[GRASSLIST:6963] Alogrithm for Hull-Quotient?

[paallen at attglobal.net]

Hi folks,

I know this is a bit off topic but considering it
is also usefull for hyper-spectral spatial data I
won't feel too bad about asking here.

I have some spectral data that consist of
wavelength (for the x-axis) and absorption (for
the y-axis) from an inferred-spectrometer
analyzing minerals.  There is filter called
"Hull-Quotient" that moves or normalizes the
jagged-inclined spectrum up to a horizontal line
so the absorption-peaks are more easily interpreted.

At the bottom of this message I have copied the
information from the PIMA (Portable Infrared
Mineral Analyzer) software about what a
Hull-Quotient is.

I would like to program this filter into my
database application.  Does anyone know a
Hull-Quotient algorithm?  I know that there are
various methods (rolling ball etc,).

Thanks,

Phillip J. Allen
Chief Geochemist South America
Anglogold Ashanti South America
e-mail: paallen at attglobal.net

 

Hull
The Hull is used to remove the slowly varying
background shape found in many spectra.  As
implemented in PimaView, the hull is a series of
straight line segments that connect reflectance
maxima within the spectrum.  The hull is
“anchored” at each end of the spectrum.  The hull
is conceptually like a string stretched tightly
over the top of the spectrum.

The Hull Quotient is calculated by dividing the
value of the spectrum by the value of the hull at
each wavelength.  This is useful if one wants to
visually enhance small absorption features.  The
Hull Difference is created by subtracting the hull
from the spectrum, and then adding 100% to the
result.  In both cases, a value of 100% represents
spectral points where the hull and spectrum had
the same reflectance value.  Absorption depth is
the percent difference from the 100% value.

The Hull line can be displayed by selecting the
Line option.  Sometimes, this can be used as a
diagnostic feature, especially for samples that
contain Iron.

The Normalise option, simple inverts the hull
difference spectrum such that it represents
“Transmission” rather than reflectance.