A new method based on the traditional method of ray scanning has been introduced to resolve the problem of judging the color in the chromaticity diagram by employing the line segments approximate curve———using polygonal approximate closed curve. Then a vector coordinate could be used to describe the curve instead of curve equations. So it is easy to judge the topology relation between an area and a point or a locus. And any data processing method has been used to save memory, which makes the improved ray scanning method is quite useful in the measurement of color.