??????Due to the special anatomy and special physiological relationship of the eyeball, laser damage to the retina is closely related to the angle of the incident eye. The reason is that the eyeball itself is a condenser lens system. When the incident laser beam enters the eye parallel to the visual axis, it is focused into a small spot in the central fovea of ??the fundus macular area, and its energy density is 3 to 4 times higher than that of the cornea . The fovea of ??the macular area is the most sensitive and important area of ??the visual function of the eye. Once damaged, the visual function will change to varying degrees. In severe cases, you will be blind for life. Because the damage of vision is caused by laser photons, the photoreceptor cells are degenerated and necrotic, causing irreversible damage.
During the day, the color vision of the human eye is completely obtained by the photosensitization of the macula. Although the area of ??the macula only occupies a small part of the total area of ??the retina, the fovea diameter is only about 0.5mm. But the reflected field of view (the total area that the eyes can see clearly in front of the gaze) accounts for a large proportion. In the physiological structure, the central fovea of ??the macula is composed of 20,000 to 30,000 long and thin pyramidal photoreceptor cells. The distribution density of the photoreceptor cells is very high, mainly responsible for visual functions. The visual function of daylight is lost after being injured. In addition, there is no blood vessel and nerve distribution in the central fovea of ??the retina, so the heat diffusion function of this part is very poor. Once the injury, the hope of repair is very slim. The fovea is the weakest part of the retina. It is more likely to be damaged than other parts of the retina after being exposed to laser light, so direct laser light is very dangerous.
When the laser light enters the eye slightly away from the visual axis, the focused spot will not fall on the macular area, but on the peripheral retina. Therefore, if the incident angle is different, the damage will be different. Even if the energy entering the eye is the same as that under direct irradiation, the damage caused is much lighter. The reason is that the photoreceptor cells on the site other than the macula are much less densely distributed than the macular area, and the retina outside the macula is thicker, receives the same energy per unit area, and its temperature rise is much smaller. In addition, the retina outside the macular area is densely covered with microvessels, and some heat can be taken away from the blood circulation, which reduces the possibility of temperature increase. The higher the temperature, the heavier the damage; conversely, the smaller the temperature increase, the less likely the damage. The main effect of laser on the acute damage to the retina is caused by thermal effects.
The incident angle of the laser is not synchronized with the visual axis. The larger the deviation angle, the less damage to the retina. The iris can block the deviated laser without entering the fundus. Because the central fovea of ??the macula plays a very important role in visual function, and this part is most susceptible to damage, the danger of looking directly at the laser beam is much greater than that of entering the eye at an angle off the visual axis. Must be absolutely avoided.