Methods of determining the contrast sensitivity function for two-photon vision

Two-photon vision relies on the perception of pulsed near-infrared laser beams as having colors like their half-wavelength counterparts. The phenomenon is due to two-photon absorption occurring in visual pigments [1]. This study is focused on methods to determine the contrast sensitivity function (CSF) for two-photon vision, which has not yet been investigated. CSF was measured for eight spatial frequencies using the tumbling E letter optotype. The optotype was projected on a white background by fast scanning the retina with a pulsed 1040 nm or 520 nm laser beam, both perceived as green. The contrast threshold was determined for the power of the beam corresponding to a minimum stimulus brightness for which the subject was able to state the correct letter orientation. Because a luminance curve for the two-photon stimulus is not available, expressing the brightness of the infrared stimulus in photometric units required finding a suitable method. Three approaches for determining contrast sensitivity for two-photon stimulus were proposed and tested to overcome this problem. The threshold contrast values, defined as Weber contrast, differ substantially between normal and two-photon vision mechanisms. Each tested method allowed qualitative comparison of the obtained contrast sensitivities. The results show that the two-photon CSF has a significantly broader range than the one-photon CSF. Determining the CSF for twophoton vision will help assess the applicability of this phenomenon to augmented reality displays.