A laser is a special kind of artificial light. Its properties differ completely from those of ordinary light. Thanks to its directivity and monochromaticity, which mean that it moves in a straight line without spreading out very much, a laser’s power is concentrated and high-density. Some lasers are so powerful that they even can melt metal in an instant. For this reason, if laser operators neglect preventive measures despite knowing of these properties, they can face a high risk of injury. Of particularly serious concern, the impact of laser light on the eye is strong even at low power, and depending on its wavelength it could cause unexpected damage to the retina through concentration of light in the crystalline lens, resulting in worsened vision or other permanent harm to the functioning of the eye.
Properties of laser light and risks to the eye
(i) Differences between lasers and ordinary light (such as light bulbs and fluorescent lamps)
- Superior directivity
- While ordinary light sources, such as white light bulbs and lamps, emit light in all directions, a laser beam is emitted in a straight line. This is referred to as having superior directivity. As a result, even at the same power level of one watt, laser light is much more powerful than ordinary light when it reaches a subject.
- Superior monochromaticity
- Ordinary light is a mixture of various kinds (wavelengths) of light. Since when it enters the eye as well the refractive index for each wavelength in the crystalline lens differs, the light is not concentrated on a single point. Since laser light has a single wavelength, its energy is extremely dense. For this reason, laser light is highly dangerous to the eye, because its power is focused in the crystalline lens and it delivers even higher-density energy to a part of the retina.
(ii) Laser light’s effects on the eye
There are two types of lasers: visible-light lasers, which can be seen by the human eye, and invisible-light lasers, which cannot. Many machine tools such as laser-beam welders, fusion cutters, and markers use invisible-light lasers, such as carbon dioxide (CO2) lasers and yttrium aluminum garnet (YAG) lasers, and some lasers are even more high-powered and dangerous. In addition, the parts of the eye susceptible to damage vary with the wavelength of laser beams. CO₂ lasers are harmful mainly to the cornea, while YAG lasers mainly damage the retina. Since the eye’s self-healing capabilities are weak, eye damage can lead to permanent vision problems.
Illustration: Effects on the eye of excessive exposure to laser rays
Lasers specified as requiring protection from contact with human beings are those with wavelengths in the range 180 nm to 1 mm (JIS C 6802). As illustrated above, the parts of the eye that are susceptible to damage vary with the wavelength of light.
Most light in the ultraviolet spectrum (400 nm and lower) is absorbed in the surface of the cornea. The portion that does penetrate is absorbed in the crystalline lens. Exposure to a high-powered UV laser can damage eye tissue through photochemical effects, causing inflammation (burning) of the cornea from short-term exposure. It is said that long-term exposure can result in photochemical effects that can cause cataracts.
The eye reacts immediately to light in the visible light spectrum (400-700 nm) by blinking as a protective reflex when it senses a flash of light. However, there are limits to the speed at which this reaction takes place, and the laser beam penetrates the eye for about 0.25 second between when the eye senses danger and when the protective reflex kicks in. Generally, lasers that can be considered safe for penetrating the eye during this period are those with outputs of 1 mw or less. It is said that anything stronger could cause permanent damage to the eye by harming parts of the retina as a result of heat and condensation of light.
Laser beams in the near-infrared spectrum (700-1400 nm), like those in the visible-light spectrum, reach the retina. Caution is required with regard to the invisible spectrum in particular, since its wavelengths are said to be very dangerous to the eye because they might not be noticed until the eye has been damaged.
- Long-term damage caused by lasers
- Many laser-caused accidents involve the near-infrared spectrum. When a visible-light or near-infrared spectrum laser beam comes into contact with the eye, it damages the retina. Since vision in the damaged portion will not recover, this leads to the serious problem of loss of vision.
Safety measures for laser light
- Be sure to check the laser class of the laser device.
- To enable safe use, all laser devices are labeled with their laser class (in line with Japan Industrial Standard [JIS] C6802, “Safety of laser products”).
- Most lasers used in laser machine tools are in class 4—the most dangerous class of lasers. The notice “Measures to prevent harm from laser rays” issued by the Ministry of Health, Labour and Welfare of Japan describes the specific content of preventive safety measures for work using lasers, separated by classes of laser device.
Basic concept of safety measures
Step 1. Check the specifications of the laser device.
If the laser protective gear you use is not compatible with each of the conditions of the laser device—its laser wavelength (nm or μm), output (mw, w, kw), and output method (continuous, pulse)—then it cannot protect you from laser light. For example, different protective gear must be used for a YAG laser vs. a CO2 laser. Even protective gear for the same laser wavelength might not be compatible with different laser outputs.
- What to check
- 1.Laser wavelength (nm or μm)
- 2.Laser output (mw, w, or kw)
- 3.Laser output method (continuous oscillation, pulse mw, w, or J second)
- Typical lasers used in laser machine tools
Step 2. Consider ways to avoid looking directly at the laser light.
It is very dangerous simply to watch the part being machined without understanding the risks of laser light and taking protective measures. In particular, most of the lasers used in laser machine tools are very dangerous because, as invisible-light lasers, they cannot be sensed by the human eye. Be fully aware that light such as that dispersed from the part being machined can damage the eyes
Step 3. Choose appropriate protective gear.
- Protective gear to use in laser management zones: Prepare a safe environment.
- If the part being machined is visible (i.e., exposed), then ideally it should be covered with a laser-shield window suited to the wavelength of the laser. If it is impossible to cover the part because it is too large or for other reasons, a laser safety fence must be used to set up a laser management zone and prevent entry into the zone. If it is absolutely essential to look inside the laser management zone, protective equipment such as a laser shield window or laser shield curtains must be set up to prevent laser light from leaking outside the management zone.
- Protective gear to use during operations: Protect workers.
- If it is absolutely essential to see the part being machined, laser protective eyewear suited to the wavelength of the laser being used must be worn. Laser protective eyewear comes in various types, including goggles, over-the-glass safety glasses, and high-powered protective eyewear for use with high-output lasers.