Twilight is the time before sunrise, or after sunset, when sunlight scattered in the upper atmosphere illuminates the lower atmosphere, and the surface of the Earth is between light and dark. Often confused with dusk, twilight is specifically defined as the period either side of night-time during which it is possible to conduct outdoor activities without the aid of artificial light. Due to the unusual, romantic quality of the ambient light at this time, twilight has long been popular with photographers and painters, who refer to it as the "blue hour", after the French expression l'heure bleue. The pictures below show this transformation, from sunset to twilight.