In this paper, we propose a mathematical model of human photopic contrast sensitivity. The model is based on a novel functional block diagram, focusing more on the information theoretical nature of human contrast sensitivity. At the same time, several other aspects of human vision are also considered. In order to obtain an estimate of the nonlinear projection of the retinal image, the numerical results of Drasdo and Fowler were used. Cone density measurements along the major meridians of the retina, performed by Curcio & Allen, were also used. Retinal midget ganglion cell receptive field density was extended to the far periphery, based on the results of Drasdo and colleagues and Curcio & Allen. Considering cell densities and optical properties of the human eye, the low-pass filtering components of human contrast sensitivity have been characterized by a simple but adequate mathematical formula. The high-pass filtering of neural origin was also estimated based on midget receptive field densities as well as other experimental results. Further models are presented for the calculation of photon noise, neural noise and spatial integration, according to the previous results in the literature. The model presented in this paper has been validated by several experimental measurements, both for foveal and peripheral vision, at several luminance levels and experimental set-ups. Results were shown to be better fitting than the model of Barten  and Rovamo et al .