Compensation method of digital image data

An image scanning system comprising:

• an image scanner for generating a digital image signal of a scanned object, and comprising a lamp tube for providing a source light to be used for scanning said scanned object, a scanning module, a color reference region and an output port for outputting said digital image signal;
• a data processing device electrically connected to said output port of said image scanner for receiving and compensating said digital image signal generated when a luminance of said lamp tube of said image scanner is unstable, wherein said data processing device is programmed to compensate said digital image signal according to the luminance values of said color reference region by a method including:
• providing a first reference luminance value W1 obtained when said scanning module scans a reference color region at a first scanning position;
• computing a first compensating gain G1 according to a target luminance value T and said first reference luminance value W1;
• providing a second reference luminance value W2 obtained when said scanning module scans said reference color region at a second scanning position;
• computing a second compensating gain G2 according to said first reference luminance value W1, said first compensating gain G1 and said second reference luminance value W2;
• compensating original luminance values of image data with said second compensating gain G2 and computing a third compensating gain G3 if said first compensating gain G1 is larger than said second compensating gain G2 and a difference between the first compensating gain G1 and the second compensating gain G2 is greater than a threshold value;
• and
• compensating the original luminance values of image data with said second compensating gain G2 without computing said third compensating gain G3 if said second compensating gain G2 is less than said first compensating gain G1 and the difference between said first compensating gain G1 and said second compensating gain G2 is less than said threshold value.

A luminance compensation method for compensating digital image data of plural scan lines generated from a scanning module of a scanner, said luminance compensation method comprising steps of:

• computing a first compensating gain according to a first luminance value of a reference color region at a first scanning position illuminated by a lamp and measured by means of the scanning module, and compensating luminance values of digital image data for N scan lines with said first compensating gain;
• computing a second compensating gain according to a second luminance value of said reference color region at a second scanning position illuminated by the lamp and measured by means of the scanning module, and compensating the luminance values of digital image data for M scan lines with said second compensating gain;
• and
• comparing said first compensating gain with said second compensating gain, and computing a third compensating gain if a difference between the first compensating gain and the second compensating gain is larger than a threshold value and not calculating said third compensating gain if said second compensating gain is less than said first compensating gain and the difference therebetween is less than said threshold value.