Vignetting compensation
View Patent ↗A method used for the compensation of vignetting in digital cameras has been achieved. The compensation for vignetting is done in two steps. The first step is done during production of the camera unit and involves taking and analyzing an image of a test screen, preferably a gray test screen. This results in a set of e.g. 5×5 coefficients describing a polynomial surface. The second step is done for each image that is taken by the camera and involves calculating and applying a gain-table based on polynomial coefficients to all pixels of the image to compensate vignetting.
1. A method to compensate vignetting in digital cameras comprising:
providing a read-Only memory (ROM), a lens system, a processing device, a RAM, a test chart, an array of pixel-based sensors, and a camera unit;
take a picture of said test chart during production of said camera;
analyze said test image in regard of brightness of pixels independently of color sensitivity of said pixels during production of said camera;
fit test image with a polynomial surface function using said brightness during production of said camera;
insert the coefficients of said polynomial function in a matrix during production of said camera;
store said matrix in said read-Only memory (ROM) during production of said camera;
retrieve said matrix from said read-Only memory (ROM);
construct a polynomial surface function using coefficients retrieved;
build a gain table using said polynomial surface for all pixels of the image; and
apply values of said gain table to all pixels of the image taken to compensate vignetting, wherein said compensation of vignetting is performed by multiplying pixel-by-pixel a normalized polynomial function with the raw pixel values of each picture taken.
2. The method of claim 1 wherein said polynomial surface function is following the equation:
g
(
x
,
y
)
=
∑
i
=
0
4
∑
j
=
0
4
k
ij
x
i
y
j
,
wherein g (x, y) is the polynomial surface (brightness) at position (x, y), k ij , are the coefficients of the polynomial surface, (i, j) are indices to k (the matrix of coefficients), and (x, y) are the coordinates of the current pixel.
3. The method of claim 2 wherein said factor k i,j is determined by using a least squares fit to the measured data.
4. The method of claim 1 wherein said matrix of polynomial coefficients are derived from a normalized surface function describing the vignetting deficiencies.
5. The method of claim 4 wherein the brightness of the center of the image is used for said normalization.
6. The method of claim 1 wherein said test chart is a gray test chart.
7. The method of claim 1 wherein said non-volatile memory is an EPROM.
8. The method of claim 1 wherein said read-only memory is an EEPROM.
9. The method of claim 1 wherein said matrix is a 5×5 matrix.
10. The method of claim 1 wherein said array of pixel-based sensors are CMOS sensors.
11. The method of claim 1 wherein said array of pixel-based sensors are CCD sensors.
12. A system to compensate vignetting in digital cameras by generating calibration data from color-independent brightness of pixels of an image taken once from a test chart and building gain tables based on said calibration data and applied to compensate vignetting is comprising:
said test chart;
a lens system;
a read-Only memory (ROM) to store coefficients of a polynomial surface describing said color-independent vignetting during production of said camera using said test chart;
a processing device building said gain tables using said coefficients of a polynomial surface wherein said gain tables are applied to all pixels of the image to compensate vignetting wherein said compensation of vignetting is performed by multiplying pixel-by-pixel a normalized polynomial function with the raw pixel values of each picture taken; and
an array of pixel-based image sensors.
13. The system of claim 12 wherein said array of image sensors array comprises CMOS image sensors.
14. The system of claim 12 wherein said array of image sensors comprises CCD image sensors.
15. The system of claim 12 wherein said read-only memory is an EPROM.
16. The system of claim 12 wherein said read-only memory is an EEPROM.
17. The system of claim 12 wherein said processing device is a microprocessor.
18. The system of claim 12 wherein said test chart is a grey test chart.
19. The system of claim 12 wherein said lens system comprises one lens.
20. The system of claim 12 wherein said lens system comprises more than one lens.