Phosphor materials are often used in illumination systems for creating white light sources. This article describes modeling a phosphorescent material, as often used to generate white light sources in illumination systems, via the Phosphor.DLL bulk scattering model in OpticStudio. Fluorescence is modeled in OpticStudio as a bulk scattering event that may take one of many distribution profiles. In a phosphorescent system, the input beam may also scatter and undergo a different distrubtion independent of "fluorescence scattering". The Phosphor DLL provided with OpticStudio has been developed to describe such a system. To model a volume as a phosphorescent material in Non-Sequential mode, select the Phosphor.DLL file under the Volume Physics tab of the object properties. This DLL models a material in which light can undergo fluorescence and “real” bulk scattering. The fluorescence distribution is given by the “Angle Scattering” model, i.e. light which fluoresces will be scattered with uniform probability into a sphere (the half cone angle is 180 degrees with respect to the input ray). The bulk scattering distribution is given by the Mie model, which is described in detail in the Knowledgebase article entitled “ How to simulate atmospheric scattering using a Mie model ”. The Phosphor DLL requires 9 inputs: 1. Mean Path, 2-5. Particle Index, Size, Density, Min. thresh., 6-7. Blue Min, Blue Max, 8-9. Fluorescent trans., Scattering trans.. An archive (ZAR) file is provided as an article attachment. In that file, a simple model of a phosphorescent LED has been constructed. The input blue source is modeled using a Source File from Osram. The wavelength of the source is defined by Wavenumber = 1: corresponding to a wavelength of 0.46 microns. The phosphor coating in which both fluorescence and scattering occur is modeled as a thin layer outside of the main bulb. This thin layer is actually modeled using another Standard Lens object, taking advantage of the nesting rules in OpticStudio. The results indicate a central emission that is white. These results may be quantified using the NSDE operand in the Merit Function Editor. The results indicate that the average chromaticity values on the detector are x = 0.33 and y = 0.35.