When a photographic emulsion is given a very brief exposure to light of high intensity, it is desensitized toward a subsequent longer exposure to light of moderate intensity. That is, the second exposure produces a fainter image than if the preexposure had not been given. This phenomenon was observed originally by Clayden when photographing lightning flashes, but it can be produced equally well by any type of light source, provided the intensity is sufficiently high and the duration short enough. See dark lightning.