Seeing nonspectral colors with single wavelength stimulation in two-photon vision.

Pulsed near-infrared (NIR) lasers can be perceived as light of approximately half their wavelength due to the process of two-photon (2P) absorption. For high intensities of light, single-photon (1P) absorption can still be perceived beyond 700 nm, so that the overall color perception in this region is the result of a mix of 1P and 2P absorption. In this study, color matching experiments were performed with seven laser wavelengths between 730 and 920 nm to investigate the interaction between 1P and 2P absorption and the range of colors that can be created by changing the laser power and repetition frequency of a ns-pulsed laser. We recorded color matches in a range from pure red for shorter wavelengths across shades of purple up to pure blue colors for the longest wavelengths, showing that nonspectral (purple) colors can be created using only one stimulating wavelength in the NIR. Changes in hue could be observed between wavelengths of 850 nm and 920 nm when laser power or repetition frequency were modified, with the highest color shifts occurring between 880 nm and 900 nm.