White light enters from the left and monochromatic light exits to the right. The angle of the monochromator selects the wavelength of light passed through the device. As the angle of the monochromator is changed, the wavelength "color" of the light exiting also changes. The second crystal surface serves to return the output light parallel to the input "white" light source. Note that the exit height of the beam changes slightly as the monochromator angle is changed. With a 6.5 mm gap, the exit height changes ~450 µm between 7 and 17.5 keV. However, the change in height during an experiment is much smaller. At the Se K edge, the change in beam height for a 200 eV change in energy is ~5 µm.