diff --git a/skimage/color/delta_e.py b/skimage/color/delta_e.py
index 9f8cd2e8..70bbb99b 100644
--- a/skimage/color/delta_e.py
+++ b/skimage/color/delta_e.py
@@ -18,6 +18,7 @@ The delta-E notation comes from the German word for "Sensation" (Empfindung).
 
 Reference
 ---------
+http://en.wikipedia.org/wiki/Color_difference
 
 """
 from __future__ import division