Visibility calculations for night-time conditions are currently based on day-time vision. Under reduced light levels, such as road lighting, the spectral sensitivity of the eye is, however, different in peripheral vision. Peripheral vision, which includes major part of the vision, plays important role in situations where presence and motion of objects outside the central vision affect the safety and judgement of actions. Currently applied models based on central vision lack the ability to describe visual performance in peripheral vision correctly. In this book, two different performance based methods were applied to evaluate the spectral sensitivity of the eye. Results reveal that spectral sensitivity of the peripheral vision should be described with its own model. A candidate model for this purpose is presented. Furthermore, a model for spectral sensitivity as a function of luminance level at mesopic and low photopic luminance levels is presented as well. These models give information on the visual performance of the human eye at low light levels. This knowledge is useful to professionals in the fields of lighting and vision.