Why does laser cause permanent damage to the eyes?

Laser technology is becoming more and more common nowadays, and people are also encouraged to wear laser safety glasses and laser protective helmets when operating lasers (especially when operating laser welding equipment). Otherwise, the eyes will be damaged by lasers and cannot be restored. Then people will have questions, why does laser cause permanent damage to the eyes?

1. What is laser? What is the principle of its generation?

Laser (LASER) is the abbreviation of "Light Amplification by Stimulated Emission of Radiation".

Generation principle

It is based on the energy level structure of atoms. Atoms are composed of atomic nuclei and extranuclear electrons. Electrons move in different orbits, and each orbit corresponds to a specific energy level. Under normal circumstances, most electrons in atoms are in low energy levels (ground state). When energy is provided by the outside world (such as through current, light, etc.), some electrons will absorb energy and transition to high energy levels (excited states).

Electrons in excited states are unstable, and they will spontaneously transition to low energy levels and release photons. In ordinary light sources, the emission of these photons is random and disordered. In the process of laser generation, a part of the photons is reflected back and forth in an optical resonant cavity by setting a reflector in the cavity. When these photons meet the atoms in the excited state, they will stimulate these atoms to emit stimulated radiation, causing the atoms to emit photons with the same frequency, phase and polarization direction as the incident photons. These photons are constantly reflected and amplified in the resonant cavity, and finally form a laser beam with high intensity and good directionality.

2. The main reasons for the permanent damage of laser to the eyes are as follows:

a. Energy characteristics of laser

High energy density

Laser has high coherence, directionality and monochromaticity, which enables it to focus extremely high energy in a very small area. The light of ordinary light sources diverges in all directions and the energy is dispersed. The laser beam can be focused on a very small point. For example, when imaging the retina, the spot diameter may be only a few microns to tens of microns.

Taking a typical continuous wave laser as an example, when it is focused on the retina, the energy (energy density) received per unit area will far exceed the range that the retinal cells can withstand. The retina is like the film of a camera, with a variety of light-sensitive cells, such as cones and rods, distributed on it. These cells will be instantly destroyed under high-energy-density laser irradiation

Energy absorption and conversion

The absorption characteristics of intraocular tissues to lasers. Different tissues in the eye have different absorption characteristics for lasers of different wavelengths. For example, the retina mainly absorbs lasers in the visible light band (400-780nm) and near-infrared (780-1400nm) lasers. When lasers of these wavelengths enter the eye and are absorbed by the retina, the light energy is quickly converted into heat energy.

Just like using a magnifying glass to focus in the sun can ignite paper, the energy of the laser is highly concentrated at the retina and produces a thermal effect. This thermal effect will cause the temperature of the retinal tissue to rise sharply, leading to protein denaturation and cell structure destruction. Moreover, this thermal damage is irreversible. Once the structure of retinal cells is destroyed, their normal visual functions such as photosensitivity and signal transmission cannot be restored, resulting in permanent vision damage.Therefore, it is very necessary to wear laser safety glasses.

Moreover, lasers with wavelengths in the infrared band (such as 1064nm Nd:YAG lasers) may be absorbed by the lens in addition to the retina. When the lens absorbs laser energy, it will also produce a thermal effect, causing lens opacity, which is cataract. This laser-induced cataract is also a permanent eye injury, which will affect the transparency and refractive power of the lens, leading to decreased vision.

b. Structural characteristics of the eye

The eye itself is a complex optical system, and the cornea and lens can focus external light on the retina. This focusing process is very important for normal visual imaging, but it also makes it easier for lasers to damage the retina.

When a laser beam enters the eye, the optical system of the eye will focus the laser on the retina like focusing ordinary light. This is equivalent to actively focusing the energy of the laser on the retina, a very sensitive area. Even a relatively weak laser will greatly increase its energy intensity on the retina after being focused by the eye, thereby increasing the risk of retinal damage.

The retina is the key part of the eye that receives light signals and converts them into nerve impulses. The retina is a thin tissue, and the photoreceptors in it are very delicate and sensitive to environmental changes.

It does not have a strong regeneration ability like skin and other tissues. Once the retinal cells are damaged by laser, it is difficult for the human body to repair these damaged cells through regeneration. For example, cone cells are mainly responsible for color vision and high visual acuity. Their damage can lead to abnormal color perception and decreased vision. And because the retinal nerve tissue is difficult to recover once damaged, this damage is often permanent.

In summary, whether it is based on the energy characteristics of the laser (high energy density, energy absorption and conversion) or the structural characteristics of the human eye (the eye is a complex optical system), the damage to the human eye caused by laser is permanent and cannot be restored. Therefore, when performing laser operations, laser safety glasses must be worn, which is also a requirement of the laser industry.