Image sensing apparatus

Abstract: An image sensing apparatus includes an image sensing circuit, an inverting circuit for inverting a polarity of an output of the image sensing circuit, and a correcting circuit for correcting a black reference level of the output of the image sensing circuit in accordance with an operation of the inversion circuit.

Description: The present invention relates to an image sensing apparatus including polarity inverting means for inverting the polarity of a sensed image. Referring to Fig. 1 which is a block diagram showing a conventional image sensing apparatus, the apparatus includes an image sensor 1 for converting an optical image of a subject or a negative image formed on a negative film into an electrical signal, a luminance signal processing circuit 2 for processing an output signal from the image sensor 1 and producing a luminance signal Y H , and a color signal processing circuit 3 for producing RGB signals. The conventional image sensing apparatus also includes gain control circuits 4 and 5 for white-balance control of the respective R and B signals, a potentiometer part 6 for white-balance control which includes potentiometers independently provided for the R and B signals, a matrix circuit 7 for producing color-difference signals R-YL and B-YL, a negative/positive inversion processing circuit 8 for the luminance signal which includes circuit elements such as an inverting circuit and a blanking processing circuit for setting a pedestal level, a potentiometer 9 for setting the pedestal level, negative/positive inversion processing circuits 10 and 11 for the color-difference signals, each of which includes an inverting circuit and so on, a switching circuit 12 which is switched in accordance with a negative/positive switching control signal, and an encoder circuit 13 for generating a composite video signal from the luminance signal and the color-difference signals. Fig. 2 is a graph showing the characteristics of densities D of individual developed photosensitive layers with respect to a typical exposure amount log H which is given to a color negative film during photography. If the negative film is illuminated with an appropriate light source having good color rendering and transmitted light is sensed by the image sensing apparatus for negative/positive inversion having the above-described arrangement, signal levels equivalent to the Ro, Go and Bo shown in Fig. 2 are obtained from the respective RGB outputs of the color signal processing circuit 3 with respect to the film exposed by, for example, a particular exposure amount Eo. From the luminance signal processing circuit 2, a luminance signal equivalent to Y 0 =0.3R 0 +0.59G 0 +0.11B 0 is also obtained. As a matter of course, the signal levels of the above-described color and luminance signals are proportional to the quantity of light incident on the image sensor 1. Fig. 3 is a graph showing the relation between the exposure amount log H which is given to the film during photography and the relative value of a luminance signal level if the quantity of light incident on the image sensor 1 is set to a predetermined value, a luminance signal level obtained at the base portion of the film being set to 100%. If a maximum exposure amount, an average exposure amount and a minimum exposure amount are, for example, E 0 , E 1 and E 2 for the film, corresponding luminance signal levels are VEO, V E1 and V E2 . In the conventional example, the quantity of light incident on the image sensor 1 has been adjusted so that V E2 reaches 100% or V E1 reaches a predetermined proper value (for example, 50%). However, since each of the signal levels V E2 and V E1 equivalent to the minimum and average exposure amounts for the film contains the signal level V E0 equivalent to the maximum exposure amount, the conventional example shown in Fig. 1 has a number of disadvantages. (i) A large error may occur if the signal level V E1 equivalent to the average exposure amount and the signal level V E2 equivalent to the minimum exposure amount are used as they stand, for the purpose of controlling an exposure amount which is given to the image sensor. (ii) Even if the luminance signal is inverted to set its black level to a portion equivalent to the minimum exposure amount, a portion equivalent to the maximum exposure amount does not reach 100% and becomes small by an amount equivalent to the signal level V E0 . Also, in the conventional example shown in Fig. 1, since a signal level proportional to the density of the negative film is obtained, the signal levels of RGB outputs with respect to a saturation exposure density level, that is, the black level of the image-sensor output, are not equal even if all the photosensitive layers are uniformly exposed. Accordingly, if the signal levels of R and B signals are to be made equal by adjusting a white-balance control circuit and controlling the gains of the R and B signals of a color signal obtained by sensing the portion of the film which has been exposed by, for example, the exposure amount Eo during photography, a deviation in white balance becomes extremely large at a portion of low density, that is, a portion corresponding to relatively high RGB signal levels. In addition, if a negative/positive inverting device such as the conventional example is to be applied to an image sensing apparatus, the following problems will arise. (i) The conventional example is arranged so that white-balance adjustment is performed by operating the potentiometer part 6 for control of white balance while an image subjected to negative/positive inversion is being viewed. However, in the case of an electronic still camera having no electronic viewfinder, it is impossible to perform white-balance adjustment while viewing an image. (ii) In the case of white-balance adjustment using an external metering method utilizing a colorimetric sensor, the colorimetric sensor is used to obtain the proportion of an R (red) component to a G (green) component to a B (blue) component which are contained in source light, and the gain control circuits 4 and 5 perform gain control on the basis of the obtained proportion. As a result, a deviation in white balance will occur if white-balance adjustment during a negative/positive inversion mode is performed on the basis of the proportion of the R component to the G component to the B component which are contained in the source light. The reason why the deviation in white balance occurs is that since the base of a negative film, which is generally called an orange base, is orange in color, gain control for white-balance adjustment during the negative/positive inversion mode must be performed so as to make uniform the proportion of the R component to the component G to the B component which are contained in source light transmitted through a white portion of a subject recorded on the orange negative film. Moreover, document US-4 635 101 discloses an image conversion apparatus for television signals, wherein a negative image on a recording medium is converted into a positive image. The negative image in the form of a first video signal includes a first luminance signal and a first color difference signal, which are to be converted into a second video signal representing the inverted (positive) image. To this end, conversion means are provided for carrying out the various conversion operations. Specifically, in a first step the first luminance signal and the color difference signal are evaluated to obtain first separated color signals, in a second step the first luminance signal is converted by a first signal processor means to obtain an inverted luminance signal as a second luminance signal, in a third step the first separated color signals are inverted to obtain second separated color signals, and in a fourth step a second color difference signal is obtained from the second luminance signal and separated color signals. The final video signal represents the converted (positive) image having corrected brightness and weight balance. Furthermore, document JP 60-176 383 (abstract) discloses an electronic camera which can perform a proper white balance adjustment depending in combination with a negative/positive converting device upon the kind of film (negative film or slide film) used. It is, therefore, an object of the present invention to solve the above-described problems. Another object of the present invention is to provide an image sensing apparatus having an inverting function which makes it possible to provide correct exposure control over an image sensor. Another object of the present invention is to provide an image sensing apparatus having inversion processing means which makes it possible to highly accurately adjust white balance for any density area of a negative film. According to the present invention, these objects are accomplished by an image sensing apparatus as set out in the appended claims. In accordance with a first aspect of the present invention having the above-described arrangement, it is possible to obtain an optimum black reference level for either of inverted polarity and non-inverted polarity. In addition, since exposure control and the like can be performed by using an output signal of corrected black reference level, it is possible to correctly control an exposure amount, a dynamic range and the like. In accordance with a second aspect of the present invention, there is provided an image sensing apparatus including color image sensing means for converting an optical image into an electrical signal and generating a plurality of color signals, inversion processing means for inverting colors carried by the plurality of color signals, respectively, and black-level shifting means for shifting black levels of the respective color signals by a predetermined amount in accordance with whether the inversion processing means is operated or not. In accordance with the second aspect of the present invention having the above-described arrangement, the black levels of the plurality of color signals are shifted by the black-level shifting means. Accordingly, it is possible to highly accurately adjust white balance for any density area. In accordance with a third aspect of the present invention, the image sensing apparatus includes first light source color detecting means for detecting a color of a source light by means of an external circuit, negative/positive inverting means, mode selecting means for selecting a negative/positive inversion mode, and controlling means for controlling a white-balance adjusting operation of white-balance adjusting means on the basis of a predetermined value if the negative/positive inversion mode is selected by the mode selecting means. In accordance with a fourth aspect of the present invention, the image sensing apparatus is arranged so that the predetermined value referred to in the above-described aspect can be switched from one value to another according to the kind of negative film. In accordance with a fifth aspect of the present invention, the image sensing apparatus includes second light source color detecting means for detecting a color of source light from an output of the image sensing means, negative/positive inverting means, and mode selecting means for selecting a negative/positive inversion mode, a white-balance adjusting operation of white-balance adjusting means being controlled by means of the second light source color detecting means if the negative/positive inversion mode is selected by the mode selecting means. In accordance with a sixth aspect of the present invention, the image sensing apparatus is arranged to correct a predetermined value such as that referred to in the above-described embodiment on the basis of a detection value provided by the second light source color detecting means if a white-balance adjusting operation of the white-balance adjusting means is to be controlled on the basis of the predetermined value. In accordance with a seventh aspect of the present invention, the image sensing apparatus is arranged so that predetermined values for white-balance control are switched from one value to another according to the kind of negative film on the basis of a detection value provided by the second light source color detecting means if the negative/positive inversion mode is selected by the mode selecting means referred to in the above-described aspect. In the image sensing apparatus having the above-described arrangement, the image sensing means converts a subject image into an electrical signal and outputs a plurality of color signals, and the first light source color detecting means detects a color of source light by means of the external circuit. The negative/positive inverting means effects negative/positive inversion, and the mode selecting means selects the negative/positive inversion mode. If the negative/positive inversion mode is selected by the mode selecting means, the white-balance adjusting operation of the white-balance adjusting means is controlled by the control means on the basis of the predetermined value. The image sensing apparatus according to the fourth aspect of the present invention is arranged to switch the predetermined value referred to in the above-described embodiment from one value to another according to the kind of negative film. In the image sensing apparatus according to the fifth aspect of the present invention, the image sensing means converts subject light into an electrical signal and outputs a plurality of color signals, and the second light source color detecting means detects a color of a source light from an output of the image sensing means. The negative/positive inverting means effects negative/positive inversion, and the mode selecting means selects the negative/positive inversion mode. If the negative/positive inversion mode is selected by the mode selecting means, a white-balance adjusting operation of the white-balance adjusting means is controlled by means of the second light source color detecting means. The image sensing apparatus according to the sixth aspect of the present invention is arranged to correct the predetermined value on the basis of a detection value provided by the second light source color detecting means if the white-balance adjusting operation of the white-balance adjusting means is to be controlled on the basis of such a predetermined value. The image sensing apparatus according to the seventh aspect of the present invention is arranged so that the predetermined values for white-balance control are switched from one value to another according to the kind of negative film on the basis of a detection value provided by the second light source color detecting means if the negative/positive inversion mode is selected by the mode selecting means referred to in the above-described aspect. The above and other objects, features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. Fig. 1 is a block diagram showing a conventional image sensing apparatus; Fig. 2 is a graph showing the relation between an exposure amount given to a negative film and the density thereof; Fig. 3 is a graph showing the relation between an exposure amount given to a film and the relative value of a luminance signal level; Fig. 4 is a block diagram showing an image sensing apparatus according to a first embodiment of the present invention; Figs. 5 and 6 are graphs each showing the relation between an exposure amount and an output signal level, and serves to illustrate the first embodiment of the present invention; and Fig. 7 is a block diagram showing an image sensing apparatus according to a second embodiment of the present invention. Detailed Description of the Preferred Embodiments: Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. A first embodiment of the present invention will be described below. Fig. 4 is a block diagram showing an image sensing apparatus according to the third embodiment of the present invention. Figs. 5 and 6 are graphs each showing the relation between an exposure amount and an output signal level, and serves to illustrate the first embodiment of the present invention. In Fig. 4, identical reference numerals are used to denote constituent elements which are identical to or correspond to those shown in Fig. 1, and description is omitted for the sake of simplicity. In the image sensing apparatus of Fig. 4, clamping circuits 34 to 36 set the black levels of RGB signals, respectively. A selecting switch 37 switches the clamping potentials of the respective clamping circuits 34 to 36 in interlocked relation to the switching circuit 12. The switching circuit 12 performs its switching operation in accordance with a negative/positive switching control signal. Clamping potentials 38 to 40 constitute black-level shifting means. During the normal mode, i.e., during normal photography, the same clamping potential (Vref) is supplied to the clamping circuits 34 to 36, and color signals and a luminance signal bypass the negative/positive inversion processing circuits 8, 10 and 11, respectively, and are outputted to the encoder 13. During the negative mode, i.e., during photography made on a negative film, a clamping potential VR is supplied to the clamping circuit 34 while a clamping potential VB is supplied to the clamping circuit 36. The operation of the above-described image sensing apparatus will be described below. Figs. 5 and 6 are graphs each showing the relations between the exposure amount log H and the relative values (100%: the signal level of G of a film base) of RGB color signal outputs which are respectively obtained from the outputs of the clamping circuits 34 to 36 when a negative film of characteristic shown in Fig. 2 is subjected to image sensing. Fig. 5 shows the case of the normal mode while Fig. 6 shows the case of the negative mode. During the normal mode, the clamping potentials for R, G and B are Vref. Accordingly, during the normal mode, if a sensed image on the negative film is subjected to signal processing to produce a positive image, the output signal levels of the respective RGB colors exhibit characteristics proportional to film density as shown in Fig. 5. For example, the output signal levels equivalent to a certain exposure amount E 0 are V R0 , V G0 and V B0 , so that an extremely large deviation occurs in white balance. In the first embodiment of the present invention, however, when the negative mode is selected, V R is supplied as a clamping voltage for an R signal, while V B is supplied as a clamping voltage for a B signal. If these V R and V B are set to: V R =V ref +(V G0 -V R0 ), V B =V ref +(V G0 -V B0 ) the black levels of the R and B signals are shifted by (V G0 -V R0 ) and (V G0 -V B0 ), respectively. Accordingly, as shown in Fig. 6, when the exposure amount E 0 is reached, the respective output signal levels are equal to one another so that no deviation occurs in white balance. Gain control circuits 4 and 5 effect gain adjustment in accordance with a white-balance control potentiometer part 6 so as to reduce deviation in the white balance of a portion which shows a small exposure amount, that is, a high signal level. Accordingly, it is possible to minimize the deviation in white balance over the entire range of signal levels. Fig. 7 is a block diagram showing an image sensing apparatus according to a second embodiment of the present invention. In Fig. 7, identical reference numerals are used to denote constituent elements which are identical to or correspond to those of the conventional example shown in Fig. 1 or those of the first embodiment shown in Fig. 4, and description is omitted for the sake of simplicity. In the image sensing apparatus of Fig. 7, lower-peak-level detecting circuits 41 to 43 are provided as black-level shifting means, and detect the lowest levels of individual RGB signals obtained by sensing an image on a negative film. Peak holding circuits 44 to 46 hold lower peak levels provided by the lower-peak-level detecting circuits 41 to 43. A comparing circuit 47 compares the output of the peak holding circuit 44 with the output of the peak holding circuit 45, while a comparing circuit 48 compares the output of the peak holding circuit 45 with the output of the peak holding circuit 46. For example, if the exposure amount E 0 of Fig. 5 is the maximum exposure amount of a negative film to be subjected to image sensing, the lowest levels of the respective RGB signals provided by a color signal processing circuit 3 is V R0 , V G0 and V B0 , and these levels are detected by the lower-peak-level detecting circuits 41 to 43 and held by the peak holding circuits 44 to 46. The comparing circuit 47 compares the value held by the peak holding circuit 44 with the value held by the peak holding circuit 45, while the comparing circuit 48 compares the value held by the peak holding circuit 45 with the value held by the peak holding circuit 46. The comparing circuits 47 and 48 generate level changes equivalent to V G0 -V R0 and V G0 -V B0 , respectively, thereby decreasing a clamping potential for the R signal and increasing a clamping potential for the B signal. In the above-described manner, the signal levels of the RGB signals equivalent to the maximum exposure amount E 0 are all set to V G0 , as shown in Fig. 6. As described above, in the second embodiment, the respective signal levels equivalent to the maximum exposure amount E 0 are detected and adjusted to reach an equal level, whereby a deviation in white balance is automatically minimized. Although the description of the second embodiment refers to white-balance adjustment which is performed when a positive image is to be produced from a general negative film by inversion, it is also possible to produce a negative image from a positive film by utilizing a negative mode selectable by the switching circuit 12 and the selecting switch 37. As is apparent from the foregoing description, in a case where color signals obtained by sensing an image on a negative film or the like are subjected to inversion processing, the black levels of the respective color signals can be shifted to highly accurately adjust the white balance of a plurality of color signals for any density region of the negative film. If the black-level shifting means is arranged in such a manner that clamping potentials which have been set in advance are switched from one value to another by means of the selecting switch, the construction of the apparatus can be simplified. If the black-level shifting means is arranged in such a manner that particularly at the time of color inversion, the lowest peak levels of a plurality of color signals are detected and the amounts of black-level shifts are set in accordance with the detection outputs, it is possible to automatically adjust white balance with high precision.

Claims: An image sensing apparatus comprising: color image sensing means (1-3) for converting an optical image into an electrical signal and generating a plurality of color signals; and inversion processing means (7, 8, 10; 8, 10, 11) for inverting colors carried by the plurality of color signals (R, G, B), respectively; being characterized by clamp-level shifting means (38-40; 41-43) for shifting clamp levels of the respective color signals by a predetermined amount in accordance with whether said inversion processing means is operated or not. An image sensing apparatus according to claim 1, characterized in that said clamp-level shifting means (38-40; 41-43) includes a clamping circuit (34-36). An image sensing apparatus according to claim 2, characterized in that said clamping circuit (34-36) includes a first clamping power source for supplying a first clamping potential and a second clamping power source for supplying a second clamping potential. An image sensing apparatus according to claim 1, characterized in that said inversion processing means (8, 10, 11) includes a first inverting circuit for inverting a polarity of a luminance signal and second and third inverting circuits for inverting polarities of R-Y and B-Y signals, respectively. An image sensing apparatus according to claim 1, characterized in that said color image sensing means (1-3) includes a charge-coupled device (1). An image sensing apparatus according to claim 2, characterized in that said clamping circuit (34-36) includes a feedback clamping circuit. An image sensing apparatus according to claim 1, characterized by further comprising exposure controlling means (20) for controlling an exposure amount of said color image sensing means (1-3), said exposure controlling means (20) being controlled in accordance with an output of said clamp-level shifting means (38-40; 41-43). An image sensing apparatus according to claim 1, characterized by first light source color detecting means (B1) for detecting a color of a source light by means of an external circuit (55); negative/positive inverting means (8, 10, 11); mode selecting means (12) for selecting a negative/positive inversion mode; and controlling means (56a) for controlling a white-balance adjusting operation of white-balance adjusting means (E) on the basis of a predetermined value if the negative/positive inversion mode is selected by said mode selecting means (12). An image sensing apparatus according to claim 8, characterized in that said predetermined value is switchable in accordance with the kind of negative film. An image sensing apparatus according to claim 8, characterized by second light source color detecting means (B2) for detecting a color of a source light from an output of said image sensing means (1-3); negative/positive inverting means (8, 10, 11); and mode selecting means (12) for selecting a negative/positive inversion mode, wherein a white-balance adjusting operation of white-balance adjusting means (E) being controlled by means of said second light source color detecting means (B2) if the negative/positive inversion mode is selected by said mode selecting means (12). An image sensing apparatus according to claim 10, characterized in that in a case where the white-balance adjusting operation of the white-balance adjusting means (E) is controlled on the basis of a predetermined value, the predetermined value is corrected on the basis of a detection value provided by said second light source color detecting means (B2). An image sensing apparatus according to claim 10, characterized in that if the negative/positive inversion mode is selected by said mode selecting means (12), a predetermined value for white-balance control is switched from one value to another in accordance with a detection value provided by said second light source color detecting means (B2). An image sensing apparatus according to claim 1, characterized by further comprising: inverting means (8, 10, 11) for inverting a polarity of an output of said image sensing means (1-3); and correcting means (38-40; 41-43) for correcting a clamp reference level of the output of said image sensing means (1-3) in accordance with an operation of said inverting means (8, 10, 11).

Inventors: TAKAYAMA TSUTOMU, YAMAZAKI YASUYUKI, TAKAYAMA, TSUTOMU, YAMAZAKI, YASUYUKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H04N9
IPC codes: H04N9, H04N5, H04N1