The possible damage caused by laser to the body mainly includes thermal damage, mechanical damage, photochemical damage and biological stimulation damage. The damaged areas are mainly the eyes, skin and nervous system. When the laser beam irradiates human tissue, it will not cause damage in all cases. Only when the laser power density or energy density incident on the body exceeds a certain critical value, it will cause damage to the body, which we call this critical value. Damage threshold. It is defined as the laser power density or energy density at the irradiated tissue when the tissue is minimally damaged under the irradiation of a specific wavelength of laser light. The unit is W/cm2 or J/cm2. The laser injury threshold is the dividing line for judging whether the tissue is damaged or not. But it should be noted that the injury threshold of the laser is not only related to the wavelength of the laser, but also related to the part of the irradiated body, such as the injury threshold of the eye and the skin is different. The injury threshold is also related to the skin color of a person, so the injury threshold measured by different countries is different; at the same time, there are differences between people with different skin colors.

Laser damage to the eye and thresholds Looking directly at high-brightness light sources such as the sun and arc lamps will damage vision. If you look directly at a laser with a higher light intensity, it will cause more serious damage. A large amount of light energy falls on the retina in a short time, and the energy is absorbed by melanin on the retina and converted into heat. A small part of this heat is taken away by the blood flow in the choroid, and the rest will rapidly increase the retinal temperature. When the temperature rises to 47 ~ 57 ℃, it will cause damage to the retina, the photoreceptor cells will coagulate and become degenerated and necrotic, and the photoreceptor will be lost, resulting in weakened vision or blindness. Laser damage to the eye is not limited to the fundus. Lasers in the infrared band can also damage the refractive system of the eye, such as the cornea, lens, and aqueous humor. The location and degree of damage are affected by many factors, the main factors are the wavelength of the incident laser, the intensity of the laser, the size of the pupil, the size of the image spot on the retina, the incident angle of the laser, and the depth of the fundus color.

The relationship between eye damage and laser wavelength The wavelength of the laser is divided into three parts, namely the ultraviolet band (short-ultraviolet 100-315nm and near-ultraviolet 315-400nm), visible light (400-700nm) and infrared band (near-infrared 700-1400nm, Mid-infrared 1400 ~ 3000nm, far infrared 3000 ~ 1 000 000nm).

• The damage of visible laser and near-infrared laser to the eye Although the eye has a self-protection function, the laser is much higher than all the strong light in nature. When it exceeds a certain threshold, the self-defense ability of the eye will fail. Among them, visible laser and near-infrared laser are particularly harmful to the eye, because the refractive medium of the eye has high transmittance, low absorption, and very strong focusing ability to the visible laser, which can increase the incident laser energy density by thousands or even tens of thousands. At times, a large amount of light energy is concentrated on the retina in an instant, causing the temperature of the photoreceptor cell layer of the retina to rise rapidly, causing the photoreceptor cells to coagulate, degenerate and necrosis, thereby losing their photosensitive effect. The coagulation or denaturation of proteins caused by overheating is irreversible, thus causing permanent blindness of the eye. This process mainly damages the retina, especially the macula of the eye.
• Far-infrared laser damage to the eyes Lasers of this wavelength are almost all absorbed by the cornea. After the light radiation is absorbed by the cornea, if the damage is confined to the outer epithelial layer of the cornea, symptoms (keratitis, conjunctivitis, eye pain) , foreign body sensation, photophobia, tearing, eye congestion, decreased vision, etc.), will gradually disappear after 2 days, because the corneal surface cells have a strong regeneration ability. If the damage reaches the internal tissue, it may cause permanent corneal opacity, resulting in serious impairment of function.
• The damage of ultraviolet laser to the eyes The ruby laser emits a deep red visible laser with a wavelength of 694.3nm, and more than 90% of the light energy is incident on the retina through the eyeball medium, which can easily damage the retina; the emission wavelength of the neodymium glass laser is 1060nm 7% of the light energy is absorbed by the cornea, 15% is absorbed by the lens, and 78% is projected on the retina. Although the cornea and lens will also be damaged, it mainly damages the retina. The CO2 laser emits far-infrared rays with a wavelength of 10.6 μm. Laser, the eyeball is basically opaque at this wavelength, and most of the light energy is absorbed by the cornea, which mainly damages the cornea. Almost all the light energy of the 300-400nm ultraviolet laser is absorbed by the cornea and lens, and the ultraviolet light has a cumulative damage effect. Even relatively weak UV light exposure can damage the eye’s refractive system over time, causing clouding of the lens and cornea.

The relationship between eye damage and incident laser intensity For lasers with a certain wavelength, the degree of eye damage is closely related to the total light energy, energy density or power density entering the eye. In the case of greater than the damage threshold, the greater the intensity of the laser entering the eye, the more serious the damage. If the incident laser intensity is small to a certain value, it will not cause eye damage. Table 3-15-2 lists the injury thresholds of several commonly used lasers for yellow eyes.

The size of the pupil of the eye has a certain influence on the degree of laser damage to the retina. When the pupil shrinks, the laser beam enters the fundus less, and the damage is small, and vice versa. Therefore, miosis has a certain positive significance in protecting the retina and preventing laser beam damage. When an average human eye adapts to a dark environment, the diameter of the pupil is 7-8mm. Under strong light exposure, it can be reduced to 1.5mm. The difference between the light transmission area of the largest pupil and the smallest pupil is more than 20 times. Therefore, the laser studio should be kept as bright as possible. When adjusting the laser in the dark room, the pupil of the eye is in the largest state, and special attention should be paid to laser damage.

After the laser is injected into the eye, it is focused on the retina by the refractive medium to form a light spot, and the vision of the brain is obtained from this light spot. This light spot is called a visual spot. The size of the visual spot is closely related to the laser damage to the eye. When the incident laser energy is near the threshold, the larger the spot, the safer it is, and vice versa. When the incident laser energy exceeds the injury threshold, the larger the spot, the larger the damage area, but the degree of damage will be lighter; the smaller the spot, the smaller the injury area and the heavier the damage.

Visible and near-infrared laser damage to the retina is closely related to the angle at which the laser beam enters the eye. Because the eyeball itself is a light-concentrating transmission system, when the laser beam is directed into the eye parallel to the visual axis, it gathers into a small spot in the central fovea of ​​the macular area of ​​the fundus, and its energy density is 4 higher than that of the cornea. order of magnitude, and the fovea of ​​the macula is the most important and most sensitive place for the visual function of the eye, because the fovea has no blood vessels and nerve distribution, and the thermal diffusion function is very poor; the fovea is the thinnest part of the retina, and it is more exposed to laser light than other parts of the retina. The parts are more easily damaged, so it is very dangerous to direct the laser to the eyes. When the laser light enters the eye at an angle slightly off the visual axis, the focused spot of the image will not fall on the macula, but on the retina outside the macula. Due to the good heat dissipation of the retina in these parts, the temperature rise is small, so even if the energy entering the eye is exactly the same as that of the eye when it is directly irradiated, the degree of damage caused will be much smaller. When the laser deviation angle is larger, it can be blocked by the iris and will not enter the fundus. In conclusion, when the same dose of laser beam is incident into the eye at different angles, direct vision is more likely to damage the retina than it is off the visual axis, and the damage is more serious.

Because pigmented tissues can easily absorb laser energy, the damage to the eyes by laser is also related to the amount of pigment in the fundus. Experiments have shown that people with dark skin, the amount of pigment contained in the fundus of their eyes is large, the degree of laser absorption is strong, and the damage is greater.

• Injury caused by diffuse reflection People who have been working with lasers for a long time often feel uncomfortable in their eyes, and their visual functions are reduced, such as reduced light perception and contrast, and increased light perception threshold. Punctate lesions appear and are prone to keratitis and conjunctivitis, both due to the cumulative effect of diffuse or weakly scattered multiple exposures.
• If the lens cataract exceeds the injury threshold, if the diffuse reflection ultraviolet laser and infrared laser repeatedly act on the eye for a long time, it will cause the coagulation and degeneration of the lens protein, make the lens cloudy, affect the vision, and cause cataract.
• Flash blindness and post-vision image eyes are suddenly irradiated by laser light, although it is certain that no organic damage is caused, but for a long period of time they cannot see things clearly, or even can’t see things. This phenomenon is called flash blindness. . When the laser stops illuminating, the eye still vaguely sees such a light source continuing to illuminate, which is called visual afterimage. They are all visual impairments caused by lasers.