From AMS Glossary
A photographic effect in which lightning discharges register as dark instead of light due to multiple exposures caused by successive members of a composite flash.
This effect, which results from a characteristic of the exposure versus illumination curve of a photographic emulsion, is sometimes called the Clayden effect after its English discoverer.
Humphreys, W. J. 1940. Physics of the Air. 3d ed., . p. 371.