We present an analytical model that allows to calculate the current response of a spatially modulated light CMOS detector (SML-detector) and compare this response with the response of a traditional CMOS photodetector. It is shown that the SML detector already yields a three orders of magnitude faster response time than a traditional CMOS detector in a 0.25 mum CMOS technology. This response time will further decrease as CM-OS technology evolves. This analytical expression is compared with a numerical solution of the diffusion equation and with experimental results. Both show an excellent correspondence. Therefore we can conclude that the SML-detector is the solution of choice for cheap, CMOS-compatible receivers in integrated opto-electronic systems