Abstract
The Effective Luminous Intensity (ELI) of two successive flashes for various durations and the intervals between the two flashes have been investigated. As the flash source, electodeless lamps and a circular bright dot with an angular diameter of 1′50″ were used. The durations of each flash were varied between 5 and 500 milli-seconds for 6 steps and the intervals between the two flashes were changed between 1 and 500 milli-seconds for 10 steps. Four observers aged from 24 to 32 participated in the observations. The brightness of the flash lights observed were with the threshold level or one of three suprathreshold levels. As a result, it was found that:
1) For flashes with a duration of 5 milli-seconds with a threshold brightness, the threshold value started to increase as the intervals increased from about 5 milli-seconds. This did not agree with Bouman’s results (1952) which started to vary from about 60 milli-seconds.
2) For flashes with one of three suprathreshold brightnesses, the ELI started to decrease as the intervals increased from about 10 milli-seconds irrespective of the durations. The ELI increased to a stable level at a duration of longer than about 500 milli-seconds, irrespective of the brightness. The two flashes were perceived as a single flash for intervals shorter than about 100 milli-seconds, irrespective of the brightness.
