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Erica Towle, Michelle Aaron, Leon McLin, Benjamin Rockwell; Influence of Thermal Lensing on Human Visual Function. Invest. Ophthalmol. Vis. Sci. 2013;54(15):586.
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It has been hypothesized that slight heating (1 - 3°C) of ocular tissue with a near-infrared (NIR) laser could induce temporary changes in visual function through an effect known as thermal lensing. If these visual distortions could be safely induced and controlled, this technology would have potential as a new form of device that can temporarily alter the visual field of the observer. A previously completed study concluded that it was possible to safely and temporarily interrupt visual function using the thermal lensing effect; however, little effort was made to measure the extent of the blurring in terms of changes to a subject’s visual acuity and contrast threshold.
Ten subjects were asked to identify the orientation of a 50% contrast Landolt ring while being exposed to various laser sources co-aligned with the stimulus. Each exposure lasted 0.5 seconds and was within the safety limits set by the American Conference of Government Industrial Hygienists. With each exposure, the size of the target was increased (from 0 logMAR) until the subject was able to correctly identify the location direction of the Landolt ring. Once the target was correctly identified, the current and previous acuity levels were repeated three times to refine the threshold of acuity. This threshold under each laser condition (none, visible only, NIR only, combined visible and NIR) was then averaged across three sessions (days) to account for any subject variability.
Results show that effects of a visible only exposures significantly alter the subject’s ability to see targets smaller than 0.25 logMAR. NIR only exposures did not significantly change the subject’s acuity. By combining two sources, the influence of the NIR on the visible obscuration was also observed to be negligible. Several subjects, however, were able to correctly identify when the NIR light was included in the exposure by a distinct “graying” or “loss of contrast” of the target.
While the blurring effect of the thermal lens was not able to obscure the target on its own, evidence suggests that a distinct loss of contrast was induced during the experiments as a result of the thermal lens introduced in the eye. This loss of contrast has the potential to reduce visual acuity of much finer targets (high resolution Gabors), and therefore, future experiments are planned to investigate the change in an individual’s contrast acuity in the presence of a thermal lens.
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