In the automobile, various information such as route guidance or road traffic information is provided. The number of displayed characters and images for the on-board displays are increasing. In order to drive safely and smoothly, visibility improvement of the on-board display will be come more and more important. The development of an on-board display which is easy to observe for older people and robust for environmental changs is strongly required.

We propose a method to determine character size and contrast between the character and the background of the displayed characters and symbols for designing on-board display images. This method estimates the legibility of the characters based on the spatial frequency property of human vision which varies with age or environmental luminance. This method will help in designing easy-to-observe images for the on-board display.