We carried out the psychophysical experiment matching and comparing the intensity of a flash light with that of a fixed light, because some papers have reported that the formula for the effective intensity of a flash light presented by Blondel amp; Rey in 1912 can not be applied when the background is bright and when the luminance exceeds a certain threshold.As a result of this experiment, we propose that the effective intensity of a flash light when background condition is 0 cd/m2 he exuressed byloge (Y) = -4.4+0.88times;loge (X) +0.7times; loge (T)Y: Effective intensity of a flash light in lx.(Illuminance of a fixed light at the observer's eye in lx.)X: Illuminance of a flash light at the observer's eye in lx.T: Lighting time period in msec.
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