Automatic visual inspection is one of the most important areas of machine vision. Color grading is one of the typical issues in automatic visual inspection

and has been broadly used in ceramic tiles

lumber etc. In order to implement grading quickly

accurately and automatically

a color grading method based on perceptually uniform color space is put forward according to human vision system characteristics. Firstly

color data in RGB like color space is transformed into perceptually uniform color space-CIE 1976 L *a *b *. In CIE 1976 L *a *b * color space

2-D RWM (radius weighted mean) cut algorithms are applied to extract DC (dominant colors)

which is not sensitive to lighting change and less computation complexity compared to 3-D RWM cut algorithms. Then

DC set as color feature

an innovative color distance metric-mapping color difference is proposed

which is identical to human visual system characteristic. The relation between mapping color difference and average color difference is also analysed. Finally

mapping color difference used as distance metric

anaminimum distance classifier is adopted to color grading. Experiments results show that the proposed method is effective and encouraging.