Invisible infrared light creates green letters that are less prone to blur
Infrared light is invisible to us. Yet under the right conditions, pulses of near-infrared light can give rise to a visible image in the eye—one that behaves in ways that defy intuition. ICTER scientists investigated what happens to that image when it is no longer perfectly focused.
Scientists at the International Centre for Translational Eye Research (ICTER) have discovered that invisible near-infrared light can create green letters that are less prone to blur compared to conventional visible-light images. This phenomenon, known as two-photon vision, occurs when two photons are absorbed almost simultaneously, resulting in a combined energy close to that of a single photon at half the wavelength.
As a result, near-infrared light with a wavelength around 1040 nm can produce a green light sensation similar to 520-nm green light.
To investigate the defocus tolerance of two-photon images, researchers conducted an experiment involving 12 healthy participants aged 25-47 with perfect or near-perfect vision. The participants' dominant eyes were tested, with one eye covered at a time, and no drugs were used to block the eye's natural accommodation process. The researchers used a scanning laser to display a small letter E, measuring the eye's amplitude of accommodation and the defocus tolerance for both two-photon and visible-light stimuli.
The main finding revealed that two-photon images tolerated greater defocus before participants perceived them as persistently blurred. The advantage persisted in both non-presbyopic and presbyopic groups, with the largest difference observed in the non-presbyopic group. Participants in this group reported the first signs of blur at a mean imposed defocus of 6.6 ± 0.7 diopters for the two-photon stimulus, compared to 4.3 ± 0.7 diopters for the visible-light stimulus.
The range over which the two-photon image remained sharp was more than 1.4 times wider than the conventional visible-light image across all participants. This property is attributed to the quadratic dependence of two-photon absorption on local light intensity, which allows the edges of defocused beams to remain well defined at greater defocus.
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