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How to prevent the eyes from being injured by laser machines?

Laser can cause irreversible and permanent injury to human eyes, ranging from eye fatigue to permanent blindness. This is a saying in the safety guidelines of laser machines that often reminds everyone. But how exactly does laser damage the human eye? This article will talk about this issue in detail for everyone.

 

When it comes to eye damage, the first thing you should be familiar with is the structure of the eye. So let's first take a look at some basic structures and functions of the eye. Figure 1 shows the basic structure of the human eye, some basic optical tissues of the eye - they are the cornea, aqueous humor, lens and vitreous humor. Among various tissues, the tissues most vulnerable to laser damage are the cornea, retina and lens.


 

Let's take a closer look at their respective functions. As shown in the figure, the outermost part of the entire eye is the cornea. The living tissue of the cornea of the human eye is directly exposed to the environment, although there is a thin tear film on the outside to protect it. The outermost surface of the tear layer is a single-layered lipid monolayer with a thickness of less than 0.5 microns, and then underneath it is a mucin layer whose mucin concentration gradually increases to make the cornea’s average refractive index 1.376. These components provide 70% refractive power of the eye. The cornea itself has a high metabolic rate and can be rejuvenated within 24 to 48 hours.

 

The pupil is the aperture of the eye, with a normal diameter and length of 2-7mm. The diameter decreases with age, but if the diameter reaches 7 mm, it may mean pupil damage. The iris is a round colored membrane used to adjust the pupil of the eye, located behind the cornea. The sclera is a denser fibrous shell that keeps the shape of the eye and bears the internal pressure of the eye.

 

The retina is probably the most familiar eye tissue. Its thickness varies, generally 0.4mm, the edge of the optic disc is the thickest, about 0.5mm, and the center is the thinnest with 0.1mm thickness. It is composed of multiple complex layers of nerve cells, of which the rod and cone layer is the photosensitive tissue. There are about 120 million rod cells in the retina, which are sensitive to weak light stimulation; in addition, there are 6.5 million to 7 million cone cells, which are sensitive to strong light and color. The macula lies in the optical central area of the human eye and is the projection point of the vision axis. The clarity of the human eye depends on the formation of the real image on the macula. The depression in the center of the macula is called the fovea. It is the most sensitive place for vision. Most of the cones are concentrated here.

 

The lens is located in front of the vitreous body, and is connected to the ciliary body by the lens suspensory ligament. It is in the shape of a biconvex lens and is full of elasticity. The lens is like the lens in a camera. It has a refractive effect on light, and it can also filter out some ultraviolet rays and protect the retina. But its most important role is to change the refractive power through the contraction or relaxation of the ciliary muscles, making the focal point of the eyeball's condensing light can accurately fall on the retina in both far and near terms. However, the metabolism of the lens is slow. The ability to adjust will gradually decrease with aging, forming a phenomenon of presbyopia. If the lens is partially or completely opaque due to various reasons, cataracts will occur.

 


What impact will the laser have on these tissues?


The damage caused by light to the eyes is mainly due to the temperature effect and photochemical reaction caused by the energy absorbed, which causes biological damage. The main form of damage depends on the wavelength of the light and the tissue exposed. For the damage of laser, the main cause of damage is tissue damage caused by high temperature caused by the absorption of light of different wavelengths by different parts.

 

Therefore, the injured part of the eye is directly related to the wavelength of the laser radiation.


The laser radiation entering the eyes and its damage can be roughly divided into:


1. Near ultraviolet wavelength (UVA) 315-400 nm, most of the radiation is absorbed in the lens of the eye. The ultraviolet rays penetrate the cornea and are absorbed by the lens, causing the soluble protein of the lens to cross-link and condense, making the lens aging or becoming opaque. Cataracts eventually occur. The effect of ultraviolet rays on the crystals is cumulative, so this effect is delayed, and problems may not appear until several years.


2. Far ultraviolet (UVB) 280-315 nm and (UVC) 100-280 nm, most of the radiation is absorbed by the cornea. Ultraviolet rays can cause acute damage to the cornea and conjunctiva through photochemical action, and cause protein coagulation and denaturation, thereby causing the corneal epithelium to fall off. Among them, ultraviolet rays with a wavelength of 280 nanometers have the greatest damage to the cornea. People only feel foreign body sensation and mild discomfort in the eyes at the first time. In severe cases, they will feel burning, severe pain, photophobia, tearing, blepharospasm (eyes cannot be opened). If the disease is repeated, it can cause chronic blepharitis and conjunctivitis, resulting in the so-called snow blindness and welded eyes.


3. Visible (400-760 nm) and near-infrared (760-1400 nm) most of the radiation is transmitted to the retina. Excessive exposure may cause flash blindness or retinal burns and lesions. The principle of retinal pathology is that when the blood flow of the choroid layer located between the retina and the sclera cannot regulate the heat load of the retina, it will cause thermal burns (lesions) in the eye, which will burn blood vessels and cause secondary bleeding of vitreous fluid, which can blur vision outside the field of view. Although the retina can repair minor damage, major damage to the macular area (the area with the sharpest vision) is one of the main causes of vision or temporary blindness, or even permanent vision loss.


4. Most of the far infrared (1400 nm-1 mm) radiation is transmitted to the cornea. Excessive exposure to these wavelengths can cause corneal burns. Infrared rays with longer wavelengths will also penetrate the tissues of the eye and fall on the retina, causing damage to the retina, especially damage to the macular area, resulting in macular degeneration.


Secondly, the duration of exposure is also an important cause of eye damage. For example, if the laser has a visible wavelength (400 to 700 nm), the beam power is less than 1.0 mW, and the exposure time is less than 0.25 seconds (anaphobic response time), the retina will not be damaged due to the short exposure time of the beam. Class 1, Class 2a, and Class 2 (see notes for laser classification) lasers fall into this category, so they usually do not cause retinal damage. Unfortunately, the beam or specular reflection observation of the 3a, 3b, or 4 lasers and the diffuse reflection of the 4 lasers may cause such damage, because the beam power is too large. In this case, the 0.25 second aphotic reaction is not not enough to protect the eyes from harm.

 

For pulsed lasers, the pulse duration also affects the possibility of eye injury. Focusing on the retina of pulses with a duration of less than 1 ms can cause sound transients. In addition to the thermal damage mentioned above, it can also cause serious other physical damage and cause bleeding. Nowadays, the pulse duration of many pulsed lasers is less than 1 picosecond. The ANSI Z136.1 standard of the American National Standards Institute defines the maximum allowable exposure (MPE) that the eye can accept under conditions that may cause eye damage (under specific exposure conditions). If the MPE is exceeded, the possibility of eye injury may be greatly increased. Because the focal magnification (optical gain) of the eye is about 100,000 times, laser retinal damage can be severe, which means that the irradiance of 1 mW/cm2 entering the eye will increase to 100 W/cm2 when it reaches the retina.

 

Finally, and the most important point: do not receive any direct laser beams under any circumstances! In addition, attention should be paid to prevent the laser beam from reflecting into the eyes, which is why it is recommended to wear laser protective glasses when working with lasers to reduce instant accidents or chronic laser damage to the eyes.