JP2545120B2 - Imaging device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image pickup apparatus, and more particularly to an image pickup apparatus suitable for using a solid-state image pickup element.

(Background of the Invention) It is known that a solid-state image sensor has a very narrow dynamic range (latitude) with respect to luminance as compared with a photographic film. For this reason, when a landscape with a large difference in brightness level, for example, outdoor shooting on a sunny day is performed, the high-luminance portion is often blown white, and conversely, the low-luminance portion is often blackened.

In a normal solid-state image sensor (CCD, MOS, etc.), the amount of incident light and the generated charge have a linear relationship (γ = 1). This is shown in FIG.

In this case, the signal has a narrow dynamic range and is saturated immediately in a high-luminance portion. For this reason, the high-brightness portion of the output image is blown out in white, and the contrast in the subject cannot be discriminated.

As a solution to such a drawback, it is necessary to give the solid-state image sensor a knee characteristic in “The Knee characteristic control of the CCD image sensor” in pp. 43-44 of the 1978 National Conference of the Television Society. Proposed. The applicant of the present application also proposes the knee characteristics of CCD in Japanese Patent Application No. 62-87393.

The knee characteristic is to widen the dynamic range with respect to the brightness by giving the knee characteristic to the relationship between the incident light amount and the generated charge as shown in FIG.

(Problems to be Solved by the Invention) Although the incident dynamic range is widened by the knee characteristics as described above, the accumulated charge amount of the light-receiving portion or the moving voltage between the light-receiving portion and the transfer portion is accurate and the variation between pixels is large. , And it is extremely difficult to control at high speed.

The present invention has been made in view of the above-mentioned problems, and an object thereof is an imaging device capable of sufficiently utilizing the dynamic range of a solid-state imaging device and obtaining photoelectric conversion output in a wide luminance range. Is to realize.

(Means for Solving the Problems) According to the present invention for solving the above problems, a light receiving unit for converting incident light from the outside into electric charges, a vertical transfer unit for vertically transferring electric charges from the light receiving unit, and a vertical transfer unit are provided. A solid-state image sensor having a horizontal transfer unit for horizontally transferring the electric charges of the solid-state image sensor and a solid-state image sensor driving unit for supplying a driving pulse to the solid-state image sensor, and the solid-state image sensor driving unit performs one vertical scanning. During the period, a pulse for reading out signals at different time intervals is output, the charges generated in the light receiving section are divided into a plurality of different time intervals and moved to the vertical transfer section, and the charges are transferred on the vertical transfer section. It is characterized in that it is configured to add and read.

(Operation) When the electric charge is read from the solid-state image sensor, the electric charge accumulated in the light receiving portion of each pixel is moved to the vertical transfer portion at a plurality of different time intervals, added on the vertical transfer portion, and then read.

(Example) With reference to drawings, the Example of this invention is described in detail below.

FIG. 1 is a configuration diagram showing a configuration of a main part of one embodiment of the present invention. In the figure, reference numeral 1 is a solid-state image sensor, and an example of an interline CCD will be described as an example.
Reference numeral 2 is a light receiving portion that generates electric charges according to the amount of received light, 3 is a vertical transfer portion that vertically transfers the electric charges generated in the light receiving portion, and 4 is a horizontal transfer portion that horizontally transfers the electric charges from the vertical transfer portion. is there. Here, 3 × 4 pixels are shown.

Further, the light receiving portion 2 and the vertical transfer portion 3 of the solid-state image pickup device 1
The potential of is as shown in FIG. 2 or FIG. In other words, it is assumed that the transfer unit can store five times the charge of the light receiving unit.

FIG. 4 is a time chart showing drive pulses when driving the solid-state imaging device 1. The solid-state image sensor drive circuit that generates this drive pulse can be realized by making some modifications to a known circuit, and detailed circuit configuration will be omitted.

The operation of the embodiment of the present invention will be described below with reference to FIGS.

Assuming that A in FIG. 4 is an exposure period and B is a reading period, the charges of the pixels corresponding to .phi.V1 to .phi.V4 are transferred to the vertical transfer section at the beginning of A, and then the charges are discarded by the reverse transfer. After that, the charges are transferred from the light receiving section 2 to the vertical transfer section 3 in five times (C1, C2, C3, C4, C5), and the addition on the vertical transfer section 3 is read in the period B. By changing the timing of the charge transfer five times, the relationship between the light amount and the voltage of the output signal changes.

Next, the relationship between the amount of light and the voltage of the output signal will be described.

FIG. 5 is a characteristic diagram showing the relationship between the charge accumulation time and the accumulated charge amount. At t ₁ , ..., T ₅ in this figure, charges are transferred from the light receiving portion to the vertical transfer portion. Further, Q ₀ is the saturated charge amount of the light receiving section 2. Here, a solid line, a broken line, etc. show how electric charges are accumulated at each luminance. The luminance ratio here is the solid line
It doubles in the order of A, broken line B, .... Further, here, the case of C1 = C2 = C3 = C4 = 4 × C5 is shown.

The output voltage of each pixel is proportional to the sum of the charge amounts of the pixels at the times of t ₁ , t ₂ , t ₃ , t ₄ , and t ₅ , and this state is shown in FIG. This figure shows the relationship between the luminance L and the output voltage V. Here, L _A is the luminance in the characteristic A of FIG. 5, L _B is the luminance in the characteristic B of FIG. 5, L _C is the luminance in the characteristic C of FIG. 5, and L _D is the luminance in the characteristic D of FIG. Is. The characteristics are the same as those when the conventional exposure is performed,
The characteristic when the exposure (C1 = C2 = C3 = C4 = 4 × C5) shown in the figure is the characteristic when the exposure of C1 = C2 = C3 = C4 = 8 × C5 is performed.

As is clear from and in this figure, by performing the exposure of FIG. 5, the output is compressed in the high luminance region (L _{A to} L _C ), which is about three times as high as that of the normal exposure. A dynamic range can be obtained.

When C1 = C2 = C3 = C4 = 8 × C5 is exposed, L _A ~
The output is compressed in the high luminance range of L _D , and the dynamic range becomes about 7 times.

By the way, the ratio of C1 to C5 is important in the above operation, and the absolute value is not important. For example, if the F value of the lens is
Although the shape of the characteristic in FIG. 6 does not change between 2.8 and when the F value is 4, the brightness is doubled. Further, if the ratio of C1 to C5 is left unchanged and the time is halved, the shape of the characteristic of FIG. 6 does not change and the luminance doubles.

As an example, t ₁ −t ₀ = t ₂ −t ₁ = t ₃ −t ₂ = t ₄ −t ₃ = 1/25
0 seconds, described as t ₅ -t ₄ = 1/1000 seconds. The pixel of brightness L _C is
_{t 0 ~t 1, t 1 ~t} 2, t 2 ~t 3, t 3 1/1000 of the beginning of each period of ~t ₄
The signal saturates in seconds (Fig. 5C) and the remaining time is meaningless as the exposure time. Since the saturation does not occur in the last 1/1000 second (t _{4 to} t ₅ ), it is meaningful as the exposure time. Therefore, the actual exposure time for this pixel is 5 × 1/1000.
= 1/200 second. Similarly, the pixel of the luminance _{L B t 0 ~t 1, t} 1 ~
t _2, t ₂ ~t _3, the signal at 1/500 second at the beginning of each period of t ₃ ~t ₄ saturates (FIG. 5 B), is meaningless as the rest of the time exposure time. Since the saturation does not occur in the last 1/1000 second (t _{4 to} t ₅ ), it is meaningful as the exposure time. Therefore, the actual exposure time for this pixel is 4 × 1/500 + 1/1000 = 1/1
It will be 11 seconds. Similarly, the pixels of the brightness L _A are t ₀ to t ₁ , t _{1 to} t.
_{2, t 2 ~t 3, t} 3 signal at 1/250 sec at the beginning of each period of ~t ₄ is saturated (FIG. 5 A), is meaningless as the rest of the time exposure time. Since the saturation does not occur in the last 1/1000 second (t _{4 to} t ₅ ), it is meaningful as the exposure time. Therefore, the actual exposure time for this pixel is 4 x 1/250 + 1/1000 = 1/59
Seconds. Pixels darker than the brightness L _A are all 1/59 seconds because all exposure times are valid. That is, the dynamic range is widened by increasing the exposure time of dark pixels and shortening the exposure time of bright pixels.

FIG. 7 is a characteristic diagram showing a case of C1 = C2 = 4 × C3 = 4 × C4 = 16 × C5. In this figure, the brightness is doubled up to A, B, ..., E. FIG. 8 shows the relationship between the luminance and the output voltage when such exposure is performed. FIG. 8 shows the characteristics when ordinary exposure is performed. With this characteristic, good output can be obtained in the dark area,
It becomes saturated when the brightness increases a little. Therefore, the dynamic range is extremely narrow. It is also a characteristic when normal exposure is performed by adjusting the aperture or shutter speed. If this is done, the dynamic range will be expanded to some extent, but the output will be small and the S / N will be poor in dark areas. Therefore, also in this case, a satisfactory result cannot be obtained. The method shown in is the method described in FIG. With this characteristic, a sufficient output can be obtained in the dark area, and it is not saturated even in the high brightness area. Then, the inclination of the intermediate portion is between the high luminance portion and the low luminance portion, and an image that is easy to see can be obtained.

FIG. 9 is a characteristic diagram showing a case of C1 = 4 × C2 = 4 × C3 = 4 × C4 = 16 × C5. In this figure, the brightness is doubled up to A, B, ..., E. FIG. 10 shows the relationship between the luminance and the output voltage when such exposure is performed. The brightness and output characteristics are the same as in the case of FIG.
Although it is bent at some points, the bending point and the inclination are slightly different.

In this way, by changing the combination of C1 to C5, the luminance-output voltage characteristic can be freely designed. Also, in the above example, there are three types of exposure time,
It is also possible to use four or five types. In addition, the exposure,
The number of times of charge transfer is not limited to 5, but can be increased or decreased depending on the ratio of the capacities of the light receiving section and the transfer section.

Note that while normal CCDs can only shoot fields, CCDs that have separate vertical transfer units for odd fields and even fields can also shoot frames.

The above description is for the case of shooting a still image, but in the case of the CCD or the FIT-CCD having the above-mentioned odd field and even field which have separate vertical transfer portions, it can be applied to a moving image.

FIG. 11 is a block diagram showing the structure of FIT-CCD. In this figure, the same parts as those in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted. The difference from the CCD shown in FIG. 1 is that it has a memory unit 5 for storing charges corresponding to each pixel.

FIG. 12 shows pulses for driving the FIT-CCD in the field storage mode. That is, charge transfer is performed a plurality of times between the light receiving unit and the vertical transfer unit, and after the exposure is completed, the charges on the vertical transfer unit are transferred to the memory unit at high speed and read from the memory unit at the video rate. While the memory unit is reading, the light receiving unit is exposing the next field. Here, in order to output as a video signal, C1 + C2
+ C3 + C4 + C5 ≤ 1/60 second condition is added, but other than that, it is the same as for still images. In this case, normally C1 + C2
+ C3 + C4 + C5 = 1/60 seconds, but in the case of a CCD that can sweep out the signal charge in the middle (for example, a CCD that can discard the charge to the overflow drain (OFD)), C1 + C2 + C3 + C4 + C5 <1/60 seconds You can also do it.

Further, FIG. 13 shows pulses for driving the FIT-CCD in the frame accumulation mode. Here, charges are first swept out by reverse transfer. Then, charges are transferred between the light receiving portion and the vertical transfer portion a plurality of times, and after the exposure is completed, the charges on the vertical transfer portion are transferred to the memory portion at high speed and read from the memory portion at the video rate. While the memory unit is reading, the light receiving unit is exposing the next field. It is the same as performing exposure of C1 + C2 + C3 + C4 + C5 ≦ 1/60 seconds for each field. Further, in FIT-CCD capable of discarding charges in OFD, a pulse to OFCG is added instead of the pulse train for reverse transfer.

As another example of the frame accumulation mode, the one shown in FIG. 15 can be considered. In this case, the reverse transfer is not swept out, and the signal of the other field cannot be moved while the signal of the field on one side is being exposed, accumulated, and moved, so that the exposure and accumulation are continued during that period. . Therefore, in this case, C1 ≧ 1/60 seconds, C1 + C2 + C3 + C4
The condition of + C5 = 1/30 seconds is added.

As another example of the frame accumulation mode, the one shown in FIG. 16 is also conceivable. This is an example of using a CCD that can discard charges in OFD. Since the charge can be dumped into OFD, the condition as in the case of FIG. 15 does not apply.

If control is possible such that only one side of the field is discarded when the charges are discarded in OFD, the pulse is driven as shown in FIG. 17, and the condition is only C1 + C2 + C3 + C4 + C5 ≦ 1/30 seconds. However, if the signals of the fields on both sides are discarded when the charges are discarded in the OFD, it is necessary to consider the effect. That is, the case shown in FIG. 16 is good, but in the case shown in FIG. 18, the exposure / accumulation time of C3 is C3 ′.

With a CCD that has separate vertical transfer sections for odd fields and even fields, each field can be moved between the light-receiving section and the vertical transfer section at any timing, so under the condition of C1 + C2 + C3 + C4 + C5 = 1/30 seconds You can freely set C1 to C5. C1 + C2 + C3 + C4 + C5 <1/30
Whether or not it can be done in seconds depends on whether the charge can be discarded in OFD.

Since the characteristic is determined by the ratio of C1 to C5, the influential luminance range can be shifted by adjusting the total exposure time while keeping the ratio fixed.

Also, the optimum luminance-output voltage characteristic curve may differ depending on the content of the image, but even in such a case, it can be easily dealt with by changing the pulse timing.

(Effects of the Invention) As described in detail above, in the present invention, the charges generated in the light receiving section are read out in a plurality of times according to the capacities of the light receiving section and the vertical transfer section, and are added on the vertical transfer section. I read it. For this reason, the output of the solid-state imaging device is not saturated even in the high brightness region, and the dynamic range is expanded. Therefore, it is possible to realize an image pickup apparatus which can fully utilize the dynamic range of the solid-state image pickup element and can obtain photoelectric conversion output in a wide luminance range.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a configuration of an embodiment of the present invention, FIGS. 2 and 3 are explanatory diagrams showing a potential of a solid-state image pickup device, FIG. 4 is a time chart showing a driving pulse, and FIGS. FIG. 10 is a characteristic diagram of the solid-state imaging device of the present invention, and FIG. 11 is
The configuration diagram showing the configuration of the FIT-CCD, FIGS. 12 to 18 are time charts showing other examples of drive pulses, FIGS. 19 and 20.
The figure is a characteristic diagram of a conventional solid-state imaging device. 1 ... Solid-state image sensor, 2 ... Light receiving unit 3 ... Vertical transfer unit, 4 ... Horizontal transfer unit

Claims (1)

(57) [Claims] 1. A light receiving portion for converting incident light from the outside into an electric charge, a vertical transfer portion for vertically transferring the electric charge from the light receiving portion, and a horizontal transfer portion for horizontally transferring the electric charge from the vertical transfer portion. A solid-state image sensor, and solid-state image sensor driving means for supplying a driving pulse to the solid-state image sensor. The solid-state image sensor driving means is for reading signals at a plurality of different time intervals within one vertical scanning period. Is configured to output the pulse of, the charge generated in the light receiving unit is moved to the vertical transfer unit at a plurality of different time intervals, and the charges are added and read on the vertical transfer unit. Imaging device.