The sensitivity of human infrared vision is age-dependent

Abstract

 

Purpose

 

Pulsed IR laser light is detectable by humans and causes sensation of color corresponding to roughly half of the stimulus wavelength. Some authors recently explained this phenomenon as resulting from two-photon isomerization of chromophore in human photoreceptors. Here we describe the effect of age on this phenomenon.

 

Methods

 

A beam from a laser source emitting a 1040 nm train of 200-fs pulses was coupled into an optical fiber of 2 m or 1 km length to obtain the different pulse lengths, 1 ps or 300 ps, respectively. After the fiber, the test set-up was arranged to create a linear scan on the fundus, perceived by the volunteer as a horizontal line (Fig. 1). To measure minimal power of the laser beam causing a visual sensation (sensitivity threshold), a neutral density filter (NDF) filter was adjusted by the subject during the test. Values measured by the power meter were saved in PC memory after the subject pressed a mouse button and measurements were repeated 10 times.<br /> The setup was isolated from residual ambient light by its dedicated housing, a subject’s head was covered by a black curtain attached to the housing. All individuals were dark adapted.<br /> The study was performed in 16 healthy volunteers of ages ranging from 24-84 years. All tests were conducted in compliance with ANSI Z136.1 and EN 60825−1. The study was approved by the Ethics Committee of the Collegium Medicum NCU. Examinations were carried out after written informed consent was obtained.

 

Results

 

The results are presented in Fig. 2. Volunteers were divided into two groups differing in age. Mean threshold power values for short pulses of the two groups were: 52±8 uW for subjects of age <50 and 169±33 uW for the more elderly. For long pulses, means for both groups were: 184±44 uW and 300±280 uW, respectively. The one-way ANOVA test performed in the two groups with significance level ≤0.05 showed that the mean values were significantly different for short pulses but not for long ones.

 

Conclusions

 

This study indicated that the sensitivity threshold for human IR vision is age-dependent for short pulses. The two-photon absorption process is stronger for short pulses causing a statistically significant correlation with age.  

 

1. Optical set-up for sensitivity measurements.

 

 

2. Sensitivity thresholds for a 1040 nm beam with different pulse lengths. Grey bars show mean values calculated for two subgroups of volunteers: <50 years old (black symbols) and >50 years old (red symbols).