number of reflecting surfaces number of wavelengths A neutron optical element to direct the path of the beam. :ref:`NXguide` is used by neutron instruments to describe a guide consists of several mirrors building a shape through which neutrons can be guided or directed. The simplest such form is box shaped although elliptical guides are gaining in popularity. The individual parts of a guide usually have common characteristics but there are cases where they are different. For example, a neutron guide might consist of 2 or 4 coated walls or a supermirror bender with multiple, coated vanes. To describe polarizing supermirrors such as used in neutron reflection, it may be necessary to revise this definition of :ref:`NXguide` to include :ref:`NXpolarizer` and/or :ref:`NXmirror`. When even greater complexity exists in the definition of what constitutes a *guide*, it has been suggested that :ref:`NXguide` be redefined as a :ref:`NXcollection` of :ref:`NXmirror` each having their own :ref:`NXgeometry` describing their location(s). For the more general case when describing mirrors, consider using :ref:`NXmirror`. NOTE: The NeXus International Advisory Committee welcomes comments for revision and improvement of this definition of :ref:`NXguide`. TODO: Explain what this NXgeometry group means. What is intended here? A description of this particular instance of ``NXguide``. TODO: documentation needed Reflectivity as function of reflecting surface and wavelength reflectivity of each surface as a function of wavelength List of surfaces. Probably best to use index numbers but the specification is very loose. wavelengths at which reflectivity was measured TODO: documentation needed TODO: documentation needed external material outside substrate The ``m`` value for a supermirror, which defines the supermirror regime in multiples of the critical angle of Nickel. TODO: documentation needed TODO: documentation needed TODO: documentation needed TODO: documentation needed TODO: documentation needed number of substrate sections (also called ``nsurf`` as an index in the ``NXguide`` specification)