The macroscopic spread function describes the extent to which radiation is spread in a semitransparent material because of surface and subsurface scattering. The objective here is to investigate the effects of optical and geometrical parameters on the macroscopic spread function of plastic reference samples and reagent strip formats. A new feature is the use of a fast raster area scattering instrument (FRASI) instrument in measuring the spread function data of the plastic samples and reagent strip formats. Experimental measurements show that least spreading is associated with the off-white plastic samples using a wavelength of 700 nm and a reflected angle of 55 deg. The results for the Glucofilm™ reagent strip formats indicate that most spreading occurred with the 45-deg reflected angle and that least spreading is associated with measuring these reagent strips at 800 nm. The spreading curve determined from the experimental data can greatly assist the reflectometer designer in selecting the optimum optical parameters and geometrical dimensions of the reference sample and the reflectometer system.