Optoelectronic device and method for brightness correction

Abstract

An optoelectronic device ( 10 ) is provided having an image sensor ( 20 ) for generating pixel images of a detection area ( 12 ) and a brightness correction unit ( 28 ) configured to modify brightness values of the pixels with a correction factor (H mul ) to obtain a more homogeneously illuminated image. A respective correction factor (H mul ) is calculated for individual pixels or groups of pixels from a perspective transformation (M) which converts geometries of an object plane ( 34 ) in the detection area ( 12 ) into geometries of the image plane ( 32 ).

The invention claimed is:  
     
       1. An optoelectronic device with an image sensor for generating pixel images of a detection area and with a brightness correction unit configured to modify brightness values of the pixels with a correction factor (H mul ) to obtain a more homogeneously illuminated image,
 characterized in that a respective correction factor (H mul ) is calculated for individual pixels or groups of pixels from a perspective transformation (M) which converts geometries of an object plane in the detection area into geometries of the image plane. 
 
     
     
       2. The device according to  claim 1 , wherein a calibration unit is provided which is configured to determine the perspective transformation (M) as the transformation which converts a known absolute geometry of a calibration code into its detected geometry in the image. 
     
     
       3. The device according to  claim 1 , wherein the brightness correction unit is configured to calculate a correction factor (H mul ) from the ratio (A Q1 , A Q2 ) of the area of a partial area (Q 1 , Q 2 ) of the image plane to the area (A T1 , A T2 ) of a transformed partial area (T 1 , T 2 ) obtained by the perspective transformation (M) of the partial area (Q 1 , Q 2 ). 
     
     
       4. The device according to  claim 3 , wherein the brightness correction unit is configured to define the partial areas (Q) by regularly dividing the image plane. 
     
     
       5. The device according to  claim 4 , wherein the image plane is divided into image squares. 
     
     
       6. The device according to  claim 1 , wherein the brightness correction unit comprises an FPGA which multiplies the pixels of a captured image with previously stored correction factors (H mul ). 
     
     
       7. The device according to  claim 1 , wherein the brightness correction unit is configured to perform an additional brightness correction with edge decrease correction factors which compensate a known or assumed brightness decrease of the images sensor in its edge regions. 
     
     
       8. The device according to  claim 1 , wherein the device is configured as a camera-based code reader comprising a decoding unit to identify code areas in the images and read their encoded information. 
     
     
       9. The device according to  claim 8 , wherein the brightness correction unit is configured to modify brightness values of pixels only in code areas. 
     
     
       10. The device according to  claim 8 , wherein a code verification unit is provided which is configured to determine whether a detected code has a predetermined code quality. 
     
     
       11. A method for brightness correction of pixel images of a detection area which are captured by an image sensor, wherein brightness values of the pixels are modified by a correction factor (H mul ) to obtain more homogeneously illuminated images, characterized in that a respective correction factor (H mul ) is calculated for individual pixels or groups of pixels from a perspective transformation (M) which converts geometries of an object plane in the detection area into geometries of the image plane. 
     
     
       12. The method according to  claim 11 , wherein the perspective transformation (M) is determined in a calibration process by capturing an image of a calibration code in the detection area and determining that perspective transformation (M) which converts a known absolute geometry of a calibration code into its detected geometry in the image. 
     
     
       13. The method according to  claim 11 , wherein a correction factor (H mul ) is calculated from the ratio (A Q1 , A Q2 ) of the area of a partial area (Q 1 , Q 2 ) of the image plane to the area (A T1 , A T2 ) of a transformed partial area (T 1 , T 2 ) obtained by the perspective transformation (M) of the partial area (T 1 , T 2 ). 
     
     
       14. The method according to  claim 13 , wherein the image plane is divided into partial areas being image squares (Q) of a mutually same size, and wherein the correction factor (H mul ) is determined from the ratio of the area (A Q ) of an image square (Q) to the area (A T1 , A T2 ) of that trapezoid (T 1 , T 2 ) into which the image square (Q 9 ) is converted in the object plane by applying the perspective transformation (M). 
     
     
       15. The method according to  claim 11 , wherein the correction factors (H mul ) include an additional component by which a known or assumed brightness decrease of the image sensor in its edge regions is compensated. 
     
     
       16. The method according to  claim 11 , wherein code areas in the images are identified and the information encoded therein is read.