diff --git a/skimage/exposure/exposure.py b/skimage/exposure/exposure.py
index a26e5630..9c50ab7d 100644
--- a/skimage/exposure/exposure.py
+++ b/skimage/exposure/exposure.py
@@ -338,7 +338,8 @@ def adjust_sigmoid(image, cutoff=0.5, gain=10, inv=False):
 
     References
     ----------
-    .. [1] http://bme.med.upatras.gr/improc/matalb_code_toc.htm#12. Adjust Contrast :
+    .. [1] Gustav J. Braun, "Image Lightness Rescaling Using Sigmoidal Contrast
+    Enhancement Functions" http://www.cis.rit.edu/fairchild/PDFs/PAP07.pdf
 
     """
     _assert_non_negative(image)