== Spectral Range ==

The electromagnetic wavelength interval or equivalent over which
observations are made is the fundamental discriminator among many
types of solar image and other data.  The model needs to apply to both
narrow-band ("monochromatic" or single-line) and broad-band data.
Different branches of the field use different units depending on their
spectral band -- frequency at the lowest ranges (of frequency), wavelength
at intermediate ranges, energy at the highest.  Again for the sake of
simplicity we define a single model, assuming that the necessary
conversions can be simply made.



==== Wave_Type ====


  type: ''menu''[[BR]]
  FITS keyword: ''WV_TYPE''[[BR]]

  The class of spectral data, relating to both the nominal spectral
bandpass and the spectral target.  Three values are recognized:

  ''' broad '''


    Indicates that the spectral range of the measurement is large
compared to the width of absorption/emission lines within the range,
and encompasses multiple lines as well as continuum (unless blanketed)

  ''' line '''


    The spectral range of the measurement is of the same order or less
than the width of the target line, and is centered on a wavelength within
the wings of the line.

  ''' narrow '''


    The spectral range of the measurement is of the same order or less
than the typical width of lines in the neighborhood, but is centered on
a continuum wavelength, outside of any significant lines.  This designation
is used to distinguish narrow-band continuum (or "white-light") data from
true broad-band data.  For data of this description, the matching spectral
range should be much broader than the instrumental bandpass, on the
understanding that the data are proxies for broadband measurements.

The exact definition of the bandpass (''e.g.'' FWHM) is not prescribed,
but is left up to the terminology of the data provider.  In the absence of
a provider definition, FWHM should be used.


==== Wave_Minimum ====


  type: ''number''[[BR]]
  unit: ''Ångström (10nm)''[[BR]]
  FITS keyword: ''WV_MIN''

==== Wave_Maximum ====


  type: ''number''[[BR]]
  unit: ''Ångström (10nm)''[[BR]]
  FITS keyword: ''WV_MAX''

  The nominal minimum (maximum) of the observing spectral bandpass
associated with the data.  As discussed above, for narrowband continuum
data, the range should be much larger than the instrumental bandpass;
it should correspond to the spectral range over which the data are
useful as a proxy, typically an octave or more.


==== Wave_Bands ====


  type: ''number''[[BR]]
  FITS keyword: ''WV_NBAND''[[BR]]

  The number of wavelength bands in the observation


==== Wave_Step ====


  type: ''number''[[BR]]
  unit: ''Ångström (10nm) / pixel''[[BR]]
  FITS keyword: ''WV_STP''[[BR]]

  The spectral dispersion