Effective Intensity of Coloured Monochromatic Flashing Light

Abstract

Flashing lights are used for visual guidance lights for aircraft navigation and traffic signs for the aid of maritime transportation, as they are more conspicuous and distinguishable than fixed lights. The brightness of a flashing light is expressed in terms of the luminous intensity of a steady light that is perceived as the same brightness. Such luminous intensity is called “effective intensity”. A mathematical method to calculate “effective intensity” was proposed by Blondel and Rey and analyzed by Douglas. However, this formula is an approximation formula based on the data obtained from a visual experiment conducted under a limited condition, namely, threshold level white flashing light observed in a dark background. In this study, a flashing light generator by which white as well as monochromatic flashing lights of 8 wavelengths (430, 470, 490, 510, 550, 580, 600 and 630 [nm]) with 34 different flash durations (from 100 [μs] to 1 [s]) is prepared. A visual experiment to evaluate the perceived effective intensity of flashing lights in comparison to reference steady light is conducted using this generator, while generating flashing lights of three perceived brightness levels (“threshold”, “minimum” and “optimum”) under three different background luminance conditions (0, 1 and 10 [cd/m2]). The objective of this study is to propose a new and practical formula to estimate the effective intensity, while comparing the perceived effective intensity obtained from the experiment (Ie) with the estimated effective intensity obtained by Blondel and Rey's formula (Is) and analyzing the characteristics of the brightness perception of flashing lights in terms of time. Upon analyzing the results, the following is clarified: a) Ie gradually becomes greater than Is as the flash duration decreases from 1 [ms]; b) in the case where both the brightness level of the flashing light and the background luminance are high, the ratio (Ie/Is) is not constant, but a local minimum appears between the flash duration of 0.003 and 0.010 [s] and a conspicuous local maximum appears between 0.03 and 0.10 [s]; and c) flash durations that provide the local maximum and local minimum are relatively short for white, blue and yellow flashing lights and long for red and green flashing lights. A new formula to estimate the effective intensity is created upon considering the photochemical response characteristics of the receptor of the visual system and characteristics of the sensitivity suppression caused by the feedback effect of the nervous system, both in terms of time. Further, it is verified that the estimated effective intensity calculated by the new formula (It) properly approximates to the perceived effective intensity.