The need to reduce energy consumption in buildings has led to the widespread adoption of energy efficient light sources, such as LEDs. Diffused light reflectors are usually preferred to distribute LED illumination uniformly across the room. In this study, unpigmented PMMA films were studied for their diffused reflectance properties. PMMA films were cast onto a cold plate under varied humidity conditions to promote the formation of a porous microstructure via solid/liquid/liquid phase separation. The relationship between casting conditions, nonsolvent content, microstructure, and diffused reflectance was studied. The diffused reflectance was evaluated using a UV/Vis spectrophotometer with an integration sphere, and the microstructure was evaluated with an SEM. It was found that increasing the nonsolvent concentration either through increased absorption of ambient humidity or by directly incorporating water into the recipe creates pores that efficiently refract visible light. Reflectance in the mid 80% was obtained just by exploiting the difference in refractive index between the PMMA and the air-filled pores. High diffused reflectance has been achieved in PMMA films purely by manipulating the film's microstructure without the use of other expensive polymers or pigments. (C) 2015 Society of Plastics Engineers