A White Color Filter

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Filters for producing very specific colors are commonly created using thin film technology. In this example, we design a filter that converts daylight (represented by illuminant CIE-D65) into pure white light -- as preceived by the human eye. We also want fairly high transmission through this filter. Our optimization targets are

> 91% transmitted luminosity

CIE-1931 transmitted color: x = 0.333 and y =0.333

In TFCalc, these color targets are entered by using the "Add Color Targets" command on the Options menu of the Targets-Discrete window. We will use TFCalc's needle/tunneling optimization to design the coating from "scratch". That is, we will begin the design process with a single thin layer of TiO2 and allow TFCalc to increase the thickness and number of layers in the design. TFCalc found a 7-layer design in a few minutes. The color chart for this design is shown below.

Plot of white color

Observe that the filter works by reflecting just enough blue light so that the transmitted light appears white to the eye. Here is the design, starting with the layer closest to the substrate, and with thicknesses given in nm:

TIO2      38.30
SIO2      36.43
TIO2      47.52
SIO2      20.40
TIO2      44.23
SIO2      55.93
TIO2      20.76
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