Merge pull request #1416 from jni/low-contrast-docstring

Clarify 'low-contrast' in docstring

skimage/exposure/exposure.py

@@ -475,7 +475,9 @@ def is_low_contrast(image, fraction_threshold=0.05, lower_percentile=1,
     image : array-like
         The image under test.
     fraction_threshold : float, optional
-        The low contrast fraction threshold.
+        The low contrast fraction threshold. An image is considered low-
+        contrast when its range of brightness spans less than this
+        fraction of its data type's full range. [1]_
     lower_bound : float, optional
         Disregard values below this percentile when computing image contrast.
     upper_bound : float, optional
@@ -489,6 +491,10 @@ def is_low_contrast(image, fraction_threshold=0.05, lower_percentile=1,
     out : bool
         True when the image is determined to be low contrast.
 
+    References
+    ----------
+    .. [1] http://scikit-image.org/docs/dev/user_guide/data_types.html
+
     Examples
     --------
     >>> image = np.linspace(0, 0.04, 100)