diff --git a/pysp2/util/normalized_derivative_method.py b/pysp2/util/normalized_derivative_method.py
index 66284ef..8550eef 100644
--- a/pysp2/util/normalized_derivative_method.py
+++ b/pysp2/util/normalized_derivative_method.py
@@ -1,7 +1,7 @@
 import numpy as np
 import xarray as xr
 
-def central_difference(S, num_records=None):
+def central_difference(S, num_records=None, normalize=True):
 
     """ 
     Compute fourth order derivative S'(t) using the 
@@ -20,6 +20,9 @@ def central_difference(S, num_records=None):
     num_records: int or None 
         Only process first num_records datapoints. 
         Set to None to process all records. 
+    normalize: bool
+        If True, normalize the derivative by the scattering signal 
+        S(t) to get (1/S) * dS/dt.
 
     Returns 
     ------- 
@@ -52,6 +55,12 @@ def central_difference(S, num_records=None):
         d[:, n-2] = (25*y[:, n-2] - 48*y[:, n-3] + 36*y[:, n-4] - 16*y[:, n-5] + 3*y[:, n-6]) / (12*dt)
         d[:, n-1] = (25*y[:, n-1] - 48*y[:, n-2] + 36*y[:, n-3] - 16*y[:, n-4] + 3*y[:, n-5]) / (12*dt)
 
+        if normalize:
+            with np.errstate(divide='ignore', invalid='ignore'):
+                d = np.where(y != 0, d / y, 0)  
+        else:
+            d = d
+
         dSdt[ch] = xr.DataArray(
             d,
             dims=('event_index', 'time'),
diff --git a/tests/test_ndm.py b/tests/test_ndm.py
index 8867890..d5a8e3c 100644
--- a/tests/test_ndm.py
+++ b/tests/test_ndm.py
@@ -5,7 +5,7 @@ def test_central_difference():
     my_sp2b = pysp2.io.read_sp2(pysp2.testing.EXAMPLE_SP2B)
     my_ini = pysp2.io.read_config(pysp2.testing.EXAMPLE_INI)
     my_binary = pysp2.util.gaussian_fit(my_sp2b, my_ini, parallel=False)
-    dSdt = pysp2.util.central_difference(my_binary)
+    dSdt = pysp2.util.central_difference(my_binary, normalize=False)
     
     np.testing.assert_almost_equal(dSdt['Data_ch4'].isel(event_index=5876, time=0).item(), 
                                    8.3333333333e6, decimal=2)
@@ -13,4 +13,12 @@ def test_central_difference():
                                    7.166666666e7, decimal=2)
     np.testing.assert_almost_equal(dSdt['Data_ch4'].isel(event_index=5876, time=19).item(), 
                                    1.5e7, decimal=2)
-    assert np.isfinite(dSdt).all()
\ No newline at end of file
+    assert np.isfinite(dSdt).all()
+
+    dSdt_norm = pysp2.util.central_difference(my_binary, normalize=True)
+    np.testing.assert_almost_equal(dSdt_norm['Data_ch4'].isel(event_index=5876, time=0).item(), 
+                                   8.3333333333e6/-30168, decimal=2)
+    np.testing.assert_almost_equal(dSdt_norm['Data_ch4'].isel(event_index=5876, time=99).item(), 
+                                   7.166666666e7/-30152, decimal=2)
+    np.testing.assert_almost_equal(dSdt_norm['Data_ch4'].isel(event_index=5876, time=19).item(), 
+                                   1.5e7/-30132, decimal=2)
\ No newline at end of file