Poland’s ICTER advances two-photon vision

06 Nov 2024

New approach offers great potential for ophthalmic diagnostics and AR/VR applications.

Scientists at ICTER, the International Centre for Translational Eye Research in Warsaw, Poland, have made a breakthrough and determined the luminance value for infrared radiation using photometric units (cd/m²). By this approach, the scientists say it is now possible to link the luminance of two-photon stimuli to a new physical quantity related to perceived brightness: two-photon retinal illumination.

Research conducted by ICTER scientists with the participation of PhD student Oliwia Kaczkoś, PhD Eng. Katarzyna Komar and Prof. Maciej Wojtkowski have shown that the luminance of a two-photon stimulus can reach almost 670 cd/m² in the safe range of laser power for the human eye. The achievement is described in Biomedical Optics Express.

Two-photon vision differs from single-photon vision primarily in the way light is absorbed. In single-photon vision, each photon of a specific energy is absorbed by molecules in the eye, which allows light to be perceived in the visible range. In two-photon vision, on the other hand, two photons with half the energy are simultaneously absorbed by visual pigments, which leads to the perception of light with half the wavelength – which theoretically should not be visible.

Furthermore, the brightness of the two-photon stimulus varies with the square of the power of optical radiation, so light scattered in the eye will not be perceived. Brightness also depends on the focus of the beam on the observer’s retina. ICTER scientists have long been studying the phenomenon of two-photon vision, were the first in the world to describe it, and now they have made another groundbreaking discovery.

Application areas

The ICTER group say that two-photon vision shows potential in two key areas: medical diagnostics and virtual/augmented reality (VR/AR). It can be used for advanced diagnostic tests, especially in neurology and ophthalmology, where infrared pulses allow for safe monitoring of visual functions without the need to use visible light.

The method described in the Biomedical Optics Express paper allows the expression of the brightness of two-photon stimuli in photometric units. The ICTER scientists were able to demonstrate the relationship between the power of the infrared beam and the power of the visible beam, which was subjectively adjusted so that both were perceived as having the same luminance.

Using the relationship between the power density of the visible laser and the luminance of the projected stimuli, it was possible to determine the subjective luminance of the infrared stimuli using photometric units (cd/m²).

Oliwia Kaczkoś, ICTER PhD student and optometrist, and lead author of the study, said, “The study aimed to develop a repeatable method for determining the brightness of stimuli for two-photon vision. Standard methods do not allow this to be done outside the visible spectrum of light, but the research opens the door to achieving this goal, which is necessary for further research and development of applications of this phenomenon in medical diagnostics and AR/VR technologies. The new approach will also enable comparison of the brightness of two-photon stimuli with traditional displays based on standard, single-photon vision.”

Further discoveries

The result of the research is the proposal of a new physical quantity, called two-photon retinal illumination, which could describe systems emitting two-photon stimuli. This relationship allowed the prediction of the luminance values of two-photon stimuli, which could reach 670 cd/m² in the safe laser power range of the human eye without adaptive optics correction.

Moreover, ICTER scientists have twice documented the repeatability for measurements made on a background with a luminance of 10 cd/m². This is necessary for the development of future technologies, such as two-photon retinal displays, which could be used in AR glasses or in advanced diagnostic tools such as two-photon microperimetry.