Selecting Curve Fit Band Types and Limits


The Curve Fit dialog provides a means for selecting the band type, limits, and number of iterations used for a curve fit. You can also save and load curve fit settings from this dialog. Use What's This help for information on the individual fields.

Select Curve Fit from the Data menu to begin curve fitting.

Band Types

Three band (curve) types are provided for curve fitting: Gaussian, Gaussian-Lorentzian, and Asymmetric Gaussian-Lorentzian. The Guassian band type is the least complex, using only three parameters per band. This allows for a faster fit and makes manual parameter adjustments easier. Asymmetric Guassian-Lorentzian is the most complex, with six parameters.

The type of curve (band) type you use depends on the shape of your data peaks. Starting with the correct band type for your data greatly enhances the chances of a good fit. In general, use the least complex type that will produce a good fit.

You can change band types at any time in the curve fit procedures, even after you’ve added bands, and your data will be retained. However, note that if you change from a more complex to less complex band type, the information represented by any unused parameters will be lost.

Limits

The default limits determine the range within which the computer adjusts a given parameter during a fit routine. The limits reflect the estimate of a particular band +/- the value entered. For example, given a position estimate of 200eV, a default limit of +/- 1eV for position allows the position to be adjusted only within the range of 199eV to 201eV. Note that some limits may be grayed out if they do not pertain to a given band type.

“Loose” limits (limits that provide more than a sufficient range for a particular estimate) gives the program more room for adjusting the estimates of each band but may result in estimates that fall outside the range of expected outcomes. “Tight” limits (limits that greatly constrain an estimate) may keep estimates closer to their expected values but may not allow enough room for adjustment to perform a good curve fit.

Important: The limits for each parameter adjust with the estimate when the estimate is adjusted manually, and after a curve fit routine is performed. This is why a second command to perform a curve fit may result in a different (and possible improved) fit after the curve fit routine has completed. Since the limits track the estimates, they are “loosened” after each fit. Click here for information on how to lock an estimate or fit the limits asymmetrically.

Maximum Iterations

The maximum number of iterations instructs the fit routine to attempt to fit a given set of bands up to the given iterations value. For each iteration, the fit routine performs a non-linear regression algorithm on the curve fit band parameters. If the fit does not improve within a specified range during an iteration and the routine has reached convergence, it stops before the maximum number of iterations is reached. A higher value for the maximum number of iterations means a longer time to fit but may result in a better fit. Note that the maximum iterations has no bearing on the way the curve fit algorithm performs, only on how many iterations it can go before it stops.

Saving and Loading Curve Fit Information

Curve fit information is saved with your acquisition files. Additionally, you can save curve fit files separately from your data. This enables you to store the most commonly used curve fit settings and apply them to different acquisitions.

Click the Save button to save the current curve fit information. All curve fit limits, bands, and band parameters are saved. If you only want to store default limit information, delete any bands before saving the file. To load, simply click the Load button. Any current curve fit information is replaced.

When loading a curve fit file that contains some bands, note that the intensities of a set of curve fit bands may not fall within the range of data for that acquisition. For example, you may have saved a set of bands for data that averaged 5,000 counts/seconds, but the current data you are fitting may have a range of 10,000 counts/seconds. The software will automatically try and compensate based on the highest peak. However, if the curve shape of the new data is sufficiantly different from that which the fit was originally created on, you may need to adjust the band intensities somewhat to compensate.

Adding and Removing Curve Fit Bands

Additional Curve Fitting Topics