'Darkness' means that there is not enough light present in the visible spectrum to activate the eye. There may still be electromagnetic radiation of longer or shorter wavelengths. Surfaces at moderate temperatures emit radiation mainly in the infrared region. Even though we cannot 'see' this radiation, it can be registered on special photographic film. On right is an example of an infrared photograph taken from the air, showing such heat-emitting surfaces.
This aerial view of part of the Malahide Road in Dublin was taken in the infrared. Roofs of buildings are clearly seen to be at a temperature higher than that of the surroundings. The picture was taken in winter, just before dawn, at which time road surfaces and bare soil fields are cooler than vegetation. Evergreen hedges along the borders of the fields are relatively warm. Notice the line of trees along the right hand side of the road.
There is no hiding place in the darkness of the night for criminals trying to escape from justice. The intruders in the images below had no idea that the radiation they were emitting was being recorded by an infrared camera!
An added advantage of infrared camerawork is that such radiation is transmitted through clouds. Not only does it work by night, but it is also unhampered by cloud cover.
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