Measured Spectra

The Measured Spectra window connects your design to the spectrophotometer. Import a measured reflectance, transmittance, or absorptance curve from an instrument file, compare it against the design on Optical Evaluation, and fit the design’s thicknesses to it. Export writes either the computed design spectrum or your imported curves to a portable file. The window is split into Import and Export tabs.

Import

Press Import Spectrum and pick a file. What the importer accepts, and the instrument quirks it handles on its own, is on the Spectrum File Formats page.

Measurement conditions

Set these before importing, because the file almost never states them and a wrong value poisons a fit without ever looking wrong:

• Angle of incidence: the angle the instrument measured at. A near-normal accessory is usually 6 or 8 degrees, not 0.
• Polarization: average, s, or p.
• Side: which face of the sample was illuminated.

Every imported curve carries its own copy of these, and you can correct them afterwards on the curve itself.

Confirming the parse

For a text table the panel shows what was detected and lets you override it: the wavelength unit, which column to take, the quantity, the Y scale, and the curve’s name. The preview beside it plots the incoming curve against the design’s own spectrum, evaluated at that curve’s angle and side, so you can see before committing whether the measurement sits where the design sits.

Add to design adds the column you configured. Add all curves appears for a file with several data columns and adds every one of them, which is what you want for a file holding T and R side by side, and not what you want for a file that also carries raw signal columns.

Editing a curve

An imported curve is stored on the design and persists with the project. Each one gets a card in the window where you can rename it, change its colour, retype it, correct the source scale, correct the measurement conditions, and trim its wavelength range. Trimming is not destructive: it moves the bounds used everywhere else and the points stay in the file, so you can widen it again.

On Optical Evaluation the curves appear as dotted lines with open-circle markers, coloured by R / T / A. The checkbox on the card hides one without removing it.

Fitting the design to a measurement

Fit… on a curve card turns that measurement into a merit-function target, so Refinement can adjust the design’s thicknesses until the computed spectrum matches what you measured. This is characterization of a coating you already know the recipe for; it is not recovering an unknown stack from an arbitrary spectrum, which is not solvable from intensity alone.

Which points to fit

• As measured uses the measured points as they are and invents nothing. Correct when the scan is dense and evenly spaced, and the default.
• Every Nth point uses measured points only, thinned. Use it when a very dense scan slows a run down for no gain.
• Even step interpolates onto a wavelength step you choose.

Interpolating a coarse scan onto a fine grid adds no information. The reason to resample is uniformity, not density: the merit function sums over its points, so an unevenly sampled scan quietly weights the fit toward wherever the instrument happened to take more readings. Interpolation is shape-preserving, so it will not overshoot at a steep band edge and ask the optimizer to chase a reflectance above 100 %.

You can also narrow the wavelength range, set the weight the fit carries against the rest of the merit function, and add minimum and maximum layer thickness constraints in the same step. Append adds the target to the merit function you already have; replace clears it first.

The target it creates

The fit becomes a single row in the Merit Function Editor holding its own copy of the measurement, so it travels with the design and keeps working if the curve is later changed or removed. Only its Enabled switch and Weight can be edited: the rest describes a measurement that was taken, not a target you choose. The value it reports is the RMS difference between design and measurement, in the same units as the curve.

The target is refused if the curve was measured on a side the design is not evaluated on, rather than quietly fitting the wrong spectrum. If a curve runs past the wavelengths your materials have data for, the target is clipped to what can be evaluated and the dialog says so.

Optical Evaluation draws the target whether or not the design still holds the curve behind it. Loading a saved merit function into another design therefore shows what it fits to; if you want the measurement back as a curve you can edit, the Import tab offers to restore it.

Export

A What to export chooser picks the source:

• Design spectrum: the computed T / R / A of the active design. Set the wavelength start, end and step, an angle-of-incidence list, the channels, and whether to split s and p (absorptance has no s/p split). It follows the active surface mode and works without Optical Evaluation open.
• Measured curves: the curves you imported. Tick the ones to write.

For either source, choose the format (CSV or JCAMP-DX), the wavelength unit (nm, µm, or cm⁻¹) and whether Y is written as a fraction or a percentage.

How to read it

The typical use is validating a deposition run: import the spectrophotometer trace and compare it against the predicted curve. Where the two diverge tells you how the as-built coating departs from the design, and fitting turns that difference into the layer thicknesses that actually came out of the chamber.

References

• McDonald & Wilks, Appl. Spectrosc. 42, 151 (1988), the JCAMP-DX XYDATA / ASDF format (AFFN, PAC, SQZ, DIF, DUP).
• Fritsch & Carlson, SIAM J. Numer. Anal. 17, 238 (1980), the shape-preserving interpolation used when resampling onto an even step.