Subject: Laser Buying Guide

Topic: Eye Hazards

Purpose: To provide a comprehensive overview of the potential Eye Hazards associated with the use of uncertified lasers.

Audience: Laser owners, intending purchasers and venue attendees where lasers are used.

Risk: Eye Damage, Blindness

Mitigation: Purchase only lasers that meet ANSI and IEC diode wavelength emission standards

The potential for laser emission causing injury to the different structures of the eye depends upon which structure absorbs the beam energy. The absorption characteristics of the different eye tissues, and the wavelength and intensity of the laser light determine whether damage occurs to the cornea, lens, or retina. The wavelengths that penetrate to the retina at the rear eye surface are determined by the overall transmission characteristics of the eye. Figure 2 illustrates eye transmission as a function of radiation wavelength over the relevant spectral range. The cornea, lens, and vitreous fluid of the eye transmit electromagnetic radiation in a wavelength range of approximately 400 to 1400 nanometers, termed the ocular focus range. Light within this range is focused onto the retina, the sensory surface that produces signals that are sent to the brain by the optic nerve. Direct viewing of a point source of light, which is the situation effectively created during intrabeam viewing of a highly collimated laser beam, produces a very small focal spot on the retina, resulting in a greatly increased power density and a high probability of damage. The dangers are similar in certain respects to those presented by direct viewing of the sun, although the potential intensity is even higher for lasers.


The optical gain of the relaxed human eye for a highly collimated beam, which is the ratio of the area of the eye's pupil to the retinal (focused) image area, is on the order of 100,000. This corresponds to five orders of magnitude irradiance increase from the corneal surface to the retina. Allowing for aberrations in the lens-cornea system, and diffraction at the iris, a well-corrected eye is capable of focusing a 20-micrometer spot on the retina. The significance of this efficiency of the eye is that even a low-powered laser beam, if it strikes the eye, can be focused onto the retina and quickly burn a hole in the tissue, permanently damaging nerves responsible for vision. The seemingly low rated power of lasers can be very misleading with respect to the damage possible when the energy is concentrated to this extent. In the case of a laser beam entering the eye directly (intrabeam viewing), a 1-milliwatt beam produces a retinal irradiance value on the order of 100 watts per square centimeter. In comparison, direct viewing of the sun produces an irradiance at the retina of approximately 10 watts per square centimeter.

The focusing effects in the eye for an extended source, such as a conventional frosted glass lamp, in comparison to a highly collimated laser beam, which has the effective properties of a point source. Because of the differences in the nature of the sources, the power density at the retina can be 1 million times greater for a focused 1-milliwatt laser than for the standard 100-watt lamp. Assuming a perfect Gaussian laser beam, directly entering an aberration-free eye, a diffraction-limited spot size of 2 micrometers diameter at the retina is possible, compared to a focused spot of several hundred micrometers for the extended source. The corresponding irradiance (power density) values at the retina, as shown in Figure 3, are approximately 10o(E8) and 10o(E2) watts per square-meter.
 
It may be thought that a burned spot on the retina measuring even 20 micrometers would not be significant to vision, since the retina contains millions of cone cells. Actual retinal injuries, however, are usually larger than the primary focused spot due to secondary thermal and acoustic effects, and depending upon the location, even an extremely small injury to the retina can significantly damage vision. In a worst-case exposure, with the eye relaxed (focused at infinity) and the laser beam entering the eye directly or from a specular reflection, the beam is focused to its minimum spot size on the retina. If damage occurs to the area where the optic nerve enters the eye, the result is likely to be complete loss of vision. Retinal burns are most likely to occur in the area of central vision, the macula lutea, having dimensions of approximately 2.0 millimeters horizontally by 0.8 millimeters vertically. The central region of the macula, termed the fovea centralis, is only about 150 micrometers in diameter and provides detailed high-acuity vision and color perception. The regions of the retina outside this tiny area perceive light and detect movement, constituting peripheral vision, but do not contribute to detailed vision. Consequently, damage to the fovea, even though the structure comprises only 3 to 4 percent of the retinal area, can result in instantaneous loss of fine vision.

The band of wavelengths that pass through the outer eye structures and reach the retina includes the entire visible light spectrum from blue (400 nanometers) to red (700 nanometers), and the near-infrared range of 700 to 1400 nanometers (IR-A). Because the retina is not responsive to radiation outside the visible spectrum, no sensation results in the eye when exposed to the near-infrared wavelengths, resulting in a much greater hazard from lasers operating in this emission range. Although invisible, the beam is nevertheless focused on the retina. As discussed previously, because of the focusing efficiency of the eye, relatively small amounts of laser radiation can injure the retina, and in some instances result in serious visual consequences. Pulsed lasers that emit high intensities can cause explosive hemorrhaging when focused in the eye, and the damage can extend a considerable distance from the focused area. Retinal injuries do not heal, and cannot generally be repaired.

Absorption in the other eye components, primarily the cornea and lens, is responsible for limiting exposure of the retina to the ocular focus range of wavelengths, which can also be considered the retinal hazard region. In the absorption process, the absorbing structures become subject to damage themselves. Only the tissue that absorbs the radiation, and tissues immediately surrounding it, are subject to injury and most instances of acute damage resulting from exposure to laser radiation outside the 400 to 1400-nanometer wavelength range do not have long-lasting effects. The cornea behaves similarly to the skin in that it is constantly undergoing replenishment, and only rather severe damage results in scarring that may have some effect on vision. Most damage to the cornea results from laser radiation in the far-infrared and ultraviolet spectral regions.



Because of the high degree of focusing that occurs within the eye, exposure to a relatively weak coherent laser beam can cause permanent, instantaneous damage. Consequently, when a powerful laser is being utilized, a specular reflection (which maintains the beam coherence) of even a few percent, for a fraction of a second, is capable of inflicting eye damage. In contrast, when the laser beam is scattered by reflection from a rough surface, or even from dust in the air, the diffuse reflection enters the eye at a larger angle. With the beam energy spread over a larger area, the reflection has the characteristics of an extended source, and produces a large image on the retina, compared to the concentrated focus produced by a point source (see Figure 3). Diffusion of the beam in this fashion reduces the likelihood of eye damage, not only by increasing the source size and reducing power density, but by disrupting the beam coherence as well.

Biological Effects of Laser Radiation

Photobiological Spectral Domain (CIE Band)   Eye Effects                                                      SkinEffects

Ultraviolet C (200-280 nm)                                    Photokeratitis                                                    Erythema (Sunburn) Skin Cancer

Ultraviolet B (280-315 nm)                                    Photokeratitis                                                    Erythema (Sunburn) Accelerated Skin Aging Increased Pigmentation

Ultraviolet A(315-400 nm)Photochemical UV        Cataract                                                            Pigment Darkening Skin Burn

Visible (400-780 nm)                                            Photochemical and Thermal Retinal Injury           Vision Degradation    Skin Burn Photosensitive Reactions

Infrared A (780-1400 nm)                                    Retinal Burns Cataract                                          Skin Burn

Infrared B(1400-3000 nm)                                    Corneal Burn  Aqueous Flare IR Cataract            Skin Burn

Infrared C (3000-1 million nm)                              Corneal Burn                                                       Skin Burn


Potential damage to the eye can be categorized with respect to the laser wavelength and the eye structures affected, with the most significant injuries being to the retina and caused by radiation in the visible and near-infrared spectral region. Thermal burn, acoustic damage, or photochemical alteration is possible depending upon the energy absorbed. The biological effects on the eye tissues, manifested within various wavelength bands, are summarized as follows, and listed in the above table.

Ultraviolet-B and C (200-315 nanometers): The surface of the cornea absorbs all ultraviolet light in this range, preventing these wavelengths from reaching the retina. A form of photokeratitis (also referred to as welder's flash) may result through a photochemical process that causes denaturation of proteins in the cornea. In addition to laser output, radiation in this range can arise from laser pump light or may be a component of blue light from a target interaction, requiring additional precautions over those specified by the ANSI standard, which only considers laser output. This type of eye damage is not usually long-lasting due to the rapid regeneration of corneal tissue.

Ultraviolet-A (315-400 nanometers): The cornea and aqueous humor transmit this wavelength range, which is then primarily absorbed by the lens of the eye. Photochemical denaturation of proteins in the lens results in the formation of cataracts.
Visible light and Infrared-A (400-1400 nanometers): This spectral region is often referred to as the retinal hazard region, due to the fact that the cornea, lens, and vitreous fluid of the eye are transparent to these wavelengths, and the light energy is absorbed in the retina. Damage to the retina is possible either through thermal or photochemical processes. Photochemical damage to photoreceptor cells of the retina can degrade overall light or color sensitivity, and the infrared wavelengths may cause cataract formation in the lens. The most likely injury when sufficient laser energy is absorbed by the eye is a thermal burn, in which absorption of light by the melanin granules of the pigmented epithelium is converted to heat. The focusing of the laser radiation by the cornea and lens within this wavelength band amplifies the irradiance by a factor of approximately 100,000 at the retina. For visible light lasers of relatively low power, the possibility of injury is reduced by the aversion reflex (taking about 0.25 second), which causes avoidance of the bright beam. If the laser energy is sufficient to produce damage in less than 0.25 second, however, this natural defense mechanism is not effective, nor does it provide any protection in the invisible near-infrared band between 700 and 1400 nanometers wavelength. Lasers operating in pulsed mode present an additional hazard from the possibility of acoustic shock wave generation in the retinal tissue. Laser pulses with duration less than 10 microseconds induce shock waves that cause tissue rupture. This type of injury is permanent and potentially more severe than thermal burn, because acoustic damage usually affects a larger area of the retina, and the required energy to produce the effect is lower. Consequently, the maximum exposure permitted in regulatory standards is reduced for short-duration pulsed lasers.

Infrared-B and Infrared-C (1400-1 million nanometers): At wavelengths longer than 1400 nanometers, the cornea absorbs energy due to water content of the tissue and the natural tear film, and the resulting temperature rise causes denaturation of proteins near the surface. Depth of penetration increases at longer wavelengths, and the thermal effects on lens proteins, at a critical temperature not much above normal body temperature, may lead to the formation of clouding, referred to as an infrared cataract. In addition to cataract formation and corneal burns, infrared radiation can produce aqueous flare, in which the normally transparent aqueous medium of the anterior chamber is compromised due to disruption of blood vessels.
In general, ultraviolet and far-infrared laser radiation is absorbed at the cornea or lens, and its effect depends upon the intensity and exposure duration. At high intensities, immediate thermal burns occur, while lower exposures may lead to the development of cataracts over a period of years. Conjunctiva tissues of the eye can also be injured by laser exposure, although conjunctival or corneal damage usually occurs at higher power levels than retinal injuries. Consequently, since retinal injuries produce more serious immediate effects, corneal hazards are generally only considered a serious concern for lasers operating at wavelengths that do not reach the retina (essentially far-infrared and ultraviolet).

Summary: Only purchase lasers that have ANSI, IEC or FDA certified diodes.

Reference: Nikon Digital Imaging - Laser Safety (Extract)

Final Comment: Lasers which have:

• No warranty

• Low stated hour rating life
  

• Short cycle times (due overheating caused by cranking the diodes)
  

• High power outputs

• Very low cost (you cannot manufacture laser products at these price points which meet and comply with safety standards and have accession numbers and certifications from governing bodies)
  

are NOT recommended for ANY entertainment application.