White balance correcting device for an image sensing apparatus

A white balance correcting device for an image sensing apparatus comprising gain control circuits for different colors positioned in respective signal lines for the colors from an image sensor to control the gains of the signal lines individually, a control signal supply circuit for supplying all of the gain control circuits with respective control signals for giving them control characteristics limited by different color temperatures from each other, wherein this control signal supply circuit has a color temperature sensor for detecting the color temperature of the outside, and the color temperatures by which the gains of the gain control circuits are limited are controlled in accordance with the output of the color temperature sensor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a white balance correcting device for video 
cameras, electronic still cameras or other image sensing apparatuses. 
2. Description of the Related Art 
FIG. 1 is a block diagram illustrating an example of construction of the 
automatic white balance correcting device for a video camera disclosed in 
U.S. patent application Ser. No. 395,102 filed on May 31, 1989. The 
circuitry of the video camera comprises an image sensor 1, a luminance and 
chrominance signal forming circuit 2, gain control circuits 3 and 4 
inserted into signal lines for red color R and blue color B, a 
color-difference signal forming circuit 5, an encoder 6, gate circuits 7 
and 8, an (R-B) signal detecting circuit 9, an averaging circuit 10, a 
comparison amplifier 11, a limit circuit 12 and a tracking correction 
circuit 13. The gain control circuits 3 and 4 and the parts beginning with 
the gate circuits 7 and 8 and terminating at the tracking correction 
circuit 13 constitute an automatic white balance correcting device 14. 
Next, the operation of the camera of this character is described. 
A photo-signal entering the image sensor 1 is photoelectrically converted 
and is taken out as an electrical signal which is supplied to the 
luminance and chrominance signal forming circuit 2 where a high-frequency 
component Y.sub.H of a luminance signal, a low-frequency component Y.sub.L 
of the luminance signal, a red color signal R and a blue color signal B 
are formed. The red and blue color signals R and B are supplied to the 
respective gain control circuits 3 and 4, where they are amplified 
individually in accordance with the characteristics controlled by 
respective control signals output from the tracking correction circuit 13. 
After this, they are produced in another form of color signals R' and B' 
which are then supplied together with the aforesaid low-frequency 
luminance signal Y.sub.L to the color-difference signal forming circuit 5, 
where color-difference signals (R-Y.sub.L) and (B-Y.sub.L) are formed. 
These color-difference signals (R-Y.sub.L) and (B-Y.sub.L) are supplied 
together with the aforesaid high-frequency luminance signal Y.sub.H to the 
encoder 6, where a standard television signal is formed and output. Here, 
the aforesaid color-difference signals (R-Y.sub.L) and (B-Y.sub.L) are 
also supplied to the automatic white balance correcting device 14. 
That is, the aforesaid color-difference signals (R-Y.sub.L) and (B-Y.sub.L) 
are supplied respectively to the gate circuits 7 and 8, where the 
unnecessary signals in the blanking period and the abnormal 
color-difference signals due to the signal collapse at the time of high 
brightness photography are removed. 
The signals output from the gate circuits 7 and 8 are supplied to the (R-B) 
signal detecting circuit 9. Here, by obtaining a difference between the 
outputs (R-Y.sub.L)' and (B-Y.sub.L)', an (R-B) signal is detected. In the 
averaging circuit 10, the (R-B) signal output from the (R-B) signal 
detecting circuit 9 is averaged, thus being converted to a DC signal. In 
the comparison amplifier 11, the signal output from the averaging circuit 
10 is compared with a reference voltage Vref1. A signal representing the 
result of this comparison is output to the limit circuit 12. In the limit 
circuit 12, the signal output from the comparison amplifier 11 is limited 
to range between a lower limit and an upper limit for color temperatures 
set at voltages V2r and V3r, so that the white balance is controlled 
within an actually acceptable color temperature range (for example, 
2000.degree. K.-10000.degree. K.). Therefore, the output of the limit 
circuit 12 lies above the voltage V2r and below the voltage Vr3. 
The output of the limit circuit 12 is supplied to the tracking correction 
circuit 13. In the tracking correction circuit 13, based on the signal 
output from the limit circuit 12, signals Rcont and Bcont for controlling 
the gains of the aforesaid gain control circuits 3 and 4 so as to correct 
the white balance are formed and output, to the gain control circuits 3 
and 4 respectively. 
Here, an example of the relationship of the signals Rcont and Bcont with 
the color temperature is described by using FIG. 2 and FIG. 3. 
In FIG. 3, a point of white color for a color temperature 6000.degree. K. 
is denoted by P1, and points of white color for color temperatures 
2000.degree. K. and 10000.degree. K. are denoted by P2 and P3 
respectively. As the voltages of the signals Rcont and-Bcont at the point 
P1 have values V1r and V1b, the required voltages of the signal Rcont and 
Bcont for controlling (correcting) the point P2 to move to the center of 
the vector diagram of FIG. 3 take values V2r and V2b respectively in FIG. 
2. Likewise, in the case of the point P3, they take values V3r and V3b 
respectively. 
Yet another point P4 in FIG. 3 is, however, not brought up to the center P1 
of the vector diagram even when the white balance is corrected, because 
the signals Rcont and Bcont are limited to the voltages V3r and V3b. 
Since, in such a manner, the negative feedback loop of the automatic white 
balance correcting device operates, for color temperatures in the actually 
acceptable range, the color-difference signal to be supplied to the 
encoder 6 can take good white balance. 
With the above-described arrangement, however, if, an object to be 
photographed has uneven color temperature distribution, there is a 
one-sided trait in its distribution, an error is produced in the white 
balance correction. Hence, there is a problem that the right correcting 
effect is not obtained. This problem is explained by taking an example of 
50% of white color and 50% of blue color. When the gain control circuits 3 
and 4 each have a gain factor of one, the white color and the blue color 
on the vector diagram have their points W0 and B0 in FIG. 4 respectively. 
Also, the outputs Rcont and Bcont of the tracking correction circuit 13 are 
assumed to lie in that relationship with the color temperatures which is 
shown in FIG. 2. 
Since, in this relationship, the operation of the negative feedback loop 
results in the (R-B) signal becoming zero, the correcting direction of the 
white balance is parallel to the R-B axis on the vector diagram shown in 
FIG. 4 or FIG. 7. Therefore, the points to which this negative feedback 
operation is stabilized are, when the limit circuit 12 does not work, 
found from B0,B1=B1,a=W0,W1 to be at B1 and W1 (where the line segment 
B1,a is parallel to the R-B axis and the point "a " lies on a line segment 
passing through the original point W0 and perpendicular to the R-B axis). 
But, because the limit circuit 12 operates, it is in reality that they 
come to points B2 and W2. In other words, the blue color and the white 
color, though, before subjected to correction, lying at points B0 and W0, 
change their coordinates to points B2 and W2 respectively after they have 
been corrected. 
This means that when, for example, the blue sky is photographed, the sky 
image gets a magenta tint as indicated close to the point B2. To avoid 
this, it may be considered to narrow the limit width of the limit circuit 
12. If it is so made, the shade portion of a white object looks in a blue 
tint. Like this, alternative drawbacks are produced. 
SUMMARY OF THE INVENTION 
The present invention has been made to eliminate such problems and its 
object is to provide a white balance correcting device for an image 
sensing apparatus which enables the visual sensation of the error of 
correction to be minimized for good white balance. 
To achieve this object, in an embodiment of the invention, the gain control 
means for individual colors inserted in the respective signal lines for 
the colors have their control characteristics given different limits in 
respect to color temperature from each other. In more detail, the white 
balance correcting device for the image sensing apparatus is constituted 
in the following ways (1) and (2): 
(1) Use is made of gain control means for individual colors put in the 
signal lines for the different color signals output from the image sensor 
to control the gains of the different signal lines individually in 
combination with control signal supply means for producing control signals 
which give the gain control means for individual colors the respective 
individual control characteristics limited by the different color 
temperatures. 
(2) In the aforesaid way (1), the control signal supply means has a color 
temperature sensor for detecting the color temperature of the outside, 
wherein the limits in color temperature of the gain control means for each 
color is controlled in accordance with the output of the color temperature 
sensor. 
And, with the use of the above-described ways (1) and (2), the gains of the 
signal lines for all colors become limited by different color temperatures 
from one another, and further, according to the way (2), the limit in 
color temperature is varied as a function of the sensed color temperature 
of the outside.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention is next described in connection with embodiments thereof. 
FIG. 5 in block diagram shows a first embodiment of an automatic white 
balance correcting device for an image sensing apparatus according to the 
invention, where the parts of the same reference characters as those of 
FIG. 1 are the same or equivalent parts and their explanation is so 
omitted here. 
In FIG. 5, a limit circuit 12r for the signal Rcont and another limit 
circuit 12b for the signal Bcont are employed. Voltages Va and Vb are 
applied to the limit circuit 12b to determine the limit voltages for the 
signal Bcont. Likewise, voltages Vc and Vd are applied to the limit 
circuit 12r to determine the limit voltages for the signal Rcont. 
In this circuit arrangement, the signals Rcont and Bcont are related to 
color temperatures by a solid line in FIG. 6. In FIG. 6, Va=V2b, Vb=V3b, 
Vc=V2r, and Vd is set at a value lower than V3r (where V2b, V3b, V2r and 
V3r are the same as shown in FIG. 2). 
Under the condition that this relationship is established, the white 
balance is corrected in the following way. Referring to FIG. 7, when an 
object of 50% white and 50% blue is photographed, according to the example 
of FIG. 1, the point W0 changes to the point W2 by .DELTA.r2 and the point 
B0 also changes to the point B2 by .DELTA.r2, as has been described above. 
But, in the present embodiment, the signal Rcont, because of being clipped 
below the voltage Vd, is corrected in the R direction by only .DELTA.r1. 
Hence, the point W0 changes to a point W3 and the point B0 changes to a 
point B3. Therefore, the amount of correction in the R direction gets 
smaller than that in the example of FIG. 1, thus making it possible to do 
good correction in visual sensation of the white balance. 
Next, a second embodiment of the invention is shown in FIG. 8. Reference 
numeral 15 in FIG. 8 denotes a color temperature sensor for detecting the 
color temperature of the outside. Depending on the information from this 
sensor 15, determination of the limit widths of the limit circuits 12r and 
12b are made with an advantage that a finer correction than in the first 
embodiment can be done. For example, when the color temperature sensor 15 
has detected a low color temperature (say, 3000.degree. K. or thereabout), 
the allowable dynamic range of the limiter is adjusted to 2000.degree. 
K.-4000.degree. K., while when a high color temperature (say, 7000.degree. 
K. or thereabout) has been detected, the range is altered to a range of 
6000.degree. K.-8000.degree. K. Hence, an optimum color reproducibility 
can be obtained. 
Though any of the foregoing embodiments has been described by taking an 
example of using the primary color filters in the image sensor, the 
invention is not confined to this. It is to be understood that the 
invention is applicable also to the case of using the complementary color 
filters. Further, it is needless to say that the invention is applicable 
to the manual white balance correcting device. 
As has been described above, the invention, when correcting the white 
balance, makes the gain control means for the different chrominance 
signals operate with such control characteristics as to be limited by 
different color temperatures from each other, thus assuring good 
correction of the white balance on the visual sensation.