The optical performance of printed matter on diffusely reflecting substrate is governed by two factors: 1) Optical dot gain due to lateral spread of light within the substrate and 2) ink spread caused by physico-chemical interaction of ink and substrate. There are a limited number of studies analyzing these factors separately. In this paper we focus on the optical dot gain aspect by measurement of lateral distribution of light, i.e. the point spread function (PSF), and its analysis in terms of the modulation transfer function (MTF). Separating the two contributions, optical dot-gain from ink spread, is necessary for realistic simulation of the inkjet printing process, which is then used for printed image quality performance comparison (e.g. optical system models, virtual image chain approach, etc.). The PSF measurement methods applied can also be used for other reflective display materials in general since they do not make use of the printing process as investigated in many of the available studies. In this study we report the optical characteristics of typical substrates used in inkjet printing industry and mention examples of possible applications.