4. Optimisations

We here present, in outline form, the results of optimised spectral designs achieved using DE. Interested readers are referred to previous publications [23, 26–29] for full details.

It is noted at the outset that it is, in principle, possible to create white-light mixtures by the use of as few as two or three wavelengths. Our early experiments showed that these practically always lead to suboptimal mixtures in the sense that either one or both of LER and Xc are unacceptably low (Xc is here used to denote any one of the colour metrics mentioned in Section 3). Further experiments indicated that mixtures of five, six or seven wavelengths gave little or no practical advantage over 4-band mixtures, while adding to the complexity (and possible unreliability) of the light source. For the sake of brevity, therefore, the following descriptions are confined solely to results for 4-band mixing.

Two types of optimisation conditions are described: (i) constrained, in which the optimiser is presented with a set of known (e.g., commercial) monochromatic spectra, and is required to find the best available mixtures in terms of defined criteria and (ii) unconstrained, when the program is given mathematical descriptions for the shapes of potential monochromatic spectra, which it then proceeds to mix and optimise as above.

In the optimisations described below, all SPDs for LEDs were computed at 5-nm intervals, and for lasers at 1 nm.
