function y = ddif(spectra)
% this file takes repetitive scans of a sample in columns as the
%  input and returns a single vector after the following treatments.
%  The expected input is ABSORBANCE data.
%     
%    1)  21-point smoothing of the original spectra
%    2)  truncation of the spectra (must be changed in the m-file)
%    3)  all the column spectra are averaged together
%    4)  this spectrum is differentiated
%    5)  the result is 21-point smoothed
%    6)  this smmothed derivative is differentiated again
%    7)  this new f" is 41-point smoothed
%    8)  the resulting spectrum is mean centered
%
%  Questions?  Call Roger @ 1-900-IMTIRED
%
ys=smooth(spectra,21);
yst=ys(200:550,:);
yst_=mean(yst')';
d1=diff(yst_);
d1s=smooth(d1,21);
d2=diff(d1s);
d2s=smooth(d2,41);
y=d2s;
end