Development of new microperimetric tools dedicated for imaging of early functional changes in the retina may help in the monitoring of various ocular diseases progression e.g. Age-Related Macular Degeneration. Recently described two-photon vision may be applied to microperimetric devices. Many subjects with well-known disease history could be investigated with newly developed instrumentation that tests ability of human eye to perceive near infrared radiation. The main limitation of this new method is a very high cost of the femtosecond laser. Facing this problem, we try to replace the femtosecond laser with lower cost fiber-optic picosecond light source. To compare these two lasers, we constructed dedicated measurement system. We performed measurements of two-photon vision threshold on healthy subjects for two different light sources - sub picosecond Kerr mode-locking solid-state laser and fiber-based picosecond laser. Experiments were conducted for an open circle flickering stimulus with 0.5 deg. diameter, for retinal locations varying from 0 deg. to 5.8 deg., using 4-2-1 threshold strategy that is well-known from classical microperimetry. Values of obtained thresholds are only 5 times higher for the fiber laser than that obtained by using the femtosecond laser, while it was expected to be about over 16 times higher. This fact requires further investigations. Nevertheless, the idea of replacement of the latter laser by relatively cheap fiber-optic one in ophthalmic devices for two-photon vision studies seems to be potentially promising.
Human subjects can detect infrared light at wavelengths over 1000 nm perceived as visible of the corresponding half wavelength. This is due to a two-photon process and requires the use of pulsed light sources well focused within the retina. We have developed an experimental system to measure, for the first time, the visual resolution of the eye when is stimulated with infrared (1043 nm) and compared with visible light (543 nm). Scanner mirrors were used to project letters of different sizes onto the retina in both wavelengths. Subjects performed a visual test to determine the smallest letter size that was distinguishable for each wavelength for a range of defocus values. An additional optical path was used to record the retinal images of the spot after reflection in the retina and double-pass through the optical media. The best visual acuity was obtained at different defocus locations corresponding to the chromatic difference between green and infrared. Although, there was some individual variability, visual acuity was found to be similar both in visible and infrared. The use of two-photon infrared vision may have some potential applications for vision in those cases were the optical media is opaque to visible wavelengths while keeping some transparency in the infrared.
Light sensation relies on photoisomerization of chromophores in rod and cone photoreceptor cells. Spectral sensitivity of these photoreceptor cells in the retina is determined by the absorption spectra of their pigments which covers a range from 400 nm to above 700 nm. Regardless the mechanism leading to visual pigment isomerization, light sensation is triggered every time visual pigment molecules change their conformation. Thus, two-photon absorption (TPA) should produce the same result (visual sensation) as single photon absorption of light. This observation was positively verified and published by our group. During human psychophysics experiments, we found that humans can perceive light in the infrared (IR) range as colors that match half of the wavelength of the applied laser beam. Other experiments and theoretical research, such as mouse electrophysiology, biochemical studies of TPA in rhodopsin or molecular modeling studies, confirmed that visual sensation can be triggered by TPA. There are few publications describing human near infrared (NIR) perception and no formal proposals to use this phenomenon to improve ophthalmic diagnosis and monitor treatment. Here we report that the use of novel instrumentation revealed that the sensitivity threshold for NIR vision depends on age.
Scattering and fluorescence images provide complementary information about the health condition of the human eye, so getting them in a single measurement, using a single device may significantly improve a quality of diagnosis as it has been already demonstrated in Spectralis (Heidelberg Eng.) OCT instrument. There is still challenge to improve quality of fundus autofluorescence (FAF) images. The biggest obstacle in obtaining <i>in vivo</i> images of sufficient quality is very low fluorescence signal. For eye safety reasons, and because of patient comfort, using highpower fluorescence excitation is not an adequate solution to the low signal problem. In this contribution we show a new detection method in the retinal autofluorescence imaging, which may improve the sensitivity. We used a fast modulated (up to 500 MHz) diode laser of wavelength 473 nm and detected fluorescence in the spectral range 500-680 nm by photomultiplier and lock-in amplifier. Average power of the collimated blue beam on the cornea used for FAF measurements was set to 50 μW, 10 μW, and even 4.5 μW.