The index fitting window is an application-wide tool that is summoned from the Tools menu. At the upper left of this window is a drop box, where you can drop glasses in order to modify them, or from where you can drag glasses to use them in a lens design. This box displays the glass name and version, just like the other glass tools.

The window starts with a default glass that you can add wavelength and index data to using the table provided. After you have entered the data you can fit to various functional forms (first enter rough parameters, or press the Guess button, then press Fit). The glass may then be dragged back to the lens design.

Instead of starting with a default glass, you can drag glasses from the lens design Glasses window or from the glass library and drop them onto the drop box. If the dropped glass contains wavelength and index data, the table in the window will be populated from that data.

index-fit

If you drag a catalog glass to this window and make changes to it, you would be advised to change its version or even its name before you drag it back out so you do not mistake it for the original glass.

The glass you are working with here is a copy. For your changes to take effect, you should drag the glass from the box in the upper left corner back to the lens design.

By default, model glasses or private glasses imported from OSLO or CODE V files are fit using the Spline function, which has no adjustable parameters. You can drag such a glass to the index fitting window, fit it with a more physical form, then drag it back where you want to use it.

Spline fitting requires four data points. But you can use fewer. The spline function degenerates to constant if there is only one data point, linear if there are only two data points, and quadratic if there are only three data points.