Image sensing device

[0002] 1. Field of the Invention

[0003] The present invention relates to an image sensing device for converting optical signals to electrical signals, and specifically relates to an image sensing device which logarithmically converts a photoelectric current generated in accordance with the intensity of incident light for output.

[0004] 2. Description of the Related Art

[0005] Color image sensing devices are provided with red, green, and blue filters, through which passes light which impinges photoreceptor sensors of red (R), green (G), and blue (B). However, these filters have different transmittance, e.g., the amount of light transmitted by each filter is different in the case of achromatic color. Specifically, the transmittance of red color is greatest, followed by green, then blue in that order. Accordingly, white balance adjustment is performed to correct differences in the amount of said transmitted light.

[0006] When sensors having linear characteristics output linear photoelectric current in accordance with the intensity of incident light, differences in the amount of light impinging the R, G, and B photoreceptors produce input/output (I/O) characteristics as shown in FIG. 1(a) (wherein the horizontal axis represents the linear scale). In the case of sensors having linear characteristics, adjustments are made to the gain of the amplification circuits connected to the output side of the sensors, so as to match R, G, B characteristics and correct white balance by changing the slope of the I/O characteristics (refer to U.S. Pat. No. 4,734,762).

[0007] On the other hand, U.S. Pat. No. 5,241,575 discloses an image sensing device provided with sensors which logarithmically convert a photoelectric current generated in accordance with the intensity of incident light. When the intensities of the incident light impinging the R, G, and B photoreceptors of this image sensing device are different, the I/O characteristics of these sensors are as shown in FIG. 1(b) (wherein the horizontal axis represents the logarithm scale). In the case of an image sensing device of this logarithm conversion type, adjustments of the gain of the amplification circuits cannot accomplish matching of the R, G, B characteristics. Accordingly, white balance adjustment is accomplished after analog-to-digital (A/D) conversion of the sensor output signals. Thus, in conventional image sensing devices of the logarithm conversion type, there are differences in the output levels of R, G, and B sensors, such that n bits less than the number of bits of A/D conversion are allocated as gradient numbers relative to amplitude A of an image signal.

[0008] An object of the present invention is to provide an image sensing device having logarithm conversion functions, which is capable of achieving white balancing using a simple construction.

[0009] A further object of the present invention is to provide an image sensing device capable of a greater number of gradients.

[0010] These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention.

[0011] In the following description, like parts are designated by like reference numbers throughout the several drawings.

[0012] FIG. 1(a) shows the I/O characteristics of an image sensing device having sensors with linear characteristics when white balance correction is not accomplished;

[0013] FIG. 1(b) shows the I/O characteristics of an image sensing device having sensors of a logarithm conversion type when white balance correction is not accomplished;

[0014] FIG. 2 shows the I/O characteristics of an image sensing device when white balance correction is accomplished;

[0015] FIG. 3 is an illustration showing the relationship between image signals and the number of bits of A/D conversion in the prior art;

[0016] FIG. 4 shows the general construction of a first embodiment of the image sensing device of the present invention;

[0017] FIG. 5 shows the circuit construction of the image sensing/transfer section within the solid state imaging portion;

[0018] FIG. 6 is a timing chart of clock signals for driving the charge-coupled device (CCD) of the charge transfer section;

[0019] FIG. 7 is a conceptual representation of the operating principle of the charge-transfer CCD;

[0020] FIG. 8 shows the relationship between image signals and the m bit of the A/D converter of the present invention;

[0021] FIG. 9 shows a modification of the circuit construction of the image sensing/transfer section of FIG. 5;

[0022] FIG. 10 is a block diagram showing the construction of a second embodiment of the image sensing device of the present invention;

[0023] FIG. 11 is a block diagram showing the construction of a third embodiment of the image sensing device of the present invention;

[0024] FIG. 12 is a block diagram showing the construction of a fourth embodiment of the image sensing device of the present invention;

[0025] FIG. 13 is a block diagram showing the construction of a fifth embodiment of the image sensing device of the present invention.

[0026] The preferred embodiments of the present invention are described hereinafter with reference to the accompanying drawings.

[0027] First Embodiment

[0028] FIG. 4 shows the general construction of an image sensing device of the present invention. Image sensing device 100 is provided with solid state image sensing section 10, amplification circuits 20R, 20G, 20B for amplifying the outputs from said solid state image sensing section 10, A/D converters 30R, 30G, 30B for converting the outputs from said amplification circuits 20R, 20G, 20B as digital signals, and voltage generating circuits 40R, 40G, 40B connected to said solid state image sensing section 10. Solid state image sensing section 10 is provided with three rows of image sensing/transfer sections corresponding to the colors red (R), green (G), blue (B). The rows of each color of the image sensing/transfer section 10 include a plurality of image sensing/transfer elements arranged in a linear array. The drawing shows rows of photoreceptor elements 11R, 11G, 11B of the image sensing/transfer section of each color. Furthermore, photoreceptors 11R, 11G, 11B are provided with a red filter, a green filter, and a blue filter, respectively

[0029] Voltage generating circuits 40R, 40G, 40B correspond to R, G, B, and supply reference voltages V_Rr, V_Rg, V_Rbto solid state image sensing section 10.

[0030] The R, G, B output signals O_Sr, O_Sg, O_Sbfrom solid state image sensing section 10 are outputted from output pins 12R, 12G, 12B. After these signals are amplified by amplification circuits 20R, 20G, 20B, they are converted to digital signals by A/D converters 30R, 30G, 30B.

[0031] FIG. 5 shows an image sensing/transfer section 13 within solid state image sensing section 10 of FIG. 4. The drawing shows only one image sensing/transfer section 13 in the construction corresponding to red (R) solid state image sensing section 10. Other image sensing/transfer sections in array R as well as image sensing/transfer sections in arrays G and B have identical constructions to that shown in FIG. 5.

[0032] Image sensing/transfer section 13 comprises PN-junction photodiode 1, n-channel MOS-FET (metal-oxide semiconductor field-effect transistor) 2, and a charge-transfer CCD. Photodiode 1 is a photoelectric signal generator which is sensitive to an incident light and generates a photoelectric signal which is logarithmically proportional to an intensity of that incident light. Cathode of photodiode 1 is supplied with a voltage V_DD, and the anode of the photodiode 1 is connected to the drain D and the gate G of MOS-FET 2. Source S of MOS-FET 2 is supplied with a voltage V_SS, and the backgate (substrate) of the MOS-FET 2 is applied a voltage V_SUB. The relationship of these bias voltages is V_DD>V_SS≧V_SUB, an opposite bias is applied to photodiode 1, an opposite bias relative to MOS-FET 2 is applied to the substrate, and an opposite bias relative to the drain is applied to the substrate. Voltage V_SUBapplied to the backgate (substrate) of MOS-FET 2 is regulated such that a subthreshold current flows to MOS-FET 2. Therefore, the MOS-FET 2 operates in the subthreshold region, such that the drain current of MOS-FET 2 is an exponential function of the voltage between the gate and source. Thus, MOST-FET 2 is a convert which is connected to the photodiode 1 so as to generate a first analog signal which is a logarithmic function of the photoelectrical signal generated by photodiode 1.

[0033] Gate of the aforesaid MOS-FET 2 is connected to the first electrode 4 of the charge-transfer CCD. The second electrode 5 of the charge-transfer CCD is connected to the voltage generating circuit 40R, and the second electrode 5 is supplied a reference voltage V_Rrfrom the voltage generating circuit 40R. Voltage generating circuit 40R is provided with a volume control VOL for manually adjusting the voltage V_Rrsupplied to second electrode 5 of the charge-transfer CCD. Volume control VOL may be, for example, a sliding resistor, but is not limited thereto. Voltage generating circuit 40R may be commonly provided to image sensing/transfer sections 13 of the array of each color, or may be independently provided to each image sensing/transfer section 13 of the array of each color.

[0034] In the image sensing/transfer section 13 of the aforesaid construction, a photoelectric current proportional to the intensity of the light impinging photodiode 1 flows from the cathode to the anode. Since voltage V_SUBis regulated so that a subthreshold current flows to MOS-FET 2, the photoelectric current is subjected to logarithm conversion to produce voltage V_Gwhich is applied to first electrode 4 of the charge-transfer CCD.

[0035] Thus, the first electrode 4 of the charge-transfer CCD is supplied with the gate voltage V_Gof the log conversion MOS-FET 2, the second electrode 5 of the charge-transfer CCD is supplied with the reference voltage V_Rr, and a signal charge is injected into the charge-transfer CCD which corresponds to the difference in the lower limit channel potential of both electrodes.

[0036] A pulse φD is applied to input diode 3 of the charge-transfer CCD, pulse φS is applied to third electrode 6, pulse φ1 is applied to fourth electrode 7, and pulse φ2 is applied to fifth electrode 8 (refer to FIG. 6). Pulses φ1 and φ2 are sequentially applied to electrodes subsequent to sixth electrode 9, so as to transfer the charge in a two-phase actuation.

[0037] FIG. 7 illustrates the channel potential corresponding to the time chart of drive pulses of FIG. 6. The areas of the drawing shaded by diagonal lines indicate portions where a charge is present.

[0038] At time t=t1, when clock pulse φD applied to input diode 3 of the charge-transfer CCD is changed from high (H) level to low (L) level, the charge passes through first electrode 4 and is injected below second electrode 5. At time t=t2, when a clock pulse φD is changed to high (H) level, the excess charge is returned to input diode 3. These actions are equivalent to a reset operation, and the charge accumulation corresponds to the difference between reference voltage V_Rrand the gate voltage V_Gof second electrode 5. Thereafter, a part of the charge of second electrode 5 passes through first electrode 4 and is emitted to input diode 3 to accomplish an integral control action.

[0039] At time t=t3, after the integral time (the time indicated by the arrow in the time chart of FIG. 6) has ended, the pulse φD changes to high (H) level at time t=t4, and the channel potential of third electrode 6 falls, such that the stored charge flows to fourth electrode 7 and fifth electrode 8 used as shift registers. The offset charge amount transferred to the shift registers is determined by the reference voltage V_Rrapplied to second electrode 5, such that the direct-current (DC) component of the output signal can be regulated by adjusting said voltage V_Rr. Thus, the charge-transfer CCD and the voltage generating circuit 40R constitute a signal adjusting device for adjusting a direct current component of the first analog signal, outputted by the MOST-FET 2, and for outputting a second analog output signal which has the thus regulator direct-current component.

[0040] Then, after time t=t5 has elapsed, when the signal to the φ2 register (i.e., the register activated by pulse φ2) changes form high (H) level to low (L) level, the channel potential of the φ2 register rises, and the charge is collected in the φ1 register (i.e., the register activated by pulse φ1) (t=t6). Thereafter, pulse φ1 changes form high (H) level to low (L) level, and pulse φ2 changes from low (L) level to high (H) level, such that the charge is transferred to the φ2 register. The charge is thus transferred to adjacent registers by inputting a reverse phase clock pulse to the φ1 and φ2 registers from the aforesaid state.

[0041] Reference voltages V_Rr, V_Rg, and V_Rbapplied to second electrode 5 of the charge-transfer CCD corresponding to each color participate in the DC component of the output of image sensing/transfer section 13, and, therefore, the output characteristics of FIG. 1(b) can be matched by adjusting the volume control VOL of said reference voltages V_Rr, V_Rg, and V_Rb. Changing the DC components of the output of the charge-transfer CCD means a horizontal migration without changing the slope of said R, G, B characteristics in FIG. 1(b). Accordingly, in the present embodiment, white balance correction can be accomplished by managing the levels of the output signals O_Sr, O_Sg, O_Sboutputted from solid state image sensing section 10.

[0042] Output signals O_Sr, O_Sg, O_Sb, the levels of which have been managed as described above, are amplified by amplifiers 20R, 20G, 20B, and then, inputted to A/D converters 30R, 30G, 30B. Thus, each A/D convert 30R, 30G, 30B, which is connected via the respective amplifier 20R, 20G, 20B to the respective signal adjusting device (combination of a charge-transfer CCD and a voltage generating circuit 40), generates a digital signal based on the analog output signal generated by the respective signal adjusting device. Therefore, the bit number m of said A/D converters 30R, 30G, 30B can be equivalent to the number of gradients, as shown in FIG. 8.

[0043] FIG. 9 shows the construction of modification 13′ of image sensing/transfer section 13 of FIG. 5. Like parts of the image sensing/transfer section 13 and 13′ are designated by like reference numbers relative to FIGS. 5 and 9, and are therefore omitted from the following discussion.

[0044] In image sensing-transfer section 13′, photodiode 101 is provided to monitor the amount of light impinging the photoreceptor section. The cathode of photodiode 101 is supplied a bias voltage V_DD, and the anode is connected to the gate and drain of MOS-FET 102. The bias voltage applied to said MOS-FET 102 is identical to that applied to MOS-FET 2. The voltage between the drain current of MOS-FET 102 is an exponential function of the voltage between the gate and source. The gate of MOS-FET 102 is connected to the input pin of source follower AMP 103, and the source voltage Vss of MOS-FET 2 and 102 is connected to the input pin of source follower AMP 104.

[0045] Source follower AMPs 103 and 104 have mutually identical constructions comprising four combined MOS-FETs. The output of source follower AMP 104 is connected via resistor 106 to the inverse input pin of differential amplifier 105. The output of source follower AMP 103 is connected via resistor 107 to the noninverse input pin of said differential amplifier 105. The noninverse input pin of differential amplifier 105 is grounded via resistor 108, and the output pin and inverse input pin are connected via resistor 109.

[0046] The output pin of differential amplifier 105 is connected to the noninverse input pin of differential amplifier 110 via resistor 111. Voltage regulator 40R is connected via resistor 112 to the inverse input pin of differential amplifier 110. The output pin and inverse input pin of differential amplifier 110 are connected via resistor 113 and variable resistor 114. The output pin of differential amplifier 110 is further connected to second electrode 5 of the charge-transfer CCD, such that the output of said differential amplifier 110 is applied to second electrode 5 as reference voltage V_Rr′.

[0047] In image sensing/transfer section 13′ having the aforesaid construction, photodiode 101 monitors the amount of light entering the photoreceptor section, and generates a photoelectric current corresponding to the intensity of the entering light. MOS-FET 102 converts the aforesaid photoelectric current via logarithm conversion and outputs it to source follower AMP 103 as gate voltage V_GM.

[0048] Differential amplifier 105, which receives the inputs of the power source voltage V_SSand the outputted signal of photodiode 101 via source follower AMPs 103 and 104, outputs a voltage corresponding to the difference in said inputs to differential amplifier 110. Differential amplifier 105 eliminates the fluctuating portion of power source voltage V_SSto achieve high error-free reliability of output V_Rr.

[0049] In differential amplifier 110, the output reference voltage V_Rr′ is regulated by suitably adjusting the regulating voltage V_Rr, thereby regulating the DC segment of the output signal output from the charge-transfer CCD. Regulation of voltage V_Rris accomplished by the voltage regulating volume control VOL of the outputted signal output from the charge-transfer CCD.

[0050] Second Embodiment

[0051] FIG. 10 shows the construction of a second embodiment of the present invention. In image sensing device 101, parts comparable to those of the first embodiment are designated by like reference numbers and are omitted from the present description.

[0052] Microcomputer 41 is provided in image sensing device 101 of the present embodiment, and said microcomputer 41 automatically calculates reference voltages V_Rr, V_Rg, V_Rbwhich are supplied to second electrode 5 of the charge-transfer CCD (refer to FIG. 5). The input pins of microcomputer 41 are connected to the output pins of each A/D converter 30R, 30G, 30B, and the output pins of microcomputer 41 are connected to D/A converters 42R, 42G, 42B provided for each color R, G, B. These D/A converters 42R, 42G, 42B apply reference voltages V_Rr, V_Rg, V_Rb, corresponding to the output signals outputted from microcomputer 41 to second electrodes 5 of the charge-transfer CCD for each color. Memory 43 connected to microcomputer 41 is a buffer provided as a work area for microcomputer 41. Furthermore, the memory 43 stores initial digital signals to be initially outputted to the D/A converters 42R, 42G, 42B beforehand.

[0053] The regulation sequences of reference voltages V_Rr, V_Rg, V_Rbin the present embodiment are described hereinafter. Microcomputer 41 outputs initial digital signals to D/A converters 42R, 42G, 42B so that D/A converters 42R, 42G, 42B apply the initial reference voltages V_Rr, V_Rg, V_Rbto the second electrode of the charge-transfer CCD of each color. Achromatic light enters solid state image sensing section 10, and output signals O_Sr, O_Sg, O_Sbcorresponding to said incident light are respectively converted to digital signals by A/D converters 30R, 30G, 30B. Microcomputer 41 fetches the digital signal of each color from the input port, and computes digital signals to be outputted to D/A converters 42R, 42G, 42B based on said digital signals. Microcomputer 41 then outputs these computed digital signals to D/A converters 42R, 42G, 42B. D/A converters 42R, 42G, 42B, which have received the computed digital signals, provide analog voltages corresponding to said signals, and apply said analog voltages to the second electrode of the charge-transfer CCD of each color as reference voltages V_Rr, V_Rg, V_Rb. In image sensing device 101 of the present embodiment, the reference voltages applied to the second electrode of the charge-transfer sections can be automatically regulated. Thus, in this second embodiment, the microcomputer 41, memory 43, D/A converters 42R, 42G, 42B, and the respective charge-transfer CCDs constitute a signal adjusting device for adjusting a direct current component of the first analog signal, outputted by the MOS-FET 2, and for outputting second analog output signals which have the thus regulated direct-current component.

[0054] In the construction of the second embodiment, a method is used wherein reference voltages V_Rr, V_Rg, V_Rb, suitable for predetermined types of objects, e.g., objects under fluorescent light, objects under incandescent light and the like, are stored beforehand in memory 43, such that reference voltages V_Rr, V_Rg, V_Rbcorresponding to said types of objects can be readout from memory 43. In this instance, selection switches are connected to microcomputer 41, and reference voltages V_Rr, V_Rg, V_Rbcorresponding to said types of objects are stored beforehand in memory 43 corresponding to said selection switches, respectively. Reference voltages V_Rr, V_Rg, V_Rbare used in conjunction with selection switch operation during image sensing. Thus, in this method, the microcomputer 41 can receive a digital signal, which was generated by the A/D converter 30R, 30G, 30B when a reference image was exposed to the photoelectric signal generator 1, and can determine an adjustment quantity of the direct-current component of the first analog signal outputted by the respective MOS-FET.

[0055] External operation keys for accomplishing micro adjustment of the white balance may be connected to microcomputer 41. In this instance, digital signals outputted to D/A converters 42R, 42G, 42B are changed in accordance with the operation of said external operation keys, and can be used to accomplish micro adjustment of the white balance during image sensing.

[0056] As can be readily understood from the aforesaid description, the image sensing device of the present embodiment is a solid state image sensing device of a logarithm conversion type capable of managing signals of each color inputted to A/D converters, wherein the bit number m of the A/D converters is equivalent to the number of gradients.

[0057] Third Embodiment

[0058] Although the first and second embodiments regulate the DC component of the signals before the charge-transfer in the charge-transfer CCD, it is to be noted that white balance adjustment may be accomplished after charge-transfer, i.e., on the output from the charge-transfer CCD.

[0059] For example, a clamp circuit may be provided as shown in FIG. 11, and a clamp voltage (bias voltage) applied to said clamp circuit may be controlled to regulate the DC component of the output signal. In this embodiment, each clamp circuit is part of the respective signal adjusting device. Furthermore, circuits may be provided to control the reference voltages (bias voltages) of amplification circuits 20R, 20G, 20B, such that the amount of offset of the signal voltage may be varied to regulate the DC component of the CCD output signal. These amplification circuits may be constructed by well known clamp circuits, and, therefore, specific description of their construction is omitted from this discussion.

[0060] Fourth Embodiment

[0061] In each of the previously described embodiments, image sensing devices have been described wherein output signals O_Sr, O_Sg, O_Sb, corresponding to each color R, G, B, are outputted in parallel from solid state image sensing section 10 used for color image sensing, and said output signals O_Sr, O_Sg, O_Sbare processed by amplifiers 20R, 20G, 20B and A/D converters 30R, 30G, 30B provided independently for each color R, G, B. However, it is to be understood that the present invention is not limited to such image sensing devices.

[0062] For example, FIG. 12 shows image sensing device 102 of the fourth embodiment. In the drawing, parts comparable to those of previously described embodiments are designated by like reference numbers and are omitted from the present description. Image sensing device 102 serially outputs the color output signals O_Sr, O_Sg, O_Sbfrom solid state image sensing section 10A, and sequentially processes said color output signals O_Sr, O_Sg, O_Sbby amplifier 20 and A/D converter 30 which are used in common for each color.

[0063] Solid state image sensing section 10A is provided with photoreceptors 11R, 11G, 11B corresponding to each color R, G, B, and image sensing/transfer sections 13 (refer to FIG. 5) linearly arrayed in correspondence with said photoreceptors 11R, 11G, 11B. Output signals O_Sr, O_Sg, O_Sbfrom solid state image sensing section 10A are stored temporarily in buffer B. The output signals O_Sr, O_Sg, O_Sbare serially outputted in sequence from output pin 12. Amplifier 20 and A/D converter 30 sequentially process the serially outputted signals O_Sr, O_Sg, O_Sb.

[0064] In image sensing device 102 of the aforesaid construction, amplifier 20 and A/D converter 30 are provided in common for the various colors R, G, B, thereby simplifying construction.

[0065] Thus, the present invention is adaptable to image sensing devices which sequentially output R, G, B color signals from a solid state image sensing section. Although the device in FIG. 12 has been described in terms of a modification of the first embodiment, it is possible that the second and third embodiments may be similarly modified.

[0066] Fifth Embodiment

[0067] FIG. 13 shows image sensing device 103 of the fifth embodiment. In the drawing, parts comparable to parts of previous embodiments are similarly designated by reference number, and further description is omitted. Image sensing device 103 serially produces color output signals O_Sr, O_Sg, O_Sbby the image sensing cycles by solid state image sensing section 10B provided with a single array of image sensing/transfer section 13 by changing filters of red, green, and blue, and wherein the serially outputted color output signals O_Sr, O_Sg, O_Sbare processed by amplifier 20 and A/D converter 30 provided commonly for each color. In this embodiment, switch S is changed synchronously with image sensing of each color, and reference voltages V_Rr, V_Rg, V_Rb, suitable for image sensing of each color are applied to second electrode 5 of image sensing/transfer section 13 (refer to FIG. 5)(reference voltages V_Rr, V_Rg, V_Rbare mutually different). Specifically, the red filter is set, and switch S is positioned at the left position. Since switch S is positioned at the left position, reference voltage V_Rris outputted from voltage generation circuit 40R and is applied to second electrode 5 of image sensing/transfer section 13. In this state, image sensing of the red image is accomplished, and output signals O_Srare outputted from solid state image sensing section 10B and processed by amplifier 20 and A/D converter 30. Then, the filter is changed to the green filter, switch S is positioned at the center position, and image sensing of the green color image is accomplished, and the output signals O_Sgare processed. Then, the filter is changed to the blue filter, switch S is changed to the right position, and image sensing of the blue color image is accomplished and output signals O_Sbare processed.

[0068] In image sensing device 103 having the aforesaid construction, solid state image sensing section 10 uses only a single array of image sensing/transfer section 13, thereby simplifying construction of the device.

[0069] Thus, the present invention is adaptable for use in image sensing devices having a single array of image sensing/transfer elements. Although the embodiment of FIG. 13 has been described in terms of a modification of the first embodiment, it is to be noted that modifications of the second and third embodiments are similarly possible.

[0070] Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.