Dust and scratch detection for an image scanner

Abstract

Multiple scans of the same object are obtained, where for any given line on the object to be scanned, the angle of the illumination is different for each scan. The different scans are obtained from different rows of photosensors that are separated. Because the angles of illumination are different, the resulting shadows in each scan are different. The multiple scans may be combined into a single composite color image. In a composite image, a dust particle may generate a series of overlapping shadows, where each shadow is a different color. Searching the composite image for the unique pattern of colors may identify artifacts or defects. Alternatively, the data for one scanned image may be compared to the data for another scanned image, and any differences may be due to shadows, which may indicate defects.

What is claimed is:  
     
         1 . A method of detecting a defect on a surface being scanned, the method comprising: 
 illuminating an area on the surface from a first direction;    scanning the area a first time;    illuminating the area on the surface from a second direction;    scanning the area a second time;    comparing the first and second scans; and    determining that a defect is present in the area when the first and second scans are different.    
     
     
         2 . The method of  claim 1 , further comprising: 
 determining that a defect is present in the area when a size of an apparent shadow is different between the first and second scans.    
     
     
         3 . The method of  claim 1 , further comprising: 
 determining that a defect is present in the area when a direction of an apparent shadow is different between the first and second scans.    
     
     
         4 . The method of  claim 1 , further comprising: 
 moving an illumination source to obtain illumination from different directions.    
     
     
         5 . The method of  claim 1 , further comprising: 
 using two different displaced illumination sources to obtain illumination from different directions.    
     
     
         6 . A method of detecting a defect on a surface being scanned, the method comprising: 
 scanning an area on the surface with a light source and a first photosensor array;    scanning the area on the surface with the light source and a second photosensor array; where the light source, and first and second photosensor arrays are stationary relative to each other, the first and second photosensor arrays are spaced apart, and where the light source and first and second photosensor arrays are moved between the first and second scans;    determining that a defect is present in the area when the first and second scans are different.    
     
     
         7 . The method of  claim 6 , further comprising: 
 determining that a defect is present in the area when an apparent shadow in the first scan has a different size in the second scan.    
     
     
         8 . The method of  claim 6 , further comprising: 
 where the first and second photosensor arrays detect substantially the same wavelengths of light.    
     
     
         9 . The method of  claim 6 , further comprising: 
 where the first and second photosensor arrays detect different wavelengths of light.    
     
     
         10 . A method of detecting a defect on a surface being scanned, the method comprising: 
 scanning an area on the surface with a light source and a first photosensor array detecting a first color;    scanning the area on the surface with the light source and a second photosensor array detecting a second color; where the light source, and first and second photosensor arrays are stationary relative to each other, the first and second photosensor arrays are spaced apart, and where the light source and first and second photosensor arrays are moved between the first and second scans;    forming a composite image from the first and second scans; determining that a defect is present in the area when a pattern of color bands, produced by overlapping shadows, is detected.