The authors give an explanation of pupillary latency in terms of nonminimum phase lag. Minimum phase lag is that phase associated with a simple linear lag element; nonminimum phase is associated with a true delay element modeled as exp (-ST). Latency can be accurately modeled by a delay element. The authors experimentally demonstrate that pupillary latency increases with reductions in intensity and increases with frequency of the square-wave stimuli. They summarize these characteristics with a model. >