Image reading apparatus and computer readable storage medium for correcting image signal

Abstract

In an X-ray film image reading apparatus, dark output distribution data of a CCD line sensor is corrected by using correction data. In order to suppress the influence of random noises on the correction data, a dark distribution correction data calculation circuit 10 calculates an average value or mode value of dark distribution outputs of all or some pixels of a CCD line sensor or a dark distribution output of the CCD line sensor subjected to a spatial low-pass filtering process, and stores it in a dark distribution memory as correction data. Next, a light source is turned on, and a subtractor subtracts the stored correction data from an output of the line sensor read from an X-ray film while the light source is turned on.

What is claimed is:  
     
         1 . An image reading apparatus comprising: 
 image pickup means for optically reading an image and converting the read image into an electrical image signal;    calculating means for calculating correction data in accordance with dark output distribution data obtained by said image pickup means; and    subtracting means for subtracting the correction data from the image signal obtained by said image pickup means by reading the image.    
     
     
         2 . An image reading apparatus according to  claim 1 , wherein said calculating means calculates an average value of the dark output distribution data of all or some pixels and uses the average value as the correction data.  
     
     
         3 . An image reading apparatus according to  claim 1 , wherein said calculating means calculates a statistical mode value of the dark output distribution data of all or some pixels and uses the mode value as the correction data.  
     
     
         4 . An image reading apparatus according to  claim 1 , wherein said calculating means calculates, if the dark output distribution data is limited to a predetermined level, a mode value having a level different from the predetermined level.  
     
     
         5 . An image reading apparatus according to  claim 1 , wherein said subtracting means does not subtract the correction data from the image signal for a specific pixel, but subtracts a pixel value of the specific pixel from the image signal.  
     
     
         6 . An image reading apparatus according to  claim 5 , wherein the specific pixel is a pixel having a random noise level such that an absolute value of an output of said subtracting means does not enter a range relative predetermined by the random noises anticipated from the physical characteristics of said image pickup means and peripheral circuits or by adding a standard deviation multiplied by k (k being an integer) of the random noises actually measured.  
     
     
         7 . An image reading apparatus according to  claim 1 , wherein said calculating means uses the dark output distribution data subjected to a spatial low-pass filtering process as the correction data.  
     
     
         8 . An image reading apparatus according to  claim 7 , wherein the spatial low-pass filtering process is a moving averaging process.  
     
     
         9 . An image reading apparatus according to  claim 1 , wherein the dark distribution output data is obtained by a single data acquisition operation.  
     
     
         10 . An image reading apparatus according to  claim 1 , wherein said image pickup means includes a line sensor using solid state image pickup elements.  
     
     
         11 . A computer readable storage medium storing a program, the program executing: 
 an image pickup step of optically reading an image with image pickup means and converting the read image into an electrical image signal;    a calculating step of calculating correction data in accordance with dark output distribution data obtained by the image pickup means; and    a subtracting step of subtracting the correction data from the image signal obtained by the image pickup means by reading the image.    
     
     
         12 . A computer readable storage medium storing a program according to  claim 11 , wherein said calculating step calculates an average value of the dark output distribution data of all or some pixels and uses the average value as the correction data.  
     
     
         13 . A computer readable storage medium storing a program according to  claim 11 , wherein said calculating step calculates a statistical mode value of the dark output distribution data of all or some pixels and uses the mode value as the correction data.  
     
     
         14 . A computer readable storage medium storing a program according to  claim 11 , wherein said calculating step calculates, if the dark output distribution data is limited to a predetermined level, a mode value having a level different from the predetermined level.  
     
     
         15 . A computer readable storage medium storing a program according to  claim 11 , wherein said subtracting step does not subtract the correction data from the image signal for a specific pixel, but subtracts a pixel value of the specific pixel from the image signal.  
     
     
         16 . A computer readable storage medium storing a program according to  claim 15 , wherein the specific pixel is a pixel having a random noise level such that an absolute value of an output of said subtracting step does not enter a range predetermined by the random noises anticipated from the physical characteristics of the image pickup means and peripheral circuits or by adding a standard deviation multiplied by k (k being an integer) of the random noises actually measured.  
     
     
         17 . A computer readable storage medium storing a program according to  claim 11 , wherein said calculating step calculates, uses output distribution data subjected to a spatial low-pass filtering process as the correction data.  
     
     
         18 . A computer readable storage medium storing a program according to  claim 17 , wherein the spatial low-pass filtering process is a moving averaging process.  
     
     
         19 . A computer readable storage medium storing a program according to  claim 11 , wherein the dark distribution output data is obtained by a single data acquisition operation.  
     
     
         20 . A computer readable storage medium storing a program according to  claim 11 , wherein the image pickup means includes a line sensor using solid state image pickup elements.