True resolution is the actual or optical resolution captured. The CCD chip itself determines the true horizontal resolution while the stepper motor that moves the scan head dictates the true vertical resolution. To create interpolated resolution software looks at two adjacent pixels in the image and averages them to create an intermediate value. However interpolation is useful for linework scans because it can smooth the jagged edges on curves and diagonal lines. Another issue is how many bits are captured and how many are used.
Most experts agree that eight bits per color is enough to produce good-quality output.
But 8 is not enough. Strange as it sounds an 8 bit scanner does not capture 8 bits of information. Because of what has been come to be known as 'noise' or 'check bits' each 8 bit scan loses about 2 bits resulting in 6 bits per color which is a significant loss. In addition, capturing more than eight bits also makes a scanner's auto-exposure feature work better. However, a third point known as the midrange threshold point, or cutoff point also significantly affects how an image will look.