Starting with object generation 5.05 the list of spectra offers a new object type called ‘Function fit’. It allows the user to define a function which may contain terms that retrieve optical functions, fit parameters and integral quantities (the latter in CODE only, but not in SCOUT). The function value is computed based on the current model and compared to a user-defined target value, or an interval of allowed target values. The fit deviation of the object is the absolute difference between current value and target value (or closest limit of the target range), taken to a user-defined power. If the function value stays within the target interval the deviation is zero.

Objects of this type can be used to impose restrictions to the model. If, for example, an optical function returns the real part n of the refractive index at a certain wavelength, you can define a target interval for n and generate a large deviation outside the target interval. This forces the fit to look for solutions that are compatible with the given range of refractive index values.

Design targets for color values or other integrated spectral values may not always be well-defined numbers. You can also search for designs where color values stay within tolerated boundaries, but it does not matter where exactly.

Such design situations can be handled in CODE using so-called ‘penalty shape functions’. However, the use of this concept turned out to be rather complicated.

We have now (starting with version 5.02) introduced a very easy definition of tolerated intervals for integral quantities: Instead of typing in the target value you can define an interval by entering 2 numbers with 3 dots in between, like ’23 … 56′ or ‘-10 … -8’. If the integral value lies within the interval its contribution to the total fit deviation is zero. Outside the interval the squared difference between actual value and closest interval boundary is taken, multiplied by the weight of the quantity.

## Hardware and software for optical spectroscopy