Recovering the continuum curve from the Continuum Removal process

In order to remove the continuum from a spectrum, ENVI first has to compute the continuum curve -- a convex hull fit over the top of the spectrum that connects local maxima with straight line segments. However, the Continuum Removal routine only outputs the continuum removed spectra, it does not show you the curve of the calculated continuum.

The continuum-removed spectrum is the original spectrum divided by the continuum curve:

S_cr = ( S / C )

where:

S_cr = the continuum removed spectrum
S = the original spectrum, and
C = the continuum curve

So, you can use the results from the continuum removal process to calculate the continuum curve:

C = ( S / S_cr )

Just apply the second equation above in Spectral Math. For example, to see the continuum for a single spectrum:

1. Display a hyperspectral image
   
2. Bring up a Z-Profile [Functions -> Profiles -> Z-Profiles (spectrum)]
   
3. From the Options menu of the plot window, select New Window: with Plots...
   
4. In this new plot window, from the Plot_Function menu, choose Continuum Removed. Re-name this plot using [Edit -> Data Parameters] to distinguish it from the one in the Z-Profile window. In the z-profile, set the plot_function back to Normal.
   
5. Then, from the Main ENVI menu, choose [Spectral Tools --> Spectral Math]
   
6. Enter the expression FLOAT(S1) / (S2)
   
7. Map S1 to the original spectrum (in the Z-Profile window) and map S2 to the continuum removed plot (in the second plot window)
   
8. Choose Output Result to Same Window
   
9. The continuum curve will plot over the original spectrum

continuum-removed spectrum

original spectrum with continuum curve

You may notice that there are some small local maxima that do not fall on the continuum curve. It is not always advantageous to place every peak in a spectrum on the continuum curve, especially if the spectrum contains noise or poorly defined features. The algorithm that ENVI uses to define local maxima has been chosen to increase the likelihood of identifying real absorption features. Also, remember that in ENVI the continuum removed spectra are used primarily for the Spectral Feature Fitting (SFF) routine where it is very important to spectrally subset the data to isolate the feature of interest. If you are investigating SFF results, you will need to zoom in on the region of the Z-Profile plot that corresponds to the spectral subset before making the new plot window and recovering the continuum with Spectral Math.