Compensation method of digital image data

A compensation method for compensating digital image data generated from a scanning module of a scanner when the luminance of a lamp tube is unstable, said compensation method comprising steps of:

• providing a target luminance value T;
• providing a first reference luminance value W1 obtained when said scanning module scans a standard color at a first scanning position;
• computing a first compensating gain G1 according to said target luminance value T and said first reference luminance value W1, wherein G1=T/W1;
• providing a second reference luminance value W2 obtained when said scanning module scans said standard color 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 the original luminance values of image data with said second compensating gain G2 and computing a third compensating gain G3 when said first compensating gain G1 is lager than said second compensating gain G2 by 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 when 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.