IMD is a free software tool for modeling the optical properties of multilayer films, including reflectance, transmittance, and electric-field intensities. It allows users to model films with up to eight independent variables and perform nonlinear least-squares curve fitting to estimate parameters from experimental data. The software includes a full graphical user interface and provides visualization tools for interactive exploration of results. IMD can account for interface imperfections such as roughness and diffuseness, and supports both periodic and graded interfaces. It also includes a database of optical constants for over 150 materials and allows users to define custom optical constants. The software is written in the IDL language and is available for popular platforms including MacOS, Windows, and Unix. IMD is used in various scientific and technological fields, including astronomy, lithography, and plasma diagnostics. The software can compute optical functions as functions of wavelength, incidence angle, and other parameters, and provides confidence intervals for parameter estimation. Examples demonstrate IMD's ability to model complex optical phenomena and accurately determine film parameters from experimental data. The software is freely available and can be downloaded from the Internet.IMD is a free software tool for modeling the optical properties of multilayer films, including reflectance, transmittance, and electric-field intensities. It allows users to model films with up to eight independent variables and perform nonlinear least-squares curve fitting to estimate parameters from experimental data. The software includes a full graphical user interface and provides visualization tools for interactive exploration of results. IMD can account for interface imperfections such as roughness and diffuseness, and supports both periodic and graded interfaces. It also includes a database of optical constants for over 150 materials and allows users to define custom optical constants. The software is written in the IDL language and is available for popular platforms including MacOS, Windows, and Unix. IMD is used in various scientific and technological fields, including astronomy, lithography, and plasma diagnostics. The software can compute optical functions as functions of wavelength, incidence angle, and other parameters, and provides confidence intervals for parameter estimation. Examples demonstrate IMD's ability to model complex optical phenomena and accurately determine film parameters from experimental data. The software is freely available and can be downloaded from the Internet.