**[size=+3]Four-Wavelength Bandpass Filter**

This idea came from a graduate student who needed a bandpass filter for detecting light at four wavelengths: 630, 690, 760, and 885 nm. Between these wavelengths the filter is allowed to transmit 10%. The filter is to be coated on glass having index 1.52. The filter operates at normal incidence. So that the demo version of TFCalc can be used to design this filter, we use SIO2 and TIO2 as the two coating materials.

We use a total of nine optimization targets: eight discrete targets of 100% transmission at each of the four desired wavelengths ±1 nm and one continuous target asking for less than 10% transmission from 600 to 950 nm. Although it may seem contradictory to have a continuous target that includes the eight discrete targets, it works in TFCalc because each discrete target has the same weight as the entire continuous target.

There is no theory for a bandpass filter of this type. Hence we use TFCalc's needle/tunneling optimization to design the filter from "scratch". That is, we start with one thin layer of TIO2 and have TFCalc grow that layer into a filter meeting the requirements. After a few minutes, TFCalc finds a 21-layer design having the transmission curve shown below.