The optical model of a starshade is illustrated. The input function f_λ(x, y) describes the flux density of a star–planet scene, as viewed from a fixed angle, and as a function of wavelength. This input function propagates past the starshade s(x, y), onto a telescope aperture mask P(x, y) at distance d, and onto a CCD in the image plane at a distance d_f. The IWA is the angular separation from the star where an exoplanet can be imaged. In the output image I_λ(x, y), the light from the host-star has been suppressed revealing the faint exoplanetary system.