Method for the determination of the luminance of two-photon vision stimuli

Two-photon vision is a new and developing field in vision science. The phenomenon is based on visual perception of pulsed infrared lasers (800-1300 nm) due to the isomerization of visual pigments caused by two-photon absorption, with color perception corresponding to a wavelength about one-half of the stimulating wavelength in the near-infrared spectral range. Future applications of this effect, both in medical diagnostics and in virtual/augmented reality (VR/AR), require the ability to determine the luminance of the two-photon stimuli. However, the luminous efficiency function V(λ) outside of the visible range is unknown, requiring a non-standard approach to quantifying the luminance of two-photon stimuli. This study proposes a brightness adjustment method to determine the subjective luminance of two-photon infrared stimuli using photometric units. The repeatability of the proposed method with the background on was approximately equal to 407 td, more than twice as good as with the background off. In this report, we present the relationship between the luminance of two-photon stimuli and a physical quantity proposed for the first time: two-photon retinal illuminance. This relationship enables the prediction of stimulus luminance that could achieve nearly 670 cd/m² within the safe range of laser power for the eye.