As spun polypropylene fibers (iPP) with different draw-down ratios (DDR) were produced at increasing take-up speeds ranging from 200 m/min to 1000 min. Two interferometric techniques (Pluta and Interphako microscopes) were used to investigate the refractive indices and birefringence of the spun fibers. The use of the Interphako microscope enabled all the fiber diameters to be studied and avoid image overlap (Sokkar et al., Opt. Lasers Eng., 45, 431 (2007)). Previously reported software (Sokkar et al., Opt. Lasers Eng., 45, 431 (2007), Hamza et al., J. Appl. Polym. Sci., 77, 3099 (2000)) was used to determine the refractive index profiles for PP fibers at different DDR. An optothermomechanical (OTM) (Hamza et al., J. Appl. Polym. Sci., 95, 647 (2005)). device attached to the Pluta microscope was used to examine the drawability of the melt-spun iPP fibers by cold-drawing. The work is concerned with the importance of the difference between physical effects of DDR and DR. The influence of DDR on the relation between optical properties and solid state draw ratio (DR) was determined. The fibers spun at low DDR give high-maximum draw ratio (DRmax) and higher birefringence (Delta n) than those of higher DDR. Total attenuation ratio (TAR) of the melt-spun iPP fibers was defined and calculated. The minimum (theoretical) birefringence (Delta n(min)) and maximum (measurable) birefringence (Delta n(max)) of iPP fibers were determined. POLYM. ENG. SCL, 49:2116-2124, 2009. (C) 2009 Society of Plastics Engineers