JP2008263339A - Imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a stable imaging signal when the signal level of a light shield area abnormally varies, particularly, even if a subject with high brightness is imaged in the light shield area. <P>SOLUTION: When an image (c) of the high-brightness subject is in the light shield area (a), the output Sout of an imaging element 201 outputs a high level d2 even for the light shield area (a) and a detecting circuit 801 of a detection and decision circuit 705 detects the output level of the imaging element in synchronism with detection timing pulses J1, but the detection result has a higher level than usual. A decision circuit 802 holds a clamp permission pulse CPENB for inhibiting clamp level update of a clamp circuit 202 at a Low level, and a clamp timing pulse CPOB1 is limited by an AND gate 706 (held at a Low level). Consequently, no clamp pulse is supplied to the clamp circuit, which does not update a clamp level (black reference) and continues to perform clamping operation on the basis of a previous clamp level, for example, d1. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an imaging apparatus that stabilizes the output of an imaging element by providing a light shielding region in a part of an imaging surface of the imaging element.

In general, an imaging apparatus includes a pixel (optical black, hereinafter referred to as OB) that is shielded from light on a part of the imaging surface against a phenomenon in which the level of an imaging signal fluctuates according to changes in the surrounding environment such as temperature. Stable by performing a so-called OB clamp in which a light-shielded area is provided and the output from the other non-light-shielded pixels (light-receiving area) is a relative level signal with the output of the OB pixel as a black level reference. An imaging signal is obtained (see, for example, Patent Document 1 below).
JP 2004-15097 A (Abstract)

  This prior art will be described with reference to FIGS. FIG. 7 shows an image pickup surface of an image pickup element 201 (see FIG. 8) in which a plurality of photoelectric conversion elements (pixels) are two-dimensionally arranged. Here, 1920 in the horizontal (column) direction and in the vertical (line) direction. 1090 pixels are arranged, and further, among the 1090 lines in the vertical readout direction, the upper 10 lines are shielded by, for example, an aluminum thin film so that incident light does not enter the photoelectric conversion element (light shielding). Area a, light receiving area b).

  FIG. 8 shows an example of an OB clamp circuit, an image sensor 201 having an image pickup surface (light-blocking area a and light-receiving area b) as shown in FIG. 1, a clamp circuit 202 that clamps the output Sout of the image sensor 201, A signal processing circuit 203 that performs desired processing as an imaging device such as white balance, gamma, and aperture control on the output CPout of the clamp circuit 202, and a horizontal (line) synchronization signal for reading the imaging signal Sout to the imaging device 201 The timing control circuit 204 outputs HD and a vertical (frame) synchronization signal VD, and outputs a clamp pulse CPOB1 to the clamp circuit 202 during a period in which pixels in the light shielding region a are read from the image sensor 201.

  The clamping operation will be described with reference to FIG. The image sensor 201 is arranged on the imaging surface (the light shielding area a and the light receiving area b) in synchronization with the vertical synchronization signal VD, the horizontal synchronization signal HD, and a pixel clock (not shown) supplied from the timing control circuit 204. Pixel signals Sout are sequentially output. As shown in FIG. 7, the upper 10 lines in the vertical readout direction are the light shielding region a. Therefore, in the first 10 line period (TOB) from the arrival of the vertical synchronization signal VD as shown in FIG. The level is a sufficiently dark (black) value d1. This value d1 is the signal level of the light-shielded pixel (black) under the current use environment conditions. Thereafter, in the 1080 line period (TAV), the light receiving region b is read, and during this period, a signal (peak value: d1 + A) corresponding to the optical image incident on the imaging surface is output. Thereafter, an electrical reference signal level (for example, 0 V) is output during a period of 35 lines (TBLK), for example, until the next vertical synchronization signal VD comes in.

  The clamp circuit 202 outputs, as a clamp output CPout, a signal obtained by clamping the image sensor output Sout input during the clamp pulse CPOB1 output from the timing control circuit 204 as a reference voltage. In FIG. 9, the clamp level of one frame before, for example, 0V is clamped for the first to fourth lines, and the clamp circuit output CPout outputs the same signal level d1 as the input signal, but the clamp pulse CPOB1 Is output to the fifth and sixth lines, the signal CPout = Sout−d1 (peak value: A) obtained by subtracting the clamp level d1 from the input signal Sout is output from the clamp circuit 202 for the seventh and subsequent lines. As a result, the signal CPout of the light receiving region b output from the clamp circuit 202 on and after the 11th line becomes a signal based on the pixel output d1 of the light shielding region a. The output CPout of the clamp circuit 202 becomes a relative signal (A) of the pixel in the light receiving region b with reference to the output of the pixel in the light shielding region a, even if the signal level (d1) of the signal fluctuates. Is obtained.

  For the sake of simplicity, FIG. 9 shows a case where the output CPout of the clamp circuit 202 is continuously (analogously) changed during the incoming period (the fifth and sixth lines) of the clamp pulse CPOB1. For example, as shown in FIG. 10, the clamp circuit 202 may be configured by a digital element. In FIG. 10, the average value calculation circuit 401 adds the image sensor output Sout during the period when the clamp pulse CPOB1 is input, and outputs the average value OBAVE. The subtraction circuit 402 subtracts the clamp average value OBAVE from the image sensor output Sout to obtain a clamp output CPout. The addition for calculating the average value OBAVE is reset for each frame by a frame synchronization signal VD (not shown).

  By the way, the conventional OB clamp circuit is premised on that the pixel output of the light shielding region a varies depending on environmental conditions such as temperature, but is a black signal that is always shielded against incident light. However, in reality, when extremely strong light is incident, leakage light due to the structure of the light shielding layer, stray light to the gap between the light shielding layer and the photoelectric conversion element, or the like is incident on the pixel (photoelectric conversion element) in the light shielding region a. There is a case where the signal level of the light shielding region a varies without being completely shielded from light.

  This case will be described with reference to FIGS. When an image c of an extremely bright subject is projected in the light shielding area a as shown in FIG. 11, the image sensor output Sout of the light shielding area a is originally as shown in FIG. The value d2 is higher than the black signal level d1. After that, the output Sout to be read out is the line where the signal level is saturated because the image c of the high brightness subject is reflected in the upper line which is the light receiving area (TAV), but the other light receiving areas b Then, the original image sensor output (peak value B) is output. More specifically, the peak value B includes the black level fluctuation due to the environmental conditions, similar to d1 in FIG. Since the clamp circuit 202 clamps the input signal Sout with this d2 as the black level, the portion of the light receiving area b where the image c of the high-brightness object is not shown is more abnormal than the original image sensor output Sout as shown in FIG. Output a dark signal CPout (peak value C).

  In view of the above-described problems of the conventional technology, the present invention provides an imaging device capable of obtaining a stable imaging signal even when a high-luminance subject is imaged particularly in a light shielding region when the signal level of the light shielding region fluctuates abnormally. The purpose is to do.

In order to achieve the above-described object, the present invention outputs an image sensor in which a light receiving area and a light shielding area are provided on an imaging surface on which a plurality of photoelectric conversion elements are arranged, and an output signal level of the image sensor in the light shielding area. A clamp circuit that clamps to a reference signal based on the black level of the imaging signal and updates it for each frame, and outputs a signal obtained by subtracting the clamped output signal level from the output signal level of the imaging element in the light receiving area as an imaging signal In an imaging device comprising:
Detecting means for detecting the output signal level of the image sensor in the light shielding region for each frame;
Means for outputting a prohibition signal for prohibiting updating of the reference signal of the clamp circuit for each frame when the output signal level of the image sensor in the light shielding region detected by the detection means is greater than a predetermined level;
Means for prohibiting updating of the reference signal of the clamp circuit for each frame by the prohibition signal;
It is characterized by having.

  According to the present invention, the update of the reference signal of the clamp circuit is prohibited when the signal level of the light shielding area is higher than a predetermined level. Therefore, when the signal level of the light shielding area fluctuates abnormally, a subject with high brightness particularly in the light shielding area. A stable imaging signal can be obtained even if an image is captured.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an OB clamp circuit according to a first embodiment of an imaging apparatus according to the present invention. In FIG. 1, the same components as those in FIG. 8 used in the description of the prior art are denoted by the same reference numerals. This OB clamp circuit includes an image sensor 201 that is the same as the prior art, a clamp circuit 202 that is the same as the prior art, a signal processing circuit 203 that is the same as the prior art, a horizontal sync signal HD, a vertical sync signal VD, and a clamp pulse that are the same as the prior art. In addition to CPOB1, a timing control circuit 704 that outputs a detection timing pulse J1 to the detection determination circuit 705, and an output level of the image sensor 201 is detected only during a period indicated by the detection timing pulse J1, and this detection result is determined in accordance with a predetermined predetermined value. The detection determination circuit 705 that outputs a clamp operation permission pulse CPENB to an AND gate (inhibiting means) 706 from the value, and the logical product CPOB2 of the clamp pulse CPOB1 and the clamp permission pulse CPENB are output to the clamp circuit 202, and the clamp circuit A prohibiting the update of the reference signal 202 Consisting of D gate 706 Metropolitan.

  FIG. 2 shows the internal configuration of the detection determination circuit 705 in detail. The detection circuit 801 is a detection circuit that detects the level of the output signal Sout of the image sensor 201 only during a period when the detection timing pulse J1 is input and outputs a detection result OBD1, for example, a period during which the detection timing pulse J1 is input. A circuit for obtaining the product sum value of the input signal Sout, or a circuit for detecting the peak of the signal after the input signal Sout has passed through the low-pass filter. The determination circuit 802 compares the output OBD1 (detection result) of the detection circuit 801 with a predetermined level TH (d1 <TH <d2), and when the detection result OBD1 is smaller than the predetermined level TH, the clamp circuit 202 In order to permit the level update, for example, the Hi level signal CPENB is output to the AND gate 706. Conversely, when the detection result OBD1 is larger than the predetermined level TH, the level update of the clamp circuit 202 is prohibited in order to prohibit the level update, for example. It is a circuit that outputs a signal CPENB.

  The operation will be described with reference to FIG. First, as in FIG. 9, in the case of an image with normal luminance, the output of the image sensor 201 in the light shielding region a is d1. The timing control circuit 704 outputs the clamp pulse CPOB1 to the 5th and 6th lines as in the prior art, but outputs the detection timing pulse J1 to the detection determination circuit 705 on the 4th line prior to this. The detection determination circuit 705 detects the image pickup element output Sout on the fourth line to which the detection timing pulse J1 is input, and compares the detection result with a predetermined value TH. In FIG. 3, the element output Sout is normal. Therefore, the clamp permission pulse CPENB becomes Hi level, and the clamp pulse CPOB1 is output as it is to the clamp circuit 202 without being limited by the AND gate 706, and the clamp operation is performed as in the conventional case.

  On the other hand, when the image c of the high brightness subject is in the light shielding area a, the output Sout of the image sensor 201 outputs a high level d2 even in the light shielding area a as shown in FIG. The detection circuit 801 in the detection determination circuit 705 detects the output level of the image sensor 201 in synchronization with the detection timing pulse J1 that enters the fourth line. At this time, the detection result OBD1 is at a higher level than usual. Therefore, the determination circuit 802 sets the clamp permission pulse CPENB to the low level so as to prohibit the clamp circuit 202 from updating the clamp level. As a result, the clamp timing pulse CPOB1 is limited (low level) by the AND gate 706, and the clamp circuit CPOB1 is not supplied to the clamp circuit 202. Therefore, the clamp circuit 202 does not update the clamp level (black reference) and before that. The clamping operation is continued with reference to a clamping level of, for example, d1. Therefore, as shown in FIG. 4, the clamp circuit output CPout is an output obtained by clamping the image sensor output Sout at the level d1 when the output of the light shielding region a is normal.

  After that, when the image c of the high-brightness subject deviates from the light-shielding region a and the signal level of the light-shielding region a returns to normal, the clamping operation similar to that shown in FIG. Even if c is applied to or removed from the light-shielding region a, the signal level of the light-receiving region b does not fluctuate and a stable imaging signal can be obtained.

  In this embodiment, the detection determination circuit 705 has only one detection circuit (detection circuit 801). However, as shown in FIG. 5, a plurality of detection circuits are used (detection circuits 1101, 1102, 1103), and the output Sout. May be detected by a plurality of detection timing pulses J1, J2, and J3, and the determination circuit 1104 may make a determination from the plurality of detection results OBD1, OBD2, and OBD3. Further, the determination method may use an average value of a plurality of detection results OBD1, OBD2, and OBD3, or may use a maximum value of the plurality of detection results OBD1, OBD2, and OBD3. In this embodiment, the output Sout detection line is the fourth line, and the clamp level update line is the fifth and sixth lines. Instead, as shown in FIG. The fourth, sixth, and eighth lines may be used, the clamp level update line may be the fifth, sixth, and seventh lines, or the output Sout detection line and the clamp level update line may be the same line.

  In the present embodiment, the light shielding region a is provided on the read start side in the vertical read direction. However, as is generally the case with conventional OB clamp circuits, it may be provided on the read end side. You may provide in the start side of a direction, or the end side like the technique disclosed by patent document 1. FIG.

1 is a block diagram illustrating an OB clamp circuit according to a first embodiment of an imaging apparatus according to the present invention. It is a block diagram which shows the detection determination circuit of FIG. 1 in detail. It is a wave form diagram which shows the main signal in the case of normal brightness | luminance in FIG. It is a wave form diagram which shows the main signal in the case of high brightness | luminance in FIG. It is a block diagram which shows the modification of the detection determination circuit of FIG. 6 is a timing chart showing main signals in FIG. 5. It is explanatory drawing which shows the imaging surface of an image pick-up element. It is a block diagram which shows the OB clamp circuit of an example of the imaging device of a prior art. FIG. 9 is a waveform diagram showing main signals in the case of normal luminance in the OB clamp circuit shown in FIG. 8. It is a block diagram which shows another example of a clamp circuit. FIG. 9 is an explanatory diagram illustrating an image pickup surface of an image pickup element at the time of high-luminance subject image pickup in the OB clamp circuit shown in FIG. 8. FIG. 9 is a waveform diagram showing main signals during high-luminance subject imaging in the OB clamp circuit shown in FIG. 8.

Explanation of symbols

DESCRIPTION OF SYMBOLS 201 Image pick-up element 202 Clamp circuit 203 Signal processing circuit 704 Timing control circuit 705 Detection determination circuit 706 AND gate (prohibition means)
801 detection circuit 802 determination circuit

Claims (1)

An image sensor in which a light receiving area and a light shielding area are provided on an imaging surface on which a plurality of photoelectric conversion elements are arranged, and a reference signal based on a black level of an imaging signal to be output based on the output signal level of the image sensor in the light shielding area An image pickup apparatus comprising: a clamp circuit that outputs a signal obtained by subtracting the clamped output signal level from the output signal level of the image pickup element in the light receiving region as an image pickup signal;
Detecting means for detecting the output signal level of the image sensor in the light shielding region for each frame;
Means for outputting a prohibition signal for prohibiting updating of the reference signal of the clamp circuit for each frame when the output signal level of the image sensor in the light shielding region detected by the detection means is greater than a predetermined level;
Means for prohibiting updating of the reference signal of the clamp circuit for each frame by the prohibition signal;
An image pickup apparatus comprising:

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