Using light onset as the stimulus in simple reaction time (SRT), the effect of stimulus intensity was studied in both between-subjects and within-subjects experimental designs. There was a strong intensity effect in both conditions but no significant interaction between the effect of stimulus intensity and the type of design. This differs from previous results with auditory stimuli where such an interaction has been demonstrated. When the criterion parameters of variable criterion theory were evaluated directly, the only significant effect was greater criterion variability in the between-subjects condition. Theoretical functions describing the growth of sensory strength for each intensity had different starting points and were largely parallel, showing only late temporal divergence. This provides an explanation of the rarity, in the SRT literature, of interactions between visual intensity and criterion variables. Correlations illustrating the relations between reaction time (RT) measures and theoretical criterion parameters are presented. Absence of the predicted relation between intensity and RT variability is evidence against theories relating RT to impulse rate treated as a Poisson process.