The objective of this report is to lay the groundwork for developing a recommendation on the measurement of effective intensity of flashing lights used for signalling applications. It recommends a convolutional method that will rest on some defined visual impulse response function to calculate the effectivity intensity. As one of such realizations of convolutional methods, the Modified Allard Method with an infinite time window is described. However it is not yet an official recommendation due to the lack of experimental verification and the need for further research. This report also provides guidance on physical measurements of effective intensity for the described method, for flashing lights using any type of light sources including xenon flash tubes, light emitting diodes (LEDs) and rotating beacons, which produce pulse widths in the range from microseconds to seconds. This report does not cover specific measurement requirements for signalling light products. While this document refers to flashing lights, the described method applies also to occulting lights, isophase lights, and groups of flashes of varying duration.