The contrast ratio of the display should be greater than 3:1; a contrast ratio of 7:1 is preferred.
Additional Information:
Either display polarity that is, dark characters on a light background or light characters on a dark background is acceptable provided it meets the requirements for resolution (1.6.1-1) and luminance (1.6.1-7). When ambient illumination in the vicinity of the VDU is high, dark characters on a light background are preferred. Contrast ratio is calculated as follows: