Abstract: A multi-layer thin film filter (36) has a transmission spectrum that is within a preset tolerance of a preselected transmission spectrum. The filter includes spaced apart layers (42a1, 42a2, 42a3, . . . ) of a material having a one refractive index; and layers of another material (42b2, 42b3, . . . ) having a higher refractive index. The latter layers (42b2, 42b3, . . . ) space apart the former layers (42a1, 42a2, 42a3, . . . ). The thicknesses of the individual layers (42a1, 42b1, 42a2, 42b2, 42a3, 42b3, . . . ) are set by selecting initial values and then repeatedly computing a transmission spectrum by solving Maxell's equations and executing a nonlinear optimization algorithm until the a computed transmission spectrum converges to within the preset tolerance of the preselected transmission spectrum. By designing a filter (36) accordingly, a transmission spectrum can be achieved having high transmittance within a desired region and very low transmittance elsewhere.