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Digital Camera Patent Abstract
A digital camera for determining the chromaticity coordinates and
the related color temperature of an object and the method thereof.
A photosensor receives the reflected light of the object and outputting
image data. An averaging unit outputs the red average, the green
average and the blue average according to the image data. A chromaticity
coordinate unit transforms the red average, the green average and
the blue average into tristimulus values using a transformation
matrix and outputs a chromaticity coordinate according to the tristimulus
values. A color temperature unit outputs the color temperature according
to the chromaticity coordinates.
Digital Camera Patent Claims
What is claimed is:
1. A method of utilizing a digital camera to obtain a set of chromaticity
coordinates and a related color temperature of an object, the method
comprising: utilizing the digital camera to capture an image of
the object; calculating a red average, a green average and a blue
average of the image according to the red, green and blue primary
colors of the image, respectively; utilizing a transformation matrix
to transform the red, green and blue averages into tristimulus values;
determining the chromaticity coordinates of the object according
to the tristimulus values; and determining the related color temperature
of the object according to the chromaticity coordinates.
2. The method according to claim 1, wherein the transformation
matrix is determined according a plurality of steps, the steps comprising:
determining a plurality of spectral sensitivity distribution curves
of the digital camera; generating a spectral distribution matrix
based on a plurality of samples obtained by sampling the spectral
sensitivity distribution curves; obtaining a plurality of samples
of a CIE color matching function to generate a stimulus matrix;
and calculating the transformation matrix according to the spectral
distribution matrix and the stimulus matrix.
3. The method according to claim 2, wherein the digital camera
comprises a plurality of pixel sensors, each of the pixel sensors
comprising a plurality of primary color sensors, the step of determining
the spectral sensitivity distribution curves comprising: generating
a single-frequency light; receiving the single-frequency light to
output separately a plurality of sensor values by the primary color
sensors of the digital camera; and generating the spectral sensitivity
distribution curves according to the sensor values and the frequency
of the single-frequency light.
4. The method according to claim 2, wherein the transformation
matrix is calculated according to the spectral distribution matrix
and the stimulus matrix using a least squares regression method.
5. The method according to claim 1, wherein the related color temperature
of the object is determined based on a point of a blackbody radiation
curve which is closest to the chromaticity coordinates.
6. A digital camera for determining a set of chromaticity coordinates
and a related color temperature of an object, comprising: a photosensor
for capturing an image of the object; an averaging unit for calculating
a red average, a green average and a blue average of the image according
to the red, green and blue primary colors of the image, respectively;
a chromaticity coordinate unit, utilizing a transformation matrix
to transform the red average, the green average and the blue average
into corresponding tristimulus values, and producing a set of chromaticity
coordinates according to the tristimulus values; and a color temperature
unit for producing the related color temperature according to the
set of chromaticity coordinates.
7. The digital camera according to claim 6, wherein the transformation
matrix is generated by a transformation matrix generator, the transformation
matrix generator comprising: a spectral distribution sensor to obtain
a plurality of samples of a plurality of spectral sensitivity distribution
curves for the digital camera so as to generate a spectral distribution
matrix; a sampling unit to obtain a plurality of samples of a CIE
color matching function so as to generate a stimulus matrix; and
a calculation unit for calculating the transformation matrix according
to the spectral distribution matrix and the stimulus matrix.
8. The digital camera according to claim 7, wherein the digital
camera comprises a plurality of pixel sensors, each of the pixel
sensors comprises a plurality of primary color sensors, the spectral
distribution sensor uses a monochromator to generate and send the
digital camera a single-frequency light, through which the primary
sensors of the digital camera based on for outputting a plurality
of sensor values, and to generate the spectral distribution curve
according to the sensor values and the single-frequency light.
9. The digital camera according to claim 5, wherein the calculation
unit of the transformation matrix generator is to calculate the
transformation matrix according to the spectral distribution matrix
and the stimulus matrix, using the a least squares regression method.
10. The digital camera according to claim 1, wherein the related
color temperature of the object is determined based on a related
color temperature point on a blackbody radiation curve that is closest
to the chromaticity coordinates.
Digital Camera Patent Description
[0001] This application claims the benefit of Taiwan application
Serial No. 093108930, filed Mar. 31, 2004, the subject matter of
which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates in general to digital cameras, and
more particularly to digital cameras for determining the chromaticity
coordinates and the related color temperature of an object.
[0004] 2. Description of the Related Art
[0005] When different observers look at the same object, their
perception of colors can vary while under the influence of the surrounding
light source and the color of the object itself. Because of these
variations, scientific standards have been developed to provide
a standard for describing colors and the related issues. According
to the trichromatic theory, a color stimulus as observed by an observer
can be formed by a combination of three primary colors. Thus, any
color stimulus can be expressed in terms of the intensity of the
three colors. FIG. 1 illustrates a color matching function determined
by the international commission on illumination (CIE). The color
matching function is used, under CIE standard observer system established
in 1931, to represent the formation of the three primary colors
X, Y and Z under different wavelengths, where X, Y, Z are also referred
to as the tristimulus values. The chromaticity coordinates x, y,
and z are then calculated respectively using the following formulae:
x=X/(X+Y+Z), (1-1)
y=Y/(X+Y+Z), (1-2)
z=Z/(X+Y+Z), (1-3)
[0006] wherein the sum of the three chromaticity coordinates x+y+z=1.
The value z can be found if x and y are known; thus, the chromaticity
coordinate systems usually only involve x and y coordinate axes.
Conventionally, a colorimeter is used to provide direct measurements
of the tristimulus values X, Y, and Z by using three broad-band
filters. The chromaticity coordinates x, y, and z are then found
using the corresponding formulae (1-1)-(1-3) for x, y, and z above.
[0007] During the research and development of digital cameras,
instruments such as colorimeters and correlated color temperature
meters are required to measure the relevant color information, such
as the chromaticity coordinates of an object and the related color
temperature of light reflected off the object, in order to incorporate
functions such as automatic white balance (AWB) and color correction
into the camera. However, due to the high prices of colorimeters
and correlated color temperature meters, it is not economically
feasible to provide each research engineer with a colorimeter and
a correlated color temperature meter.
SUMMARY OF THE INVENTION
[0008] It is therefore an object of the invention to provide a
digital camera for determining the chromaticity coordinates and
the related color temperature of an object, and a method thereof,
such that the aforementioned problems are solved.
[0009] The invention achieves the above-identified object by providing
a method of utilizing a digital camera to determine a set of chromaticity
coordinates and a related color temperature of an object. The first
step includes utilizing the digital camera to capture an image of
the object and to output image data. Then, the following step is
to respectively calculate a red average, a green average and a blue
average according to the red, green and blue primary colors of image
data. Next, the red, green and blue averages are transformed into
tristimulus values. Then, the next step includes determining the
chromaticity coordinates of the object according to the tristimulus
values. Then, the related color temperature of the object is determined
according to the chromaticity coordinates.
[0010] In addition, the invention achieves the other above-identified
object by providing a digital camera for determining a set of chromaticity
coordinates and a related color temperature of an object. First,
photosensor receives light reflected off the object and outputs
image data. Then, an averaging unit calculates a red average, a
green average and a blue average from the red, green and blue primary
colors of the image data. Then, a transformation matrix transforms
the three color averages separately into tristimulus values, and
outputs the chromaticity coordinates. Then, a color temperature
unit outputs the related color temperature of the object according
to the chromaticity coordinates.
[0011] Other objects, features, and advantages of the invention
will become apparent from the following detailed description of
the preferred but non-limiting embodiments. The following description
is made with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a graph showing a CIE color matching function.
[0013] FIG. 2 is block diagram showing a digital camera for determining
the chromaticity coordinates and the related color temperature of
an object according to a preferred embodiment of the invention.
[0014] FIG. 3 shows a flow chart illustrating the utilization of
a digital camera to determine the chromaticity coordinates and the
related color temperature of an object.
[0015] FIG. 4 shows spectral sensitivity distribution curves for
respective red, green and blue primary colors of a digital camera
according to a preferred embodiment of the invention.
[0016] FIG. 5 is CIE chromaticity diagram with a black body radiation
curve.3
DETAILED DESCRIPTION OF THE INVENTION
[0017] FIG. 2 shows a block diagram of a digital camera for determining
the chromaticity coordinates and the related color temperature of
an object according to a preferred embodiment of the invention.
The digital camera 200 includes a photosensor 210, an averaging
unit 215, a chromaticity coordinate unit 220, a color temperature
unit 230, and a display screen 240. Photosensor 210, such as charge
coupled device (CCD), includes a plurality of pixel sensors, each
including 3 primary sensors. The primary sensors are such as red,
green and blue sensors, for receiving light reflected off an object
and outputting image data. Averaging unit 210 receives image data,
and calculates a red average, a green average and a blue average
according to the red, green and blue primary colors of image data,
respectively. Chromaticity coordinate unit 220 utilizes a transformation
matrix to transform the red average, the green average and the blue
average separately into the corresponding tristimulus values, and
then utilizes the tristimulus values to output the set of chromaticity
coordinates to be displayed on display screen 240. The transformation
matrix is generated by a transformation matrix generator, which
includes a spectral distribution sensor, a sampling unit, and a
calculation unit. The spectral distribution is used to obtain a
plurality of samples of a spectral sensitivity distribution curve
for the digital camera so as to generate a spectral distribution
matrix. The sampling unit obtains a plurality of samples of a CIE
color matching function so as to generate a stimulus matrix. The
calculation unit is for calculating the transformation matrix according
to the spectral distribution matrix and the stimulus matrix.
[0018] Then, color temperature unit 230 calculates the related
color temperature according to the chromaticity coordinates, and
outputs the color temperature to be displayed on display screen
240.
[0019] FIG. 3 shows a flow chart illustrating the utilization of
a digital camera 200 to calculate the chromaticity coordinates and
the related color temperature of an object. The first step involves
utilizing the digital camera 200 to capture an image of the object,
and causing photosensor 210 of the digital camera 200 to output
data image, as indicated in step 310. Next, averaging unit 215 calculates
color averages (Rm, Gm, Bm) according to the red, green and blue
primary colors of image data, respectively, as shown in step 320.
Next, chromaticity coordinate unit 220 utilizes transformation matrix
M to transform color averages (Rm, Gm, Bm) into tristimulus values
(X,Y,Z), as shown in step 330, and transforms the tristimulus values
(X,Y,Z) into chromaticity coordinates (x,y,z) as shown in step 340
by using the above mentioned formulae (1-1), (1-2) and (1-3). Lastly,
color temperature unit 230 determines the related color temperature
of the object according to the chromaticity coordinates (x,y,z),
as shown in step 350.
[0020] Matrix M used in step 350 is to transform color averages
(Rm, Gm, Bm) of image data into tristimulus values (X,Y,Z), as shown
in below formula (2): 1 [ X Y Z ] = [ m 11 m 12 m 13 m 21 m 22 m
23 m 31 m 32 m 33 ] [ Rm Gm Bm ] ( 2 )
[0021] Matrix M is pre-determined according to spectral sensitivity
distribution of the digital camera and CIE color matching function.
The properties of photosensors vary from camera to camera, where
one of the properties of photosensors refer to their spectral sensitivity
distribution, which is an indicator for the sensitivity and responsiveness
of the digital camera under light with different frequencies. FIG.
4 shows spectral sensitivity distribution curves of a digital camera
for use in the embodiment. The spectral sensitivity distribution
curves shows the intensity of light detected by the red, green and
blue sensors of the digital camera over a range of light frequencies.
For example, the spectral sensitivity distribution curves of a digital
camera can be determined by successively emitting single-frequency
lights over a frequency range to the photosensors of the digital
camera and obtaining the intensity of corresponding electrical signals.
With respect to human visual perception, spectral sensitivity distribution
curves can thus be determined by performing the above described
method over the human visual perceptual range from 400 to 700 nm.
In addition, signal-frequency light can be generated using turnable
monochromators.
[0022] After the determination of the spectral sensitivity distribution
curves, a transformation matrix M can be obtained according to a
stimulus matrix C and a spectral distribution matrix S. For example,
by sampling each of the curves for the primary colors at a rate
of 10 nm, a total of 31 samples with respect to a primary color
are obtained. Hence, a 3.times.31 spectral distribution matrix S
is obtained for the three primary colors: 2 S = [ R G B ]
[0023] Similarly, a 3.times.31 stimulus matrix C is determined
by sampling the CIE color matching function, and C is as the following:
3 C = [ X Y Z ]
[0024] Stimulus matrix C is the dot product of spectral distribution
matrix S and transformation matrix M (C=S.multidot.M). A 31.times.31
transformation matrix M can thus be calculated mathematically according
to a spectral distribution matrix S determined experimentally from
the digital camera, and a stimulus matrix C determined from above
method. For example, the transformation matrix M can be determined
by using the least squares regression method to minimize .parallel.SM-C.parallel..sub.F
such that M=(S.sup.TS).sup.1S.sup.TC is desired.
[0025] In step 340, after formulae (1-1), (1-2) and (1-3) are applied
to transform tristimulus values (X,Y,Z) into chromaticity coordinates
(x,y,z), the set of values (x,y,Y) can be displayed on camera display
screen, and the color temperature can be calculated in step 350
according to (x,y).
[0026] In step 350, the related color temperature of the object
is determined according to a blackbody radiation curve. FIG. 5 is
the CIE chromaticity diagram with a blackbody radiation curve, where
every point on the black radiation curve represents a color and
the color temperature of the color is also indicated on the curve.
A point on the blackbody radiation curve which is closest to the
chromaticity coordinates (x,y) obtained in step 340 is to be determined.
The corresponding color temperature of this point is then taken
as the related color temperature of the object, and displayed on
the display screen of the digital camera.
[0027] The embodiments of the invention above discloses the digital
camera for determining chromaticity coordinates and related color
temperature of an object and the method thereof, in which the digital
camera can be manufactured with low costs. Thus, as compared to
the conventional use of expensive colorimeters and correlated color
temperature meters, using the digital camera under the invention
can result in economic efficiency.
[0028] While the invention has been described by way of example
and in terms of a preferred embodiment, it is to be understood that
the invention is not limited thereto. On the contrary, it is intended
to cover various modifications and similar arrangements and procedures,
and the scope of the appended claims therefore should be accorded
the broadest interpretation so as to encompass all such modifications
and similar arrangements and procedures.
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