Scientists decipher two-photon vision

optical system. The IR and VIS beams were generated by a femtosecond laser (pulse width τ=200 fs, repetition rate F = 76 MHz). BS, beam splitter. CAM, camera; GS, galvanometer scanner; IF, 940 nm bandpass filter; IL, illuminator. L, lens. LED, white light emitting diode. M, mirror. NDF, neutral density filter. NDV, neutral density gradient filter; OBJ, objective. PM, power meter. Pupillary plane PP and retinal plane RP. The conjugate planes (PP* and RP*) are indicated by blue and red lines, respectively. Inset graph: Optical spectrum of the stimulation laser and white LED (background) measured in the pupil plane. Credit: Biomedical Optics Express (2024). DOI: 10.1364/BOE.525180
Two-photon vision is a new method with great potential for the future of ophthalmological diagnosis. Although it has many advantages, improvements are needed in key areas. Scientists at the International Center for Ophthalmology Research (ICTER) have taken a step forward by refining this technology and opening new perspectives in ophthalmology.
Instead of looking at images through a lens, imagine looking through a kaleidoscope that focuses invisible light to obtain a new range of colors. Photons, ephemeral messengers of light, usually appear singly, but here they appear as double photons, which is the basis of two-photon vision. This is an unusual phenomenon in which the human eye, instead of perceiving conventional light, receives pulses of infrared lasers, a gateway to the invisible world.
However, the key is to measure the brightness of two-photon stimuli, which until now could only be measured using visible light. ICTER scientists have made a breakthrough and determined infrared brightness values using photometric units (cd/m2). Thanks to this approach, it is possible to link the luminance of two-photon stimuli to a new physical quantity related to perceived brightness: two-photon retinal illumination.
Research – carried out by scientists from the International Center for Ophthalmic Research (ICTER) with the participation of Drs. Student Oliwia Kashkos PhD Engineering Katarzyna Komar and Professor Maciej Wojtkowski showed that the brightness of two-photon stimulation can reach almost 670 cd/m2, within the range of eye-safe laser powers.
The result was a paper titled “Method for determining the brightness of two-photon visual stimuli” and was published in the journal Biomedical Optics Express.
see the invisible world
The human eye can receive stimulation from the world around it in the form of electromagnetic waves ranging from approximately 380 nm to 780 nm (violet to red). Waves outside this range, such as infrared (above 780 nm) and ultraviolet (below 380 nm), are invisible to us without special equipment, but can affect our senses in other ways .
All visual processes follow the same path when photons of visible light are absorbed by the visual pigments of the photoreceptors in the retina (the light-sensitive part of the eye). This event sets off a series of chemical reactions that convert photons into electrical signals that are processed in the brain.
Two-photon vision is a phenomenon in which the human eye can perceive ultrashort pulses of infrared lasers with wavelengths ranging from 800 to 1300 nm by absorbing two photons. This process causes the visual pigments to become isomerized, allowing us to perceive light at wavelengths that are half of infrared wavelengths. Although these lasers are outside the visible range of the spectrum, their effect on visual pigments allows infrared light to be recorded as different colors.
Two-photon vision differs from single-photon vision primarily in the way light is absorbed. In single-photon vision, each photon with a specific energy is absorbed by molecules in the eye, allowing us to perceive light in the visible range. In two-photon vision, on the other hand, two photons with half the energy are absorbed simultaneously by visual pigments, resulting in the perception of light of half the wavelength that theoretically should be invisible.
Furthermore, the brightness of the two-photon stimulus varies as a function of the square of the power of the optical radiation, so that the light scattered in the eye is not perceived. Brightness also depends on the focus of the beam on the observer’s retina. The received stimulus is clearer and has better contrast than in “normal” single-photon vision.
ICTER scientists have been studying the phenomenon of two-photon vision for many years and are the first in the world to describe it. And now they have made a new breakthrough.


Schematic diagram of simultaneous display of stimuli in the brightness adjustment method. Credit: Biomedical Optics Express (2024). DOI: 10.1364/BOE.525180
A new method to determine the brightness of two-photon stimuli
Two-photon vision shows potential in two major areas: medical diagnostics and virtual/augmented reality (VR/AR). It can be particularly used for advanced diagnostic tests such as neurology and ophthalmology, where infrared pulses allow visual function to be safely monitored without the use of visible light. On the other hand, this phenomenon makes it possible to create new realistic visual experiences by manipulating optical stimulation in the infrared region, opening up new possibilities for interaction with virtual images (VR/AR).
All future applications of this phenomenon will require knowledge of the luminance of the two-photon stimulus, but the luminous efficiency function V(λ) outside the visible range is unknown. Quantifying the brightness of two-photon stimuli requires non-standard approaches, such as using infrared light. ICTER scientists have done just that.
The method presented in this paper makes it possible to express the brightness of two-photon stimuli in photometric units. Thanks to the measurements carried out, scientists were able to demonstrate the relationship between the power of the infrared beam and the power of the visible beam. This relationship was subjectively adjusted so that both were perceived to have the same brightness.
By exploiting the relationship between the power density of the VIS laser and the brightness of the projected stimulus, we were able to determine the subjective brightness of the infrared stimulus using photometric units (cd/m2). These results highlight the nonlinearity of two-photon vision, which is consistent with previous studies.
This study aimed to develop a repeatable method for determining stimulus brightness for two-photon vision. Standard methods do not allow this to be done outside the visible light spectrum, but our work opens the door to achieving this goal. This is necessary for further research and development of the application of this phenomenon in medical diagnostics and augmented reality (AR). Virtual reality (VR) technology.
The new approach also makes it possible to compare the brightness of two-photon stimuli with traditional displays based on standard single-photon vision, says ICTER’s Dr. Oliwia Kashkos. student, optometrist, and lead author of the study.
A platform for further discovery
The result of the study is the proposal of an entirely new physical quantity, called two-photon retinal illumination, sufficient to describe systems that emit two-photon stimuli. This relationship allows prediction of the luminance value of two-photon stimulation, which can reach 670 cd/m2 in the safe laser power range for the human eye without adaptive optics (AO) correction.
Furthermore, the scientists recorded that measurements made against a background with a luminance of 10 cd/m2 were twice as reproducible. This is critical for the development of future technologies such as two-photon retinal displays that can be used in augmented reality (AR) glasses and advanced diagnostic tools such as two-photon microperimetry.
The paper’s authors include Oliwia Kaczkoś, Agnieszka Zielińska, Jacek Pniewski, Maciej Wojtkowski, and Katarzyna Komar.
Further information: Oliwia Kaczkoś et al, How to determine the brightness of two-photon visual stimuli, Biomedical Optics Express (2024). DOI: 10.1364/BOE.525180
Provided by Polish Academy of Sciences
Citation: Scientists Decipher Two-Photon Vision (November 4, 2024), Retrieved November 4, 2024 from https://phys.org/news/2024-11-scientists-decipher-photon-vision.html
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