[R] filtering spectra

Ritter, Christian C SRTCL-CTGAS Christian.C.Ritter at OPC.shell.com
Mon Aug 7 14:52:39 CEST 2000

Hi all,

I'll have to filter Raman spectra and I would like to do so in R. I'm trying
to figure out how. Maybe one of you could help me ...

This is what I have to do:

	separate the real spectrum (the set of peaks) from two
contaminations: a drifting background and (white) noise

Constraints (which should help the task):
	A - the spectrum is positive
	B - the spectrum is long (covers about 1000 wave numbers)
	C - the background changes slowly (on the order of 10 equivalent
degrees of freedom over the 1000 wave numbers).
	D - the spectrum changes fast (a peak comprises 10-20 wave-numbers)
	E - the noise changes very fast (can be assumed uncorrelated)
	F - sometimes, repeat measurements are available on the same (or
essentially the same) analyte: such repeats are
		supposed to share the same spectrum but may have different
backgrounds (and the noise is certainly different)

Ideas: (on the lines of smoothing)
	Use a positive smoother to model signal (maybe a finite support
kernel of appropriate bandwidth)
	Use a smoothing spline with low degree of freedom (10 or so) for the
	Use a generalized additive model type setup for fitting (with
Gaussian error)

Assistance would be greatly appreciated.

Christian Ritter, Statistical Services, Belgian Shell Chemicals
Tel. +32 10 477349, fax +32 10 477219, email christian.c.ritter at opc.shell

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