layer (SiO ₂ ) 32 and the hole accumulation layer for dark current reduction. The aluminum layer (A
1) Light is shielded at 36. Due to this shading, the light receiving area as the CCD is not effectively utilized, and the aperture ratio is reduced. In this embodiment, a condenser lens 31 for compensating for a decrease in the aperture ratio is provided for each pixel.

The charge generated by the photoelectric conversion is V-CCD for vertical transfer.
(Vertical transfer register) 38, sequentially transferred on a two-dimensional plane, and output as a voltage value from the read amplifier.

A channel stop portion 39 is provided for each pixel, and is separated so that charges generated in each pixel are not mixed. A P layer 34 and an N-SUB layer 35 are provided below the sensor unit 33, and a substrate bias potential Vs applied to both layers.
Unnecessary charges are processed by ub30.

FIG. 7 is a conceptual diagram showing a charge discharging operation of a VOD type CCD by a potential potential. The line indicated by “CHARGE” is a potential potential diagram in the state of accumulation of photocharge, and “DISCHA
A line indicated by “RGE” is a potential potential diagram in an unnecessary charge discharging state. In the "CHARGE" state, charges are accumulated in the concave portion above the electrogram. In the “DISCHARGE” state, ΔVsub is further applied to the substrate voltage Vsub to make Vsub + ΔVsub, so that the above-mentioned concave portion disappears and the accumulated electric charge is discharged to the lower part.

Incidentally, the P layer is disposed below the N-type V-CCD in order to prevent unnecessary charges having a long recombination time from escaping from below and entering the vertical transfer register 38 as noise when unnecessary charges are discharged.

FIG. 8 is a timing chart showing changes in the Vsub potential during the high-speed shutter operation described above. First,
Within one field period of the standard television signal,
As shown in (c), the Vsub potential is changed in a pulsed manner at predetermined time intervals, and the discharge of the unnecessary charges accumulated in the sensor unit 61 is repeated until a predetermined time. Thereafter, in the latter half of one field period (one screen), photocharge accumulation is performed for the time t, and the accumulated charge indicated as “HIGH” is read out as image information by the readout pulse shown in FIG. In this way, a high-speed shutter effect for accumulating the time t is obtained. The difference in the state of charge accumulation is shown in (a), with "NORMAL" during normal accumulation and "HIGH" during high-speed shutter operation.

The characteristic operation of this embodiment will be described below based on the basic operation of the CCD described above.

It has already been stated that the shape of the concave portion of the potential potential can be varied by changing the substrate voltage Vsub in the VOD type CCD.However, not only can the ΔVsub be changed in a pulsed manner but also by applying a small potential change The amount of photocharges that can be stored in the recesses can be controlled. Tenth
The figure shows how a plurality of levels of the Vsub potential are set and selectively controlled. The concave portion of the potential potential is set deeper in LEVEL2 than in LEVEL1, so that more charges can be stored than in LEVEL1.

Conceptually, the ease of charge overflow is controlled by controlling the Vsub potential, and the black circle in the figure becomes the difference between LEVEL1 and LEVEL2, indicating that overflow is occurring. The storable amount is LE
In VEL1, the charge is indicated by a white circle, and in LEVEL2, the sum of the charge indicated by a white circle and the charge indicated by a black circle is used. Thus, Vsub
By controlling the potential, the saturation charge level (the amount of storable charge) can be intentionally made variable.

An example in which the above-described Vsub potential is controlled and the state of LEVEL1 and the state of LEVEL2 are switched at a predetermined timing within one screen is shown in a timing chart of FIG.

After discharging unnecessary charges, in the LEVEL1 state, photocharge accumulation is performed for the time t1 up to the saturation charge level of MAX, and then in the LEVEL2 state, photocharge accumulation is performed for the time t2 up to the saturation charge level of the MAX level raised state. ,read out. here,
Is the saturated charge level (the amount of storable charge) when the potential value of Vsub is in the LEVEL1 state, and is the saturated charge level when the Vsub is in the LEVEL2 state.

According to the above-described operation, the charge exceeding the saturation charge level overflows during the first time t1 in one screen, and the charge exceeds the saturation charge level.
11 Like the characteristics of the flat part indicated by the solid line in FIG.
Clipping occurs in the high luminance area. For example, assuming that the saturation charge level is 100%, all object light having a luminance higher than this is clipped. Next, the control is switched in the latter half of one screen, the Vsub potential is set to the LEVEL2 state, the saturation charge level is also raised, and the accumulation of the photocharge is continued for the time t2 with the accumulation margin.

Here, for example, as an NTSC television signal,
If t1 + t2 = 1/60 second, t1 = 1/6 if t2 = 1/500 second
It can be regarded as 0−1 / 500 ≒ 1/60 seconds.

This is usually 1/60 second, 1/125 second, 1
Considering that the steps of / 250 sec. And 1/500 sec. Correspond to one step (one stop) in terms of aperture, respectively, the subject with the proper exposure at t1 and the subject with high brightness by 3 stops Indicates that the proper exposure is given by t2.

Here, it is assumed that there is a luminance difference of three stops between the main subject and the background in the backlight state. During time t1, a screen corresponding to the BLC-ON state (backlight correction state) of the conventional example is captured, the main subject is properly exposed, and the background portion is clipped due to high luminance. In the exposure during t2 hours, an image corresponding to the BLC-OFF state (state in which backlight is not corrected) of the conventional example is imaged, and the background portion brighter by three stops than the main subject is properly exposed, and almost all main subjects are not exposed. No image is captured. The charge stored during the time t1 and the charge stored during the time t2 are added in the photocharge storage unit of each pixel, and as a result, a screen as shown in FIG. 2B is obtained.

Next, FIG. 9 shows a configuration example of the controller 3 described in FIG. As described above, the controller 3
Is supplied with, for example, luminance histogram information from the image analysis circuit 2, and in addition, controls the driving of the image sensor 1 in accordance with the mode state selected by the mode switching switch 5.

In response to the selection signal of the mode switching switch 5, the control mode switch 301 changes the drive mode of the controller 3 to N.
Set to one of ORMAL-MODE (normal shooting mode) and BLC-ON-MODE (backlight correction mode).

In the case of NORMAL-MODE (normal shooting mode), the LE potential in FIG. 10 is set so that the Vsub potential (substrate voltage value) becomes the maximum value of the saturation charge level determined by the performance of the image sensor 1 itself.
Set to the state shown in VEL2.

In the case of BLC-ON-MODE (backlight compensation mode), Vsub
Is switched between LEVEL1 and LEVEL2 in FIG. 10 during shooting of one screen. Here, the setting of the switching timing (t1 and t2) and the setting of the saturated charge level of LEVEL1 are based on the image analysis results such as the above-described luminance histogram, respectively.
This is performed by the timing control circuit 303 and the level control circuit 302. As shown in FIG. 12, the overall photoelectric conversion characteristic that changes according to the setting of the level and the timing is a non-linear characteristic. The state changed by the level setting is indicated by a white arrow, and the state changed by the timing setting is indicated by a black arrow.

Next, the relationship between the backlight correction effect and the image analysis according to the present embodiment will be described using FIG. 13 showing an example of a luminance histogram.

An automatic exposure adjusting mechanism of a video camera or the like usually controls an iris or the like so that an integrated value of a luminance signal for one screen of a video signal approaches a predetermined value. Therefore, as shown in curves a and b of the luminance histogram shown in FIG.
Even when the brightness distribution in the screen is different, if the integrated value of the luminance signal for one screen is the same, the iris 11 is controlled in the same manner. Curve A is a luminance occurrence frequency distribution pattern that tends to occur during backlit shooting, and curve B is a luminance occurrence frequency distribution pattern that tends to occur during forward light shooting.

The BLC-O is switched by the mode switch for backlight compensation.
When the N-MODE (backlight correction mode) is selected, the BLC switch 19 is turned on, and the curve A changes to a curve C as shown in FIG. Since the imaging signal that has been “blackened” has been raised to a value close to the above-mentioned predetermined value (indicated by a dashed line), which is a guideline for proper exposure, the imaging signal in the bright part causes high-brightness clipping. The frequency is a pattern biased toward the bright part.

Next, the overexposed portion of the curve C is converted to an appropriate exposure state by the non-linear processing of FIG. As a result, the shaded portion in (b) is converted into a shaded portion of the bright portion in the curve (b) as in the dead zone, and as a result, an image close to a normal light state is obtained.

Therefore, utilizing the features of image analysis as described above,
By appropriately performing predetermined arithmetic processing in the timing control circuit 303 and the level control circuit 302, it is possible to set the timing of switching between LEVEL1 and LEVEL2 and the saturation charge level of LEVEL1.

After the backlight correction, discontinuity of the luminance level or level inversion may possibly occur partially, but this does not pose a problem at the time of backlight because the probability of occurrence is extremely low.

Next, “high-luminance color signal suppression” that suppresses the occurrence of general false colors will be described. FIG. 14 is a configuration diagram of a color video camera having a single-panel type color filter configuration.

The subject light incident from the imaging optical system 141 removes infrared light, which is an unnecessary wavelength region, by an infrared cut filter 142, and a color filter 143 assigns each pixel of the image sensor 144 a color image for color imaging. Distribution is performed, and an imaging signal including color information is output from the imaging element 144. By the distribution amplifier 145, the information on the brightness is transmitted to the luminance signal processing circuit 1.
The color information is distributed to the color signal processing circuit 147, and so-called Y and C signals are output to 46.

Here, as shown in FIG. 15 (a), one of the primary colors (or complementary colors, etc.) distributed by the color filter has a high luminance, so that the saturation charge level (accumulated charge) of the pixel shown in FIG. When saturation exceeds the possible charge amount and other colors do not saturate, the color balance is lost and a color different from the actual subject light is output. Therefore, the gain control signal generation circuit 148 controls the gain variable amplifier 149 to reduce the amplification factor of a color signal having a luminance level exceeding the threshold value TH2 shown in FIG. A control signal for realizing the gain characteristic as shown in b) is generated. Then,
The luminance signal and the chrominance signal are mixed by the mixer 140 and output as a composite TV signal.

In the present embodiment, since a plurality of saturation charge levels of pixels are set within one screen, the above-described point of reducing the color signal amplification factor must be provided at a plurality of locations correspondingly.

FIG. 16 shows a configuration diagram of only a portion related to a color signal. As in the case of FIG. 14, each signal of Y and C is output, and Y is input to the threshold value generation circuit 160. The threshold generation circuit 160 generates a plurality of thresholds corresponding to the saturation charge level of the pixel, and inputs the threshold to the gain control signal generation circuit 161. These saturation charge levels (,) and thresholds (TH1, TH2, TH
17 (a) and 17 (b) show the relationship between 3) and the relationship between the threshold and the gain characteristic.

The level that saturates earlier in time is of the window comparator type, so two thresholds (TH1 and TH2) are set for one level, and the gain decreases only during that time. Therefore, generation of a false color signal at an intermediate luminance level on the imaging screen can be suppressed. Also,
The occurrence of a false color signal at high luminance can be suppressed by the threshold value TH3 as in the case of FIG.

As explained above, the VOD type C that is currently becoming mainstream
By adding a simple control processing circuit to a CD, it is possible to effectively perform backlight correction, which is a major problem when shooting a video camera.

The background portion, which flew white during conventional correction and could not be reproduced, can be imaged simultaneously with the main subject, and this effect is enormous.

In particular, in the present invention, by performing the false color signal suppression processing at a plurality of points, an unnatural false color clinging around the main subject at the time of backlight shooting can be removed, and an effect of shining white can be obtained. However, there is also an effect that a wide gradation feeling can be reproduced.

In addition, the present invention can be applied to a modification or a modification of the above embodiment without departing from the gist thereof.

[Effects of the Invention] As described above, the imaging apparatus having the gradation control function of the present invention can be used when the high-luminance color signal overflows beyond the storable charge amount due to the change in the storable charge amount. ,
By lowering the amplification factor of the color signal, it works to prevent the generation of a false color signal.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of the present invention, FIGS. 2 (a) and 2 (b) are diagrams showing an example of imaging in a backlight state, and FIG. 3 is a configuration example of a VOD type CCD. Fig. 4 is a conceptual diagram of an interline transfer type CCD, Fig. 5 is a diagram showing a high-speed shutter operation, Fig. 6 is a conceptual diagram of a frame interline transfer type CCD, and Fig. 7 is a VOD. FIG. 8 is a timing chart of the vertical reset operation of the VOD type CCD, FIG. 9 is a detailed configuration diagram of the controller, FIG. 10 is a conceptual diagram of the potential level control, FIG. FIG. 12 shows a timing chart of the potential level control, FIG. 12 shows a non-linear characteristic diagram of photoelectric conversion, FIGS. 13 (a) and 13 (b) show examples of a luminance histogram, and FIG. Block diagram of a color video camera with a plate-type color filter FIG. 15 is a diagram showing the saturation state of the color signal, FIG. 16 is a diagram showing only the configuration relating to the color signal, FIG. 17 is a diagram showing the relationship between the threshold value and the saturation charge level, and FIG. FIG. 19 is a conceptual diagram showing the effect of a conventional backlight correction circuit. In the figure, 5 ... mode switching switch, 10 ... optical system, 11 ...
Iris, 15 adder, 17 ig meter, 19 BLC switch.

Claims (2)

(57) [Claims] 1. Sensing means for receiving an optical signal and performing photoelectric conversion to accumulate information, and control means for changing the storable charge amount of the sensing means, wherein one screen is constituted by the control means. An image pickup apparatus having a gradation control function of changing the amount of storable electric charge during a unit light accumulation period for performing the amplification, wherein an amplification rate of a color signal is changed in relation to the change of the amount of storable electric charge. An imaging apparatus having a gradation control function, characterized by having a rate changing means. 2. The control means according to claim 1, wherein said storable charge amount is:
2. The image pickup apparatus having a gradation control function according to claim 1, wherein the amount of light is changed so as to decrease at the front part and increase at the rear part during the unit light accumulation period.