Method and apparatus for correcting the white balance of a video color picture signal

The color quality of a received color picture signal is detected by a device for measuring the intensity of at least the red component and the green component of the picture signal. A microprocessor determines a correction value for the white balance correction on the basis of the detected color quality and corresponding to selectable correction values, stored in a memory, for each color component. The method and apparatus permit an application specific correction of the white balance, particularly for use in endoscopic systems, such as video endoscopes and endoscope cameras, to be used in different body organs which cause different color shifts.

FIELD OF THE INVENTION 
The invention pertains to a method and a device for the correction of the 
white balance of a video color picture signal in which the color quality 
of a received signal is first detected. 
BACKGROUND OF THE INVENTION 
In the past, the primary field of usage of endoscopy was in diagnostics. In 
diagnosnics the doctor observes with the naked eye the hollow organ to be 
examined. The eye possesses high brightness and color dynamics, which 
results in the fact that the attainable imaging quality, with respect to 
resolution and color representation, is almost entirely dependent on the 
endoscope that is being used. 
In surgery, however, other requirements are placed on an endoscopic system 
different from those in diagnostics. In the case of a surgical operation, 
the picture of the operating field must be available to the entire 
operating team, since several people carry out the surgical operation in 
cooperation. 
As a result, observing the operating field with the naked eye through an 
endoscope is not adequate. For that reason, a video camera is mounted on 
the proximal end of the endoscope, or else a video endoscope with a 
distally mounted CCD sensor is used, so that the operating field can be 
shown on a monitor. With the use of CCD cameras, however, several problems 
arise as well, since a CCD sensor does not possess the dynamics, the 
sensitivity, and the spectral characteristics of the eye. This is 
especially important with regard to the representation of color. According 
to one possibility it is worthwhile to try to optimize the representation 
of color from the point of view of the color variations that occur in an 
endoscopic system. The optimization should be carried out as automatically 
as possible, so that the operating team can concentrate on the operation, 
and not have to concern itself with the technology. 
In the case of an endoscopic examination of a hollow organ of the body, the 
endoscope is introduced into the body cavity or the hollow organ to be 
examined. Illumination light makes its way through light guide fibers in 
the endoscope and into the body cavity, where it illuminates the hollow 
organ. In order to obtain a true reproduction of the color, the 
illumination light should not be spectrally influenced by the hollow 
organ. However, depending on the nature of the hollow organ, that is not 
the case. A portion of the illumination light penetrates, for example, 
into the mucous membrane inside the hollow organ, and this membrane acts 
as an absorption filter. As a result, the mucous membrane acts in turn as 
a narrow-band spectral illumination source, since the penetrating 
illumination light is re-radiated in a filtered fashion from the surface 
of the mucous membrane, and in turn illuminates with red light the object 
to be observed. 
This light absorption behavior differs, depending upon the hollow organ 
being examined. Thus, the light absorption behavior plays no role in the 
joint area, whereas in the stomach, where mucous membranes that are 
heavily supplied with blood can be found, it leads to a color shift in the 
red direction. For any given organ, the strength of the red shift depends 
upon the observation distance and the observation and illumination angle 
of the endoscope. The shorter the observation distance or the closer the 
endoscope is to the mucous membrane, the more strongly the mucous membrane 
is trans-illuminated, which leads to a stronger red shift. 
If the angle of observation or illumination becomes greater, then the 
intensity of the illumination light becomes greater in the edge areas. In 
this case, the mucous membrane is strongly trans-illuminated there. This 
effect occurs in a particularly severe way in gastroscopy and coloscopy, 
for which the viewing and illumination angle amounts to at least 
120.degree.. In such cases, tube-like hollow organs (i.e., esophagus, 
intestines) are being examined in which the mucous membrane is located 
very close to the instrument. In combination with the large viewing and 
illumination angle, this leads to a very large red shift in these places. 
However, this application-specific color shift is not taken into account 
by industrial video cameras. 
U.S. Pat. No. 5,111,281 describes a color correction device for a video 
endoscope, having means for detecting the color quality of a color picture 
signal and means for the pixel-by-pixel execution of a dynamic color 
correction. Since the correction is carried out pixel-by-pixel, the known 
device is not able to distinguish between strong colors, red in 
particular, that occur in a point-like fashion, and increases in color 
that affect the entire picture. As a result, the known device also 
corrects increases in color that occur in a point-like fashion, and thus 
has poor color differentiation. 
In U.S. Pat. No. 4,831,437 a video endoscope is described which is provided 
with a device for color balance. By means of this device, however, only 
the static color variations, such as those caused by light guide cable, 
endoscope, and illumination source, can be regulated in the endoscopic 
system. This means that dynamic color variations, such as those which can 
be caused by the mucous membrane during an operation, are not compensated 
for. 
In addition, U.S. Pat. No. 4,831,437 shows a specialized device for 
carrying-out an automatic white balance which eliminates the color error 
caused by the ambient light. Spectral differences of the components being 
used in the endoscopic system, such as light guide, light source and 
endoscope, are also compensated for. For the white balance, the device has 
a balloon-shaped balance accessory with a white coating on the inside, 
through the opening of which the endoscope is inserted. It is thereby 
ensured that the ambient light that disturbs the balance does not mix with 
the illumination light. As a result, a white field of observation located 
in front of the endoscope is also shown in white on the monitor. If, 
however, a hollow organ is examined with the endoscope after the white 
balance has been carried out, a shift in the red direction, results from 
the mucous membrane. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a method and an 
apparatus for correction of the white balance of a video color picture 
signal which can ensure a maximum color differentiation of the color 
picture and can compensate for a specific red shift in the color picture 
signal. 
A method for correcting the white balance of a video color picture signal 
which accomplishes the above-mentioned object comprises the following 
steps: 
a) detecting a color quality of the color picture signal; 
b) determining correction values for correction of the color picture signal 
present after the white balance, for each individual color component in 
dependency upon pre-set operating conditions; 
c) correcting the color picture signal corresponding to the white balance 
by means of the correction values determined in step b), in dependency 
upon a relationship of the red component detected in step a) to the 
brightness portion of the detected color quality of the color picture 
signal. 
Preferably, several sets of correction values that correspond to the 
operating conditions are stored ahead of time in a storage step, whereby 
in step b) the correction values that correspond to these sets are read 
from a storage medium. 
For optimization of the color differentiation, the method in accordance 
with the invention additionally includes an integration step d) for 
integration of at least a portion of the color components of the video 
color picture signal over one field period thereof. Preferably, the 
integration step d) integrates only red color components R and green color 
components G, and the correction step c) corrects the color picture signal 
following the white balance, in dependency upon the relationship R/(R+G). 
It is expedient if the sets of correction values stored in the 
above-mentioned storage step, or several correction value functions stored 
ahead of time, are read in dependency on the relationship R/(R+G). 
In conjunction with this, the stored sets of correction values or 
correction value functions can be linearly or non-linearly dependent upon 
the relationship of the red component to the brightness portion of the 
color picture signal. 
The method in accordance with the invention can also be used, in 
particular, for the correction of the white balance of video color picture 
signals with video endoscopes or endoscope cameras. 
The apparatus in accordance with invention for the correcting the white 
balance of a video color picture signal includes a detection device for 
detecting the color quality of a received color picture signal, and is 
characterized by: 
Means for determining correction values for correction of the white balance 
for each individual color component in dependency upon pre-set operating 
conditions, and 
Correction members for correcting the color components of the color picture 
signal corresponding to the white balance, by means of the determined 
correction values in dependency upon a relationship of the red component 
to the brightness portion corresponding to the color quality detected by 
the detection device. 
The detection device has members for measuring the intensity of the red 
component R and the green component G of the color picture signal, and 
integration and retention members for integration and retention of the 
intensity value of both the red component R and the green component G over 
one field period of the received color picture signal. 
The correction device includes an arithmetic unit for calculating an 
approximated brightness signal, both from the red component R that was 
integrated and retained over one field period by the integration and 
retention members and from the green component G that was integrated and 
stored over one field period, for calculation of a quotient R/(R+G) from 
an instantaneously measured intensity of the red component and the 
brightness signal and for determination of the correction value. The 
arithmetic unit may comprise, for example, a microprocessor. The latter 
can have a write-read memory as the storage means. 
This write-read memory stores several pre-set sets of correction values 
dependent upon the quotient R/(R+G) and upon the pre-set operating 
conditions in the form of look-up tables, whereby the microprocessor 
reads, from the look-up table that corresponds with the current operating 
conditions, the appropriate correction value on the basis of the quotient 
R/(R+G) calculated from the instantaneously measured red component R and 
brightness signal R+G. 
Functionally connected with the input of the microprocessor is a control 
unit for selection of one of several sets of correction values stored in 
the write-read memory or of one of several correction value functions 
dependent on the quotient of the instantaneous red component R and the 
approximately calculated brightness signal R+G. For generation of a DC 
voltage corresponding to the respective color components, the 
microprocessor is connected on the output side to correction voltage 
generators, one of which is allocated to each of the color components and 
receives from the microprocessor the respective selected correction value 
for the particular color component. Each correction voltage generator has 
a digital-analog converter or a digital potentiometer, which generates an 
analog DC correction voltage. 
In addition, the output of each digital analog converter generating the 
respective correction voltage is connected with the input of a 
voltage-controlled amplifier, from which a different input is connected 
with a particular output of a white balance device, which is in turn 
functionally coupled with the control unit for carrying-out an automatic 
white balance. The outputs of the three voltage-controlled amplifiers each 
lead to inputs of a matrix circuit for generation of component signals.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
In FIG. 1 video camera output signals R, G, and B are present at the input 
of a device 17 for automatic white balance. The device 17 is known per se, 
and is described in EP-0530738A, for example. It is controlled by a 
control unit 21. When a white balance button is actuated, an automatic 
white balance is carried out, as will be described in more detail later. 
The balanced signals, that is the color components R, G, and B, proceed 
from the output of the device 17 respectively to an input of 
voltage-controlled amplifiers 18a, 18b and 18c, which are designated "VCA" 
in the following. The control inputs are each connected with a correction 
voltage generator 22b, 22c, 22d. With the aid of the VCAs 18a, 18b and 18c 
and the correction voltage generators 22b, 22c and 22d, the white balance 
can be corrected in an application-specific manner, in order to compensate 
for the color shift that occurs in the hollow organ of the body as a 
result of the mucous membrane. The VCAs 18a, 18b and 18c are triggered by 
the correction voltage generators 22b, 22c and 22d, respectively, with a 
correction voltage U.sub.RW, U.sub.GW, U.sub.BW. The level of 
amplification of the VCAs is adjusted with the aid of these voltages. The 
thus-corrected signals RW, GW, BW are present at the input of a matrix 19, 
and are there converted into the component signals Y, U, V there. A 
modulator 20 connected behind the matrix 19 converts the component signals 
Y, U and V into the brightness signal H, the chromatic signal C and the 
video signal (video). The matrix 19 and the modulator 20 are common 
integrated complex components. 
As shown in FIGS. 1 and 2, the detection device 22 receives at its input 
the red color component R and the green color component G of the color 
picture signal, along with a synchronization pulse V.sub.sync. In 
accordance with the invention, the correction device, as it is shown in 
FIGS. 1 and 2, carries out a dynamic white balance correction. The 
detection device measures the red portion and the green portion of the 
color picture and forms therefrom a signal that corresponds to the color 
quality, such that the measured red portion is set relative to a measured 
brightness portion of the picture. If the quotient of the red portion and 
the brightness portion exceeds a certain value, then it can be assumed 
that an increased red portion is present which is unusually large in a 
picture, that is a color shift in the red direction caused by the mucous 
membrane of the hollow organ of the body. 
In the embodiment, only the red portion and green portion of the color 
components are measured. The green portion of the picture contributes most 
to the brightness signal H of the color picture, the blue portion least. 
The brightness signal H is composed of the color components R, G and B in 
the following way: 
EQU H=0.30R+0.59G+0.11B. 
This equation results from the eye sensitivity curve for a light-adapted 
eye. Since the blue portion thus contributes least to the brightness 
signal, only the red portion R and the green portion G are measured, and 
the blue portion B is ignored. 
The device 22 includes an arithmetic unit with a microprocessor 22e. The 
latter is functionally connected to the control unit 21, which is shown 
schematically in FIG. 2 by means of the buttons or keys 21a, . . . 21n. On 
these keys, n different operation- or application-specific settings can be 
selected, and, in addition, the color setting can be manually adjusted. 
Depending upon the setting selected, the microprocessor 22e calculates or 
reads from a table, which is stored in a write-read memory 22a, the 
correction values which correspond to the current application or operating 
condition for each color component, and which are output to the correction 
voltage generators 22b, 22c and 22d, which act as the digital-analog 
converters. 
This write-read memory 22a can be located in an endoscope, for example, and 
contains, in addition to balance data for the video endoscope, an 
identifier by means of which the field of use of the endoscope, and thus 
also the optimum white balance function therefor, can be specified and 
automatically switched to a correction value function stored in the 
write-read memory 22a of the microprocessor 22e. In the write-read memory 
22a, can be stored, in addition, data which do not relate to the optimized 
correction values for the instrument being used at present. 
FIG. 3 shows several possible functions which can be stored in the 
write-read memory 22a for the correction value k in dependency upon the 
relationship of the red portion to the brightness portion of the color 
picture signal, more specifically the quotient R/(R+G). 
It is expedient if each color is allocated its own correction value 
function, even though FIG. 3 only shows one curve for each color. The 
dotted straight line 32 corresponds to a static white balance correction, 
that is, the correction value k' is constant and is not dependent on the 
relationship R/(R+G). The use of such a white balance correction would 
have the consequence that even a picture with a small red portion would 
also undergo a white balance correction, and would thus exhibit a color 
cast outside of the operating field, or, even in the case of a long 
observation distance, where there was no red shift. 
The straight line 33 shows a linear dynamic function as it can be used as 
an application-specific correction function by the correction device in 
accordance with the invention. Here, the correction value k increases in a 
linear fashion with an increasing ratio of red portion/brightness portion, 
or with the increasing ratio R/(R+G), of the picture. This correction 
value function is particularly advantageous, since in the case of a longer 
observation distance, the mucous membrane is not so strongly 
trans-illuminated, and thus the red shift is smaller than in the case of a 
shorter observation distance where, depending on the application, a strong 
red shift occurs. The white balance is thus corrected only as strongly as 
a shift in the red direction occurs. 
In contrast with that, the functions 34 and 35 exhibit a pronounced 
non-linear behavior. This has the consequence that small red portions, 
which, for example, are not caused by a red shift, are tolerated in the 
picture; that is, they bring about no correction of the white balance, or 
only a small one. Stronger red portions, such as are brought about by a 
red shift that results from the mucous membrane, cause a contrastingly 
stronger white balance correction than in the case of linear, dynamic 
behavior. While the curve 34 grows continuously from a specific 
relationship R/(R+G), the curve 35 has an asymptotic approximation of a 
constant correction value; for example, in accordance with FIG. 3, of the 
correction value k', which corresponds to the axis-parallel straight line 
32. This has the consequence that, in the case of larger red shifts, a 
white balance correction is carried out with an almost constant correction 
value k'. 
However, it should be expressly mentioned that FIG. 3 only represents 
examples of the functions of the correction value k. Which correction 
value function is ultimately stored or selected depends upon the nature of 
the hollow organ being examined, and on the selection subsequently carried 
out at the control unit 21, or more specifically its keys 21a, . . . 21n 
by the examining person. Thus, it is entirely possible for other dynamic 
curves of the correction value k to be stored in the write-read memory 22a 
as well. 
The microprocessor 22e thus generates the correction value k for the color 
signals RW, GW and BW in dependency upon the measured quotient R/(R+G) and 
on the correction value function selected from the write-read memory (FIG. 
3), while the microprocessor 22e generates the respective correction 
values on the basis of the look-up tables corresponding to the correction 
value functions stored in the write-read memory. This is significantly 
faster than a calculation. 
For the detection of the quotient R/(R+G), the video signals R and G are 
present at the input of the integration and retention members 22h, 22i, 
and are integrated over one field period; that is over 20 ms. With the 
negative flank of the vertical synchronization pulse V.sub.sync, the 
voltage integrated over a field period is stored at the output of the 
integration and retention members 22h, 22i. These voltages then correspond 
to the intensity of the red portion and green portion of the color 
picture. The positive flank of the vertical synchronization pulse 
V.sub.sync reduces the integrator of the integration and retention members 
22h, 22i and triggers an interruption at the microprocessor 22e. The 
microprocessor 22e switches a multiplexer 22g at the output of the 
integration and retention member 22i. As a result, the voltage of the red 
channel is present at the input of an analog-digital converter 22f and is 
digitized there. This digitized value is temporarily stored in the 
microprocessor 22e. With the aid of the newly detected red portion and 
with the aid of the green portion which is one field period older, an 
approximate value of the brightness signal is calculated in accordance 
with the sum R+G. Thereafter the quotient of the new red portion is 
calculated in accordance with R/(R+G). 
According to this embodiment, on the basis of the quotient R/(R+G) 
calculated in this way, the microprocessor 22e reads the white balance 
corrections selected at the control unit 21 from the look-up table in the 
write-read memory 22a. Alternatively, the microprocessor 22e can 
calculate, by means of arithmetic operations, the correction values k that 
correspond to the quotient R/(R+G) for each color. However, this requires 
a longer computation time than the time needed for reading from the 
look-up table. 
With the next vertical synchronization pulse V.sub.sync, the described 
read, integration and conversion process is repeated in the same way, 
except that now only the green channel G is read in. To do this, the 
multiplexer 22g is switched over to the integration and retention member 
22h. With the aid of the new green portion and with the aid of the red 
portion which is now one field period older, an approximation value is 
again calculated for the brightness portion; that is the sum R+G. 
Subsequent to the calculation of the quotient R/(R+G) and making use of 
the selected white balance correction function, the correction values k 
are again determined for each color once, or are read from the correction 
value function look-up tables stored in the write-read memory. It is thus 
sufficient to measure the intensity of a color component R or G and to 
read it into the microprocessor once per field period, since within a 
field period of 20 ms, the color integrated over a field period only 
changes by fractions. 
It would also be conceivable to measure the R portion and the G portion 
after each frame period; that is, to read the R channel and the G channel 
into the microprocessor 22e and to calculate the correction value k again 
from that. The load on the microprocessor 22e would thereby be reduced, 
since it would have to carry out only half as many operations per unit of 
time. However, this would have the consequence that the correction voltage 
generators 22b, 22c, 22d are reset into a 25 Hz rhythm, and would no 
longer be in a 50 Hz rhythm. In extreme cases, this can lead to a flicker 
effect, since 25 Hz pictures lie close to the threshold of perception of 
the human eye. 
Preferably, therefore, the red components and the green components are 
detected, and the digital value corresponding to their intensity is read 
into the microprocessor 22e, displaced from each other by one field 
period. 
The correction voltage generators 22b, 22c and 22d represent digital-analog 
converters or digital potentiometers. They receive from the microprocessor 
22e the digital correction values k that correspond to the individual 
color components and convert them into the correction voltages U.sub.RW, 
U.sub.GW, U.sub.BW which are directed to the control inputs of the VCAs 
18a, 18b and 18c. 
In addition to the selection of the correction value functions by pressing 
the keys 21a, . . . 21n of the control unit 21, the examining person who 
is handling the endoscope can also manually change the 
application-specific correction voltages to values that do not correspond 
to the values in the write-read memory 22a. By pressing one of the keys 
21a, . . . 21n, this manual change can also be canceled again, so that the 
correction values k corresponding to the preset functions are again 
generated at the output of the microprocessor 22e. 
Shown in FIG. 4 is an auxiliary device for the white balance carried out 
with the white balance device 17 shown in FIG. 1. With the auxiliary 
device, a white balance known as EWC (Endo White Control) is performed. In 
addition, the spectral differences of the components being used in the 
endoscopic system, such as light guides, light source and endoscope, are 
compensated for. The endoscope 10 is inserted through an opening into a 
balloon-shaped balance accessory 16 with a white coating on the inside. In 
this way, it is ensured that ambient light 4 that disturbs the balance 
cannot mix with the illumination light and lead to a false result. With 
the balance aid 16 and the white balance device 17 as it is known from 
EP-0530738A, the difference in the components being used in an endoscopic 
system can now be compensated for. As a result, a white field of 
observation located in front of the endoscope is also shown in white on 
the monitor. 
It will be appreciated by those skilled in the art that changes could be 
made to the embodiments described above without departing from the broad 
inventive concept thereof. It is understood, therefore, that this 
invention is not limited to the particular embodiments disclosed, but it 
is intended to cover modifications within the spirit and scope of the 
present invention as defined by the appended claims.