Rendering Iridescent Colors of Optical Disks

The iridescent colors observed on objects such as optical disks are caused by light diffraction from their surface microstructure. We show that when an optical disk is illuminated by a light source it typically displays a major highlight strip of colors diagonally across the disk, consisting of saturated colors that vary along the strip, and a pair of weak strips of unsaturated colors that change across the strip. In this report, we propose a diffractive illumination model and use it to render such iridescent colors. Our model is based completely on the physical structure of optical disks and includes contributions due to both diffractive and non-diffractive factors. For the diffractive part, we first model the pit periodicity for optical disks by using identical spheres and we then approximate their distribution within tracks by uniform groups of spheres. Utilizing this representation, we carry out analytic, self-contained calculations from first principles based on the superposition principle of light waves, and derive the distribution of light intensity due to diffraction. We also propose and prove the condition for highlights on illuminated grooved surfaces; this condition provides the non-diffractive contribution within our model. The model successfully accounts for the complex behavior of optical disks — rendered images achieve excellent agreement with corresponding photographs of real disks. While this report specifically focuses on optical disks, the key idea in our illumination model is generally applicable to various diffractive structures.