JPH04168887A - Image pickup device having gradation control function - Google Patents

Abstract

PURPOSE:To realize satisfactory exposure in both of a body to be photographed of low luminance and a body to be photographed of high luminance in one screen by varying the accumulatable charge quantity to plural stages by a control means in the accumulatable charge quantity in a unit light accumulation period for constituting one screen, and also, executing the control so that the timing for varying to plural stages concerned is set in accordance with image information. CONSTITUTION:In the case of NORMAL-MODE (regular photographing mode), Vsub potential (substrate voltage value) is set to a state shown by LEVEL2 so as to become the maximum value of the saturation charge quantity determined by the performance of an image pickup element 1 itself. In the case of BLC-ON-MODE(back light correction mode), Vsub is switched on the way of photographing one screen between LEVEL1 and LEVEL2. When a prescribed arithmetic processing is executed suitably in a timing control circuit 303 and a level control circuit 302, a timing for switching the LEVEL1 and LEVEL2, and a saturation charge level of LEVEL1 can be set.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to, for example, an imaging device for imaging a video signal, and particularly to an imaging device having a gradation control function suitable for use in backlight correction and the like.

[Prior Art] When photographing a subject with a video camera or the like, the gradation within one screen becomes extremely wide, such as when photographing against strong illumination.

Imaging devices that use solid-state imaging devices, such as those used in video cameras, do not have a dynamic range wide enough to convert all of these wide gradations into video signals, so they can Each of them will generate a clip. This is a phenomenon called "overexposure" or "explosion of shadows," in which the gradation of extremely bright or dark areas cannot be expressed in one screen.

In particular, when shooting against the light, the main subject is in the shadow and becomes extremely dark compared to the brightness of the background, resulting in "blackout".

As a countermeasure to this problem, a method called BLC (back light control) has been used in the past to correct the backlight by increasing the exposure amount. Hereinafter, backlight correction using a conventional method will be explained in detail with reference to the drawings.

First, the BLC in the exposure adjustment mechanism using an iris
The operation will be explained using the backlight correction circuit configuration diagram shown in FIG.

The light ray from the subject that enters from the optical system 10 is the iris 1
1, the amount of light 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,
Output as a video signal.

On the other hand, the above-mentioned 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 A via the adder 15.
A drive signal for driving the ig meter 17 is generated in response to an iris control signal from the E circuit 14, thereby realizing automatic adjustment of the amount of light by the iris.

In the above-mentioned concept of automatic adjustment of light amount, generally,
Since the amount of light is adjusted according to the integrated value of the brightness level of the video signal for one screen, the focus is only on the average brightness of the entire one screen. Therefore, in the case of backlit photography, the amount of light incident on the image sensor 12 is adjusted to match the extremely bright background, resulting in the main subject becoming extremely dark, resulting in "blackout".

In such a case, the photographer needs to turn on the BLC and issue a backlight correction instruction. In the circuit configuration diagram of FIG. 14, when the BLC switch 19 is turned on, a signal that directs the iris to the "open" side is supplied from the correction signal generator 18 to the driver 16 via the adder 15. The driver 16 drives the ig meter 17 in the direction of opening,
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 backlight correction using the conventional method, the image changes from the BLC-OFF (before backlight correction) state shown in FIG. 15(a) to the state shown in FIG. 15(b). The state changes to BLC-ON (after backlight correction). As you can see from the diagram, BLC-OFF
So, the background is good (it appears in the picture), but the main subject is not well captured due to "blackout".
With BLC-ON, the gradation of the main subject is well expressed and captured well.

[Problem to be Solved by the Invention] However, in the conventional imaging device described above, as can be seen from FIG. 15(b), by performing backlight correction, the amount of light received by the imaging element increases. Although the main subject can be photographed well, the background cannot be photographed well due to ``overexposure''.

In other words, with backlight correction using conventional methods, the problem is that due to the dynamic range limitations of the image sensor, clipping occurs at either the upper or lower level, making it impossible to capture both the main subject and the background well. there were.

Therefore, the imaging device of the present invention has been made in view of the above-mentioned problems, and its purpose is to achieve good exposure for both the main subject and the background, and to improve the appearance of the image.
An object of the present invention is to provide an imaging device having a gradation control function capable of expressing a wide range of gradations.

[Means for Solving the Problems] In order to solve the above-mentioned problems and achieve the objectives, an imaging device having a gradation control function according to the present invention receives an optical signal, performs photoelectric conversion, and accumulates information. It has a sensing means and a control means for changing the amount of charge that can be stored in the sensing means, and the amount of charge that can be stored is controlled in a plurality of stages by the control means during a unit light accumulation period for forming one screen. The timing of the change and the change to the plurality of stages are set according to the image information.

[Function] In the above configuration, the imaging device having the gradation control function of the present invention sets the amount of charge that can be accumulated to a small value during the unit light accumulation period to perform exposure suitable for a low-luminance subject. By setting a large amount of charge that can be stored and performing an exposure suitable for high-brightness subjects, and by making the timing for switching each operation be performed according to the image information, it is possible to reduce the It works to achieve good exposure for both bright and high-brightness subjects.

[Example] Hereinafter, a preferred example 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 this embodiment will be explained based on FIG.

The light rays from the subject that are incident through the optical system 10 have their light intensity adjusted by the iris 11, and form an image of the subject on the image sensor 1. The signal photoelectrically converted and outputted by the image sensor 1 is converted into a video signal by the signal processing circuit 13 and outputted to the outside as a video signal, while being supplied to the AE circuit 14 and the image analysis circuit 2. Ru.

The image analysis circuit 2 analyzes the brightness information of the video signal and supplies, for example, a brightness histogram to the controller 3. Depending on the conditions of the subject, select the mode switch 5.
By switching the NORMAL-MODE (
One of the shooting modes (normal shooting mode) and BLC-ON-MODE (backlight correction mode) is selected. The controller 3 receives a photographing mode selection signal from the mode changeover switch 5 and information such as a brightness histogram from the image analysis circuit 2, and adjusts the image sensor 1 according to the information.
The drive control is performed.

On the other hand, an iris control signal corresponding to the video signal is generated from the AE circuit 14, and output from the adder 15 to the driver 1.
Added to 6. The driver 16 receives the iris control signal, generates a drive signal for driving the ig meter 17, and automatically adjusts the amount of light using the iris.

A correction signal used in situations such as backlighting is generated by a correction signal generator 4. The BLC switch 19 is linked to the mode changeover switch 5 described above, and when the subject is backlit, that is, the mode changeover switch 5
When BLC-ON-MODE (backlight correction mode) is selected, it is in the ON state. When the BLC switch 19 is turned on, the correction signal from the correction signal generator 4 is added to the adder 15, as in the case of conventional backlight correction, and the output of the AE circuit 14 is shifted in the direction in which the iris 11 opens. .

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

Incidentally, in the case of BLC-OFF, as in the case of BLC-OFF described in the prior art, "closed black" as shown in FIG. 2(a) occurs.

Next, the image sensor 1 controlled by the controller 3 will be described below.

There are many types of image pickup devices depending on the principle of photoelectric conversion, such as CCD, MOS, and BASIS. The explanation will be given by taking COD, which is currently most commonly used for video cameras, as an example.

Among CCDs, FT type (frame transfer type)
Semiconductor device structures are classified based on differences in charge readout methods such as IT type (interline transfer type) and FIT type (frame interline transfer type), and differences in unnecessary charge processing such as VOD type.

Here, VOD is a vertical overflow drain (Ver.
tical overflow drain).

First, interline transfer type (IT type) CCD
At the same time, we will explain the high-speed shutter operation, which is one of the basic operations of COD.

FIG. 4(a) is a conceptual diagram of an interline transfer type (IT type) CCD, where 41 is a sensor section (41) that performs photoelectric conversion.
42 is a vertical transfer register, 44 is a horizontal transfer register, and 45 is an output amplifier. A- in the diagram
The cross-sectional view and potential diagram along line A' are shown in Figure 4 (b
).

In FIG. 4(b), 46 is a channel stop (CS) for pixel separation, and 47 is a readout gate (R) for transferring the charges accumulated in the sensor section 4 to the vertical transfer register 42.
OG), 48 is a substrate, and 49 is an oxide film.

The operation during high-speed shutter will be explained using FIGS. 4 and 5. FIG. 5 is a diagram for one field of a standard television signal (for example, about 1/60 second in NTSC), where φROG is a pulse applied to the readout gate 47, and when the logic level is "H" Then, the potential of the readout gate 47 decreases, and the charge in the sensor section 41 is transferred to the vertical transfer register 42. The removal pulse φSUB is a pulse applied to the substrate 48, and is “H
”, the charge accumulated in the sensor unit 41 is expressed as φSUB
Sweep out (remove) it to the outside through the terminal.

In this example, in FIG. 5, φROG is during the vertical retrace period and φSUB is during the horizontal retrace period. time to
After reading the electric charge of the sensor unit 41 at , the next period starts, but during the horizontal retrace period at time t1, φ5UB="H
”, so t.

No charge from tl remains in the sensor section 41.

Since φ5UB="L" from time t1 to tl,
Charges during this period are accumulated in the sensor section 41 and transferred to the vertical transfer register 2 at the φROG=“H” pulse at time t2. After all, the exposure time in this case is (tl - tl)
becomes.

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

Next, frame interline transfer type (FI
The operation of the T type) COD will be explained.

Figure 6 shows the frame interline transfer type (F
It is a conceptual diagram of an IT type) CCD. The difference from the interline transfer type CCD shown in FIG. 4(a) is that a storage section 63 is provided.

The number of memory cells in the memory section 63 is the same as the number of cells in the sensor section (sensing means) 61. The charge from the sensor section 61 is transferred to the vertical transfer register 62, then transferred to the storage section 63 during the vertical retrace period, then transferred to the horizontal transfer register 64 at a predetermined timing, and read out through the output amplifier 65. I'm going to be done.

Furthermore, the cross-sectional view and potential diagram along the line AA' in FIG. 6 are the same as those in FIG. The charge readout mechanism is also similar.

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 explained. The object light indicated by the upper arrow is passed through the oxide layer (SiO□) 32 and the hole storage layer for reducing dark current to the photodiode (PD) of the sensor part 33.
I will take the uke. An aluminum layer (Al) 36 is used to block unnecessary light from entering areas other than the sensor section. Due to this light blocking, the light-receiving area of the CCD is not effectively utilized, and the aperture ratio decreases. In this embodiment, a condenser lens 31 is provided for each pixel to compensate for the decrease in aperture ratio.

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

A channel stop section 39 is provided for each pixel, and the charges generated in each pixel are separated so as not to be mixed. 2 layers 34 below the sensor part 33, N-5UB layer 3
5, and unnecessary charges are processed by the substrate bias potential Vsub30 applied to both layers.

FIG. 7 is a conceptual diagram showing the charge discharging operation of the VOD type COD in terms of potential potential. The line marked rCHARGEJ is a potential potential diagram in a photocharge accumulation state, and the line rDISCHARGEJ is a potential potential diagram in an unnecessary charge discharge state. rcHARGE
In the J state, charge is accumulated in the recessed part at the top of the electrogram. rDIS
In the CHARGEJ state, ΔV sub is further applied to the substrate voltage V sub, and V sub+ΔV su
As b, the above-mentioned concave portion is eliminated and the accumulated charges are discharged to the bottom.

Incidentally, two layers are arranged below the N-type V-CCD in order to prevent unnecessary charges having a long recombination time from going around from below and mixing into the vertical transfer register 38 as noise when discharging unnecessary charges.

FIG. 8 is a timing chart showing changes in the Vsub potential during the aforementioned high-speed shutter operation. First, within one field period of a standard television signal, as shown in (c), the Vsub potential is changed in a pulse manner at a predetermined time interval, and unnecessary charges accumulated in the sensor section 61 are discharged in a predetermined manner. (Repeat until time t. Then, in the latter half of one field period (one screen), photocharge accumulation is performed for time t, and with the reading pressure pulse shown in (b), "H
The accumulated charge indicated as IGHJ is read out as image information. In this way, a high-speed shutter effect is obtained that accumulates the time t. In addition, the normal accumulation time is rNORMALJ
, the difference in charge accumulation is shown in (a) when high-speed shutter operation is defined as rHI GHJ.

Based on the basic operation of the CCD explained above, the characteristic operation of this embodiment will be described below.

It has already been mentioned that the shape of the potential recess can be varied by changing the substrate voltage ■sub in a VOD type CCD.
By not only greatly changing b but also by applying a minute potential change, it is possible to control the amount of photocharge that can be stored in the recess. FIG. 10 shows how a plurality of levels of the V sub potential are set and selectively controlled. Compared to LEVELI, LEVEL2 has a deeper potential recess, making it possible to store more charges than LEVELI.

Conceptually, by controlling the V sub potential, the ease of charge overflow is controlled, and the black circles in the diagram indicate LEVELl and LEV
It becomes a difference in EL2, and you can see that it is overflowing. The amount of storage that can be stored is the charge shown by the white circle in LEVELl, and the charge that can be stored in LE
In VEL2, the charge is the sum of the charge shown by the black circle in addition to the charge shown by the white circle. In this way, by controlling the V sub potential, it is possible to intentionally saturate the charge and make the bell (the amount of charge that can be stored) variable.

The timing chart in FIG. 11 shows an example in which the above-described V sub potential is controlled and the LEVELI state and LEVEL2 state are switched at a predetermined timing within one screen.

After discharging unnecessary charges, in the LEVELI state, photocharge accumulation is performed for a time ti to the saturation charge level of MAX■, and then in the LEVEL2 state, photocharge is accumulated for a time t2 to the saturation charge level of the MAX■ level raising state. and
Read out, where ■ is when the potential of V sub is LEVE
Saturation charge level (storable charge amount) in LI state
, and ■ is the saturation charge level in the LEVEL2 state.

According to the above operation, within the first tl time in one screen, charges exceeding the saturation charge level ■ overflow and become high as shown in the characteristics of the flat part shown by the solid line in Fig. 11(a). Clipping occurs in the brightness area. For example, if the saturation charge level ■ is set to 100%, all subject light having a brightness higher than this level will be clipped. Next, control is switched in the second half of one screen, the V sub potential is set to LEVEL2 state, the saturation charge level is also raised to ■, and photocharges are continuously accumulated for a time t2 with an accumulation margin.

For example, if tl + t2 = 1/60 seconds as an NTSC television signal, then t2 = 115
In the case of 00 seconds, tl = 1/60-11500 sen 1/6
It can be considered as 0 seconds.

This is usually 1/60 second, 1/1 second for camera exposure.
Considering that the stages of 25 seconds, 1/250 seconds, and 11500 seconds each correspond to one step (-stop) in aperture terms, for a subject that is properly exposed at TL, the height is increased by 3 stops. It is shown that appropriate exposure is given by t2 for a bright subject.

Assume here that there is a luminance difference of three stops between the main subject and the background in a backlit state.

During the 31st hour, a screen corresponding to the conventional BLC-ON state (backlight correction state) is captured, the main subject is properly exposed, and the background is clipped due to high brightness. During exposure during t2 time, BLC-OFF of the conventional example
The screen corresponding to the state (without backlight correction) is imaged, the background part that is three stops brighter than the main subject is properly exposed, and the main subject is hardly imaged. The charges accumulated during these 31 hours and the charges accumulated during time t2 are added in the photocharge accumulation section of each pixel, and as a result, a screen as shown in FIG. 2(b) is obtained.

Next, FIG. 9 shows an example of the configuration of the controller 3 explained in FIG. 1. As mentioned above, the controller 3
For example, brightness histogram information is supplied from the image analysis circuit 2, and in addition to this, the mode changeover switch 5
The drive control of the image sensor 1 is performed according to the mode state selected by.

In response to the selection signal from the mode selection switch 5, the control mode switch 301 changes the drive mode of the controller 3 between NORMAL-MODE (normal shooting mode) and BLC.
-ON-MODE (backlight correction mode).

In NORMAL-MODE (normal shooting mode), set the V sub potential (substrate voltage value) to the maximum saturation charge amount determined by the performance of the image sensor engineer itself, as shown by LEVEL 2 in Figure 10. Set to .

In the case of BLC-ON-MODE (backlight compensation mode), set V sub to LEVELI and LEVEL in Figure 1O.
Switch between 2 and 2 while shooting a single screen.

Here, the setting of the switching timing (tl and t2) and the setting of the saturation charge level of LEVELI are based on the above-mentioned image analysis results such as the brightness histogram, respectively.
This is performed by a timing control circuit 303 and a level control circuit 302. The overall photoelectric conversion characteristic, which changes depending on the level and timing settings, becomes a nonlinear characteristic as shown in FIG. 12. White arrows indicate changes due to level settings, and black arrows indicate changes due to timing settings.

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

An automatic exposure adjustment mechanism of a video camera or the like normally controls an iris or the like so that the integrated value of a luminance signal for one screen of video signals approaches a predetermined value.

Therefore, even if the brightness distribution within one screen is different, as shown in curve A and the mouth of the brightness histogram shown in Figure 13(a), if the integral value of the brightness signal for one screen is the same, The iris 11 is controlled in the same way. Curve A is a brightness occurrence frequency distribution pattern that tends to occur when shooting against the light.
The curved line is a brightness occurrence frequency distribution pattern that is likely to occur during front-light photography.

For backlight correction, mode changeover switch 5 allows BLC
-ON-MODE (backlight correction mode) is selected, the BLC switch 19 is turned on, and from curve A to (b
) changes to the curve shown in By raising the level of the imaging signal that had been showing "closed up shadows" and bringing the overall value closer to the above-mentioned predetermined value (indicated by the - dotted chain line), which is a guideline for proper exposure,
The imaging signal in the bright area causes high-intensity clipping, and the frequency of occurrence is a pattern that is biased towards the bright area.

Next, the overexposed portion of curve C is converted to a proper exposure state by the nonlinear processing shown in FIG. As a result, the shaded area in (b) is converted into a bright shaded area at the curved opening in (a), like a dead zone, and as a result, an image close to the front-lit state is obtained.

Therefore, by utilizing the characteristics of the image analysis described above and appropriately performing predetermined arithmetic processing within the timing control circuit 303 and the level control circuit 302, the timing of switching between LEVELI and LEVEL2 and the saturated charge of LEVEL1 can be determined. It is possible to set the level.

After backlight correction, brightness level discontinuity or level reversal may occur in some areas, but this does not pose a problem since the probability of occurrence is extremely low during backlighting.

As explained above, by adding a simple control processing circuit to the VOD type CCD, which is currently becoming mainstream, it is possible to effectively perform backlight correction, which is considered to be a major problem when shooting with a video camera. Become.

This makes it possible to capture images of the main subject at the same time as the background, which was blown out during conventional correction and could not be reproduced, and the effect is tremendous.

In particular, since tl and t2 are associated and controlled according to image information, if the brightness difference within one screen is small, the high brightness storage time t2 will be long (and t1 will be short (set)). However, in imaging a low-luminance part, as long as overflow does not occur, a photoelectric conversion time equivalent to that for normal imaging is ensured, so there is an effect that S/N deterioration in the low-luminance part is less likely to occur.

Further, the present invention can be applied to modifications or variations of the above embodiments without departing from the spirit thereof.

[Effects of the Invention] As described above, in the imaging device having the gradation control function of the present invention, the amount of charge that can be accumulated is set to a small value during a unit light accumulation period to provide exposure suitable for a low-luminance subject. By having the camera perform each operation and setting the amount of charge that can be stored in a large number to achieve exposure suitable for high-brightness subjects, and by having the timing for switching each operation be performed in accordance with the image information, This has the effect that good exposure can be achieved for both low-luminance objects and high-luminance objects within one screen.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the configuration of an embodiment of the present invention. Figures 2 (a) and (b) are diagrams showing an example of imaging in a backlit state. Figure 3 is the configuration of a V, OD type COD. Figure 4 is a conceptual diagram of an interline transfer type COD, Figure 5 is a diagram showing high-speed shutter operation, Figure 6 is a conceptual diagram of a frame interline transfer type COD, Figure 7 is a conceptual diagram of a frame interline transfer type COD, and Figure 7 is a conceptual diagram of an interline transfer type COD. Fig. 8 is a timing chart of the vertical removal reset operation of VOD type COD, Fig. 9 is a detailed configuration diagram of the controller, Fig. 10 is a conceptual diagram of potential level control, Fig. 11 Figure 12 shows a timing chart of potential level control, Figure 12 shows nonlinear characteristics of photoelectric conversion, Figures 13 (a) and (b) show examples of brightness histograms, and Figure 14 shows conventional FIG. 15 is a conceptual diagram showing the effect of conventional backlight correction. In the figure, 5...Mode changeover switch, 10...Optical system, 11...Iris, 15...Adder, 17...
...1g meter, 19...BLC switch. Fig. 1 BLC-OFF Fig. 2 (G) F* E Iman・) I-X Rohinako [-Iku') Fig. 2 (b) Fig. 4 (l"lOG of (1) ? 8th Figure b lN 9 Figure $ Figure 10 @ Figure 11 Seagull - Light @ Figure 13 Figure 14

Claims (2)

[Claims] (1) Unit optical storage for forming one screen, which has sensing means that receives optical signals, performs photoelectric conversion, and stores information, and control means that changes the amount of charge that can be stored by the sensing means. The storable charge amount is changed by the control means into a plurality of stages during a period, and the timing of changing into the plurality of stages is controlled to be set according to image information. An imaging device with a control function. (2) Imaging with a gradation control function according to claim 1, characterized in that the amount of charge that can be accumulated is changed such that it is smaller at the front and larger at the rear during the unit light accumulation period. Device.