# Plate Theory and Extensions - Effective Plate Number > Page 65

Giddings [16]
proposed the function () to define the resolving power (R) of a
column. This function is analogous to a very similar expression used in
spectroscopy to define spectroscopic resolution, *i.e.,* (), where (Dl) is the minimum wavelength increment that
can be resolved at a mean wavelength (l).
The value that would be analogous to (Dl)
in spectroscopy was (Dk'), the band
width at the base of the eluted peak. Consequently, (Dk') will be equivalent to twice the peak width at the points of
inflexion or four times the standard deviation (4s).
From the plate theory, (R_{r}) the concept of resolution as introduced
by Giddings, will be given by

Dividing
through by (v_{m} ), and noting that ,

(58)

Equation (58)
shows that the resolving power as defined by Giddings is directly proportional
to the square root of (N_{E}),
the number of effective plates. Thus, (R_{r}) can also be used to
compare the resolving power of different columns of any size or type. However,
the magnitude of (R_{r}) will still depend on the (k') of the solute
used for measurement and thus comparison between columns must be made using
data for solutes that have the same, or very similar, (k') values.