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+
+.. algorithm::
+
+.. summary::
+
+.. relatedalgorithms::
+
+.. properties::
+
+Description
+-----------
+
+This algorithm sums the Y values of histograms given by *InputWorkspaceIndexSet* into a single single histogram. The summation is done using the method proposed by Cubitt et al. [#CUBITT]_ This involves a projection to an
+arbitrary reference angle, :math:`2\theta_R`, with a "virtual" wavelength,
+:math:`\lambda_v`. This is the wavelength the neutron would have had if it had
+arrived at :math:`2\theta_R` with the same momentum transfer (:math:`Q`).
+
+Counts are considered to be spread evenly over the input pixel, and the
+top-left and bottom-right corner of the pixel are projected onto
+:math:`2\theta_R` giving a range in :math:`\lambda_v` to project onto. Counts
+are shared out proportionally into the output bins that overlap this range, and
+the projected counts from all pixels are summed into the appropriate output
+bins.
+
+The input workspace should have wavelength as the X units as well as an instrument.
+
+To produce a reflectivity, the input workspace has to be the reflected beam workspace where each histogram is individually divided by the summed (in lambda) direct beam data.
+
+Usage
+-----
+
+**Example - basic usage with reflectometry-like data**
+
+.. plot::
+   :include-source:
+
+   import numpy as np
+   from mantid.simpleapi import *
+   import matplotlib.pyplot as plt
+
+   # Direct beam
+   noBackground = 'name=LinearBackground, A0=0'
+   direct = CreateSampleWorkspace(
+       Function='User Defined',
+       UserDefinedFunction=noBackground,
+       NumBanks=1,
+       XUnit='Wavelength',
+       XMin=0., XMax=20., BinWidth=1.)
+   # Move the detector such that the beam is right at its center.
+   MoveInstrumentComponent(
+       Workspace=direct,
+       ComponentName='bank1',
+       X=-0.008 * 4.5, Y= -0.008 * 4.5, Z=0.)
+   # Fill intensity for pixels in the beam
+   for i in [44, 45, 54, 55]:
+       direct.dataY(i).fill(1.)
+       direct.dataE(i).fill(0.1)
+   # Group detectors to form a 'line detector'. The line is vertical in this case.
+   groupingPattern=''
+   for row in range(10):
+       for column in range(10):
+           groupingPattern = groupingPattern + str(column * 10 + row)
+           if column < 9:
+               groupingPattern = groupingPattern + '+'
+       if row < 9:
+           groupingPattern = groupingPattern + ','
+   direct = GroupDetectors(
+       InputWorkspace=direct,
+       GroupingPattern=groupingPattern,
+       Behaviour='Sum')
+
+   #Reflected beam
+   reflected = CreateSampleWorkspace(
+       Function='User Defined',
+       UserDefinedFunction=noBackground,
+       NumBanks=1,
+       XUnit='Wavelength',
+       XMin=0., XMax=20., BinWidth=1.)
+   # Move the detector. This reflectometer moves vertically.
+   MoveInstrumentComponent(
+       Workspace=reflected,
+       ComponentName='bank1',
+       X=-0.008 * 4.5, Y= 0.008 * 4, Z=0.)
+   # Create some fake reflected beam data.
+   Xs = reflected.readX(0)
+   Xs = (Xs[1:] + Xs[:-1]) / 2  # Bin edges -> points
+   decay = np.exp(-(Xs - 4.) / 3.)
+   span = decay < 1.
+   for i in [44, 45, 54, 55]:
+       Ys = reflected.dataY(i)
+       Ys.fill(1.)
+       Ys[span] = decay[span]
+   reflected = GroupDetectors(
+       InputWorkspace=reflected,
+       GroupingPattern=groupingPattern,
+       Behaviour='Sum')
+
+   # Now we have somewhat realistic data.
+   # Sum the direct beam (in lambda).
+   direct=SumSpectra(direct, ListOfWorkspaceIndices=[4, 5])
+   # Calculate (sum in Q) the reflectivity.
+   reflected /= direct
+   reflectivity = ReflectometrySumInQ(
+       InputWorkspace=reflected,
+       InputWorkspaceIndexSet=[4, 5],
+       BeamCentre=4,
+       WavelengthMin=0.1,
+       WavelengthMax=19.)
+
+   fig, axes = plt.subplots(subplot_kw={'projection': 'mantid'})
+   axes.plot(reflectivity)
+   axes.set_ylabel('"Reflectivity"')
+   # Uncomment to show the plot window.
+   #fig.show()
+
+References
+----------
+
+.. [#CUBITT] Cubitt et al., *J. Appl. Crystallogr.*, **6** 2006 (2015)
+             `doi: 10.1107/S1600576715019500 <http://dx.doi.org/10.1107/S1600576715019500>`_
+
+.. categories::
+
+.. sourcelink::
+