1. A method for converting an original color image into a monochromatic image said conversion being made with a minimal lose of information, the method comprising the following steps: a computer for performing the steps of: converting the original color image made of pixels, each pixel being represented by hue H, saturation S and value V components in a color space, or by hue H, luminescence L and saturation S components in a HLS color space into an intermediate image made of pixels, each pixel being represented by components X and Y depending solely on the H and S components of the pixel in the original color image; wherein components X and Y are functions of the H component determined by: 
 X=G X ( H ) and Y=G Y ( H ); wherein these functions verify the following relationships: 
 G X (0)= G X (1) and G Y (0)= G Y (1); (b) generating a new monochromatic image from the intermediate image; wherein the intermediate image each pixel is represented only by a component D 1 ; D 1 being a function of components X and Y of the pixel in the intermediate image.