Closed loop requirements definition is a key concept to avoid over-designing system sub-components. For improved system performance, minimized energy consumption and environmental impact, it is crucial to enable system architects with the tools to evaluate various hypothesis in terms of sub-system components. This presentation will introduce sensor simulation as a tool to support closed loop requirements definition. A framework is proposed to introduce physically consistent noise and other non-idealities to real-life or synthesize images. These images may be introduced in a system model to evaluate the impact of sensor noise, QE and MTF on such KPI as eye gaze detection accuracy.