We present a virtual digital camera sensor, whose aim is to simulate a real (physical) image capturing sensor.To accomplish this task, the virtual sensor operates in two steps. First, it accepts a physical description of agiven scene and simulates the entire process of photon sensing and charge generation in the sensor device. This process is affected by noise, mostly photon noise. Second, it adds to the image the noise that results from the electronic circuitry. We present a model for the different sources of noise relative to each sensor-based image formation step, and use measurements of real digital camera images to validate the model.