An interactive graphic system is described in which a designer using a light pen creates an optical filter design at an IBM 2250 Graphic Console controlled by an IBM 1130 computer. The designer can observe the reflectivity (or some other property) of the filter as a function of wave number plotted on the CRT. The application was developed using an experimental graphic version of the 1130 Continuous System Modeling Program (CSMP), a general purpose, block-oriented simulation language in which the functional blocks represent the elements and organization of an analog computer. The designer has available a full set of operators for further analyzing the behavior or modifying the design of the filter. In addition to providing a highly flexible analytic tool, the system is intended to explore means for makingt he interactive computer terminal an important element in the inventive process.