When the eye is exposed to pulsed infrared (IR) light, it is perceived as visible of the corresponding half wavelength. Previous studies have reported evidence that this is due to a non-linear two-photon absorption process. We have carried out a study which provides additional support to this nonlinear hypothesis. To this end, we have measured the spectral sensitivity at 2 different pulse repetition rates and have developed a theoretical model to account for the experimental observations. This model predicts a ratio between the minimum powers needed to detect the visual stimulus at the 2 pulse repetition rates employed of 0.45 if the stimulus were detected through a nonlinear effect and 1 if it were caused by a linear effect as in normal vision. The value experimentally found was 0.52 ± 0.07, which supports the hypothesis of a nonlinear origin of the two-photon vision phenomena.

中文翻译：

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脉冲红外光的视觉是由非线性光学过程介导的

当眼睛暴露于脉冲红外（IR）光中时，可以看到相应半波长的可见光。先前的研究已经报道了证据，这是由于非线性的两个光子吸收过程所致。我们进行了一项研究，为这种非线性假设提供了额外的支持。为此，我们测量了2种不同脉冲重复频率下的光谱灵敏度，并建立了理论模型以说明实验结果。该模型预测，如果通过非线性效应检测到刺激，则在采用2个脉冲重复率0.45时检测视觉刺激所需的最小功率之间的比率，如果该刺激与正常视觉一样是由线性效应引起的，则为1。实验得出的值为0.52±0.07，