An ultra-low noise, ultra-sensitive fully differential CMOS imager for background illumination rejection is presented. Fabricated by TSMC35 technology, the pixel is able to detect very low light signals and discern them from backgrounds exceeding their intensity. Measurements indicate a common mode rejection ratio (81.6 dB), very high sensitivity (200 V/Lux.s), and noise floor of 4 electrons. Obtained figures agree with theoretical assessments and simulations, proving the pixel to be a basis of a sensing element in ultra-sensitive continuous time or gated applications. As a second contribution, to stabilize the DC output, a transistor based common mode feedback (CMFB) is added (AMSp35 technology). By moving the load and CMFB to column periphery, number of transistors per pixel have reduced to 12, and improvements in pixel size and fill factor are achieved. Simulations promise a functional imager with high gain (50 dB) and noise floor of 98 µV.