diff --git a/common/plotters/PDF_plotter.cpp b/common/plotters/PDF_plotter.cpp
index 6286bc83e6..9eb46d32f7 100644
--- a/common/plotters/PDF_plotter.cpp
+++ b/common/plotters/PDF_plotter.cpp
@@ -424,8 +424,9 @@ void PDF_PLOTTER::PlotImage( const wxImage & aImage, const wxPoint& aPos,
             }
             else
             {
-                // Grayscale conversion
-                putc( (r + g + b) / 3, workFile );
+                // Greyscale conversion (CIE 1931)
+                unsigned char grey = KiROUND( r * 0.2126 + g * 0.7152 + b * 0.0722 );
+                putc( grey, workFile );
             }
         }
     }
diff --git a/common/plotters/PS_plotter.cpp b/common/plotters/PS_plotter.cpp
index 7b2425814d..52353577a4 100644
--- a/common/plotters/PS_plotter.cpp
+++ b/common/plotters/PS_plotter.cpp
@@ -737,7 +737,12 @@ void PS_PLOTTER::PlotImage( const wxImage & aImage, const wxPoint& aPos,
             if( colorMode )
                 fprintf( outputFile, "%2.2X%2.2X%2.2X", red, green, blue );
             else
-                fprintf( outputFile, "%2.2X", (red + green + blue) / 3 );
+            {
+                // Greyscale conversion (CIE 1931)
+                unsigned char grey = KiROUND( red * 0.2126 + green * 0.7152 + blue * 0.0722 );
+
+                fprintf( outputFile, "%2.2X", grey );
+            }
         }
     }