The modulation transfer function (MTF) of an optical or electro-optical device is one of the most significant factors determining the image quality. Unfortunately, characterization of the MTF of the semiconductor-based focal plane arrays (FPA) has typically been one of the more difficult and error-prone performance testing procedures. Based on a thorough analysis of experimental data, a unified model has been developed for estimation of the overall CMOS active pixel sensor (APS) MTF for scalable CMOS technologies. The model covers the physical diffusion effect together with the influence of the pixel active area geometrical shape. Agreement is excellent between the results predicted by the model and the MTF calculated from the point spread function (PSF) measurements of an actual pixel. This fit confirms the hypothesis that the active area shape and the photocarrier diffusion effect are the determining factors of the overall CMOS APS MTF behavior, thus allowing the extraction of the minority-carrier diffusion length. Section 2.2 presents the details of the experimental measurements and the data acquisition method. Section 2.3 describes the physical analysis performed on the acquired data, including the fitting of the data and the relevant parameter derivation methods. Section 2.4 presents a computer model that empirically produces the PSF of the pixel. The comparisons between the modeled data and the actual scanned results are discussed in Section 2.5. Section 2.6 summarizes the chapter.