Information loss that occurs in real systems means that the MTF figure is always less than one. For these scanners, we measured the MTF by scanning a precision-printed target of black lines on a white surface. Measurements from such a pattern can be used as a reference level for further measurements at higher line densities. As the number of line pairs per inch increases, the peak values of the scan move closer to an average grey. 70lppi, 95lppi, 140lppi and 180lppi.
Line width equals the gap between lines.
The peak distribution of light and dark pixels in a scanned image was measured using Photoshop's Histogram tool. This yields four MTF values for red, four for green, and four for blue. MTF varies with colour because refraction in optical systems differs according to the frequency of the light. This means that lenses have different focal points for red, green and blue light. Scanner optics are usually optimised for green light because this best suits our eyes.