This article explains why lasers can cause harm to the human body and eyes, how to use laser safety glasses and the categories of laser safety glasses.

Laser is a kind of light, any damage that can be caused by laser, light of the same wavelength emitted by other light sources, as long as its intensity is similar to that of laser, can also cause the same damage as laser. The most sensitive organ of the human body to light is the eyes. The eyes have two self-protection abilities, which enable the eyes to see things clearly and prevent the eyes from being damaged by strong light. One is that the pupils can be automatically narrowed and enlarged, and the other is that the eyelids can be closed or opened. In the daytime, the sunlight is very strong, and the pupil shrinks, and the minimum diameter can reach about 1.5mm, which greatly reduces the light energy entering the eyeball. If the bright light still hurts, you can also close your eyes. When there is no lighting at night, the pupils of the eyes are very large, and the diameter of the pupils can be as large as 7-8mm, which increases the light energy entering the eyes by 20-30 times, so that the eyes can see things clearly in the dark. If there is a sudden burst of strong flash, such as the arc light of electric welding or the lightning light of thunder, people’s eyes can be closed immediately reflexively, thus preventing the damage of the strong light to the human eyes.

The most sensitive organ of the human body to the laser is also the eyes, and the focus of protection is the eyes. Can the eyes be protected by the constriction of the pupil and the blink reflex? The answer is no. Because it takes at least 0.1 second between when the light enters the eye and when the eye is closed due to conditioned reflex, and the Q-switched laser pulse radiates all the energy within 10-1 to 10-12 seconds, as long as the eye is closed Seeing the laser, before blinking, the laser energy has all entered the eye. As for the constriction and dilation of the pupil, it takes longer. And the difference between the largest pupil and the smallest pupil is no more than 30 times at most, while the intensity of the lasers can differ by more than a dozen orders of magnitude. Therefore, to prevent laser damage to the eyes, we must rely on laser protective glasses.

To provide effective protection against laser light, a reasonable selection of laser protective glasses must be made according to specific use requirements. When choosing protective goggles, first determine the maximum irradiance or maximum irradiance of the laser output according to the maximum output power, beam diameter, pulse time and other parameters of the laser used. Then, determine the minimum optical density value required by the glasses according to the maximum allowable radiation amount of the corresponding wavelength and irradiation time, and select the appropriate protective glasses accordingly. Laser protective glasses should make the protected laser attenuate below the safety threshold, but the laser must still be seen; in addition, the light in the non-protective band should still have good light transmittance, so as to avoid pupil dilation due to too dark, so that entering the The laser energy in the eye is increased by 20 to 30 times to avoid greater damage to the eye.

Several common laser protective glasses are protective glasses that prevent laser damage to the human eye. According to their protection principles, they can be divided into reflection type, absorption type, diffraction type and compound type.

Reflective protective goggles are laser protective goggles made of a filter material coated with a dielectric film that reflects the wavelength of the corresponding radiation laser. The method of vacuum coating is used to alternately spray multiple layers of dielectric films with high refractive index and low refractive index on the outside of the ordinary lens, and the optical thickness of each layer is 1/4 of the specified wavelength of the laser to be protected. Due to the reflection and interference of the laser light, the laser light of some specific wavelengths is nearly totally reflected, and the rest of the light is allowed to pass through the lens. Because reflective safety goggles reflect light rather than absorb it, they can withstand strong laser energy better than absorbing goggles, and have better light-sharp cut-off performance. However, since the reflectivity of reflective protective glasses is related to the incident angle of the laser, they can only have an effective protective effect within a certain angle range. When the incident angle is 15° to 30° or more, a phenomenon called “blue frequency shift” occurs. At this time, the wave light of the beam is frequency shifted downward. For example, the 694nm ruby laser can be frequency shifted to 685nm, and the 532nm Nd:YAG frequency-doubling laser will frequency shift to the green passband of the protective glasses, so that the reflective protective glasses lose their anti-laser properties. ability. Therefore, most of the actual protective glasses are both absorbing and reflective, that is, absorbing protective glasses with a reflective coating. In addition, in order to solve the “blue frequency shift” problem of reflective protective glasses, experts have made various efforts. Ttown, an American association company, has developed three-color optical interference protective glasses, which overcome the “blue shift” problem due to the use of dye filtering, and increase the light transmittance in 3 passbands. In theory, as long as the number of dielectric film layers is increased, the attenuation index of the reflective protective mirror can be correspondingly increased to any specified value. In actual use, it is not meaningful to simply pursue the high attenuation index of protective glasses, because glasses can only protect the eyes, and other parts of the head and face cannot be protected. The difference between the allowable standards is limited. Some U.S. commercial laser protective glasses have an attenuation index as high as 50, but wearing such protective glasses cannot directly look at lasers that are higher than the allowable standard of 50 orders of magnitude, because even if the eyes are not damaged, other parts will be injured.

Absorptive protective glasses are protective glasses made of filter material that absorbs the wavelength of the corresponding laser radiation. The lenses are mainly made of color filter glass or colored plastic. It is the easiest and cheapest to absorb laser light of certain wavelengths. protective goggles. Because colored glass or polycarbonate plastic can absorb specific wavelengths of laser light, and allow other wavelengths of light to pass through. A typical example is a blue-green filter against a ruby laser. Blue-green filters are transparent to blue and green light, but block red and yellow light from passing through. Absorptive protective glasses have a high absorption rate of laser radiation in a certain band, and the optical density increases with the thickness, but with the increase of optical density, the transmittance ratio will decrease significantly. Although absorbing protective glasses are simple, lightweight and inexpensive, they also have many defects, and they will be burned by high-power lasers due to heat generated by absorbing lasers. Durability tests show that if a 10W power laser continuously irradiates various filters, the plastic protective goggles will explode in a few seconds, while some glass protective goggles can withstand exposure for more than 3 minutes. Another serious drawback is that it lacks the ability to transfer from strong absorption at the laser wavelength to weak absorption at adjacent wavelengths, ie the light transmission curve does not have a steep “sag” at the laser wavelength. The sharp cut-off performance of protective glasses is not good, which leads to the absorption of light of nearby wavelengths while absorbing the laser wavelength, which reduces the transmittance of visible light. Also, it fails due to aging, oxidation, and exposure to sunlight, so the protective effect of absorbent goggles is unsatisfactory.

These are protective glasses that combine the above two types of protective glasses. It is made by sandwiching a reflective lens between two absorptive lenses.

Coherent protective glasses are made by depositing multiple layers of dielectric materials on a substrate. Due to the high coherence of the laser, the coherent cancellation of the laser after passing through the coherent protective glasses plays the role of filtering. Coherent safety glasses are available as bandpass, cutoff, and narrowband filters. This protective limiting lens has a larger optical density than ordinary absorbing protective glasses, and its filtering performance is also related to the incident laser angle.

Holographic protective glasses are produced by the method of recording image dry information with two kinds of coherent light on photosensitive materials, and are divided into holographic notch type protective glasses and ultra-narrow notch type protective glasses. It is different from ordinary coherent protective glasses. Common coherent protective glasses are made by depositing materials with different refractive indices. Holographic protective glasses can be designed with high optical density and narrow filter bandwidth, and have better light cutoff than common electrolyte material protective glasses, which can reduce insertion loss.

The optical density of holographic safety glasses at a specific wavelength is related to the angle of the incident laser light. Therefore, the design of the optical system must ensure that the protective glasses are in the position with the highest density in the optical path, and there is no adverse effect caused by the off-axis incident laser. Holographic protective glasses have been used for laser protection in the visible light and near-infrared bands. Infrared spectral region development. The Pilkington Optronics holographic safety glasses produced by the American company are made of dichromated gelatin sandwiched between glass. A further improvement is the sandwiching of optical film polymers between polycarbonate plastics. Currently, holographic filters are quite expensive to manufacture, but it is possible to manufacture holographic protective glasses inexpensively in large quantities using plastic molding techniques.

The first generation of laser protective glasses adopts a single absorption principle or reflection principle, that is, absorbing protective glasses or reflective protective glasses. The second generation of protective goggles uses a combination of absorption and reflection principles, namely absorbing and reflecting goggles. They are all based on the principle of linear optics. The third-generation protective glasses are optical switch-type protective glasses, which are made of nonlinear optical materials. It works like a watch’s LCD display. It does not respond to light of a specific wavelength, but only to the energy level of the laser, which must have a sub-nanosecond response speed. Nonlinear optical materials can change their optical properties in strong light or electromagnetic fields. When irradiated with intense laser light, the polarity of its molecules changes rapidly, almost becoming opaque; and when the laser pulse disappears, it returns to a transparent state. Therefore, optical switch safety glasses can block strong laser light and allow weak laser light to pass smoothly. Commonly used CO2 laser can be protected by ordinary optical glasses; Nd:YAG laser can be protected by green filter; He-Ne laser can be protected by dark glasses. Reflective coated glasses and color-changing glasses can protect against a variety of visible lasers, but they are easily broken down by pulses with very high peak power, or the protection fails due to too long a color-changing reaction time. No matter what kind of laser goggles, it can only protect the laser of certain wavelengths and certain power, and you should not look directly at the laser while wearing goggles.

Intense pulsed noncoherent light (IPL), commonly known as pulsed light, is not a laser, but its working principle is the same as that of a laser, and it also follows the principle of selective photothermal action. It is a kind of strong composite light with a wavelength of 500-1200nm generated and emitted by a flash lamp. This light is very similar to sunlight in nature, and is partly visible light and partly near-infrared. It also has two properties: particle-like (the energy of photons is released in units of photons) and wave-like (with a certain frequency and amplitude). Clinically, according to different treatment requirements, different filters can be used to filter out the short-wave light source, so as to obtain light in different intervals for treatment. Due to the coexistence of a variety of strong lights of different wavelengths, although the protective goggles using neutral attenuation sheets can simultaneously attenuate lasers of different wavelengths, the attenuation can only be controlled within a certain range. If the attenuation is too large, it may affect the operator’s safety. Therefore, the protection ability of IPL protective glasses for human eyes is limited.

There are many factors to consider when choosing laser safety glasses, such as laser output wavelength. Laser protective glasses are designed to prevent specific wavelengths of laser radiation from reaching the eyes, so the laser wavelength is the main factor in determining the type of laser protective glasses. Many lasers emit more than one wavelength, and each specific wavelength or spectral band must be considered separately. Laser safety goggles must attenuate every harmful spectrum or additional output light for eye safety. After the yellow light with a wavelength of 585nm hits the surface of the object, because the human eye is more sensitive to it, as long as the laser is a little bright, we will feel dazzling, and the eyes will feel dry after a long time. If you wear an eye with a gray lens, the yellow light scattered to the human eye will be greatly attenuated, and the eye will feel much softer. High-quality protective goggles, the yellow light can be attenuated to the point that the eyes can hardly feel it, so the 585nm laser protective goggles can effectively prevent laser damage to the eyes.