The Need For Laser Safety
First we need to explore: what are the mechanics of laser light and the interaction of those with the eye. While laser light may also adversely affect skin, the focus here will be on the eye.
The propagation of laser light possesses unique properties, two related elements of which are: coherence and often monochromaticity. Coherence is defined by light in which the phases of all electromagnetic waves (visible light, IR, etc.) at each point on a line normal to the direction of these same sources are identical. As such, their frequency is said to be “in phase” with one another. Monochromatic light is characterized as consisting of waves of one wavelength (694.3nm, 1064nm, etc.) only. Another unique element to the propagation of laser light is that it has a high degree of spatial coherence. That is to say that the light exiting the laser will disperse (termed diffraction) minimally as a function of the distance at which it has exited the laser cavity. In a sense, we call this light – collimated. How then do these unique properties of laser light interact with the eye for its consideration of safety? For that – a simplified description of the elements of the eye are in order. Proceeding from the exterior of the eye and progressing to its most interior portions they are:
- Cornea – a clear, dome-shaped surface that covers the front of the eye and is the first significant lens of the eye’s optical system
- Iris – the colored part of the eye consisting of a ring of muscle fibers which expand and contract to open and to close the pupil in response to brightness
- Pupil – the hole in the center of the iris allowing the passage of light
- Lens – a crystalline structure responsible for keeping images in focus on the retina
- Vitreous humor – a jelly-like liquid (principally composed of water) that fills the eye from the lens back to the retina
- Retina – a unique cellular structure of the eye which converts light rays into electrical signals
- Macula – the part of the retina responsible for central vision (20/20) – or in the case of Macular Degeneration 20/200 to 20/800 vision
- Fovea – an indentation in the center of the macula responsible for the highest degree of visual acuity
- Optic Nerve – the “nerve bundle” connecting the eye to the brain (principally to the occipital lobe of the brain) and consisting of approximately 1.2 million nerve fibers!
What happens when laser light comes into contact with the human eye? ANSI Z136.1, Section 1.1 Scope states: This standard provides recommendations for the safe use of lasers and laser systems that operate at wavelengths between 0.18 µm (180nm) and 1 mm (1,000,000nm). Lasers operate in the ultraviolet (UV), visible and infrared (IR) regions of the electromagnetic spectrum. ANSI Z136.1 correlates these regions in wavelengths as: UV – 180nm to 400nm Visible – 400nm to 700nm IR – 700nm to 1mm Potentially, all of these regions can interact with the various structures of the eye. The most dangerous of which are those capable of creating degrees of eye damage or blindness by their interaction with the retina, macula or fovea. Laser radiation is able to transmit to these three areas in the wavelength region ranging from 400nm to 1400nm. This same area is also termed the ocular focus region. The mechanisms responsible for damage to the eye are photochemical, thermal or both in nature. Photochemical Damage: occurs when the energy of the light is sufficient to damage or to destroy the chemical bonds of the cells involved in vision and so diminish or eliminate their ability to function properly. Thermal Damage: occurs when the energy of the light possesses sufficient power to heat the cells involved in vision to a level which either damages or destroys their ability to function properly. In either case, testing of damage threshold values has established the Maximum Permissible Exposure (MPE) limits the eye is able to process safely for the full range of laser wavelengths and laser operations. If the laser does exceed the MPE, then appropriate control measures are evaluated by the LSO and implemented accordingly. These control measures are:
- Engineering Controls – physical constraints on the output of the laser with the intent to establish a Class 1 laser operating condition.
- Administrative and Procedural Controls – methods or instructions which specify rules, work practices or both which implement or supplement engineering controls and which may specify the use of personal protective equipment (PPE)
- Protective Equipment Controls – the use of laser protective eyewear (LPE), barriers, curtains, windows etc. to block hazardous radiation incident onto the eye.
Frequently, LPE is used to ensure that the MPE is not exceeded. To do so, laser protective eyewear possesses an optical density (OD) which attenuates the output of the laser reaching the eye to levels at or below the laser’s appropriate MPE value. Optical Density is the defined as the log to the base 10 of the reciprocal of the transmittance. A simple outline of which would show the following OD and Transmission values.
OD and Transmission
| Optical Density | Transmission of Light (%) |
|---|---|
| 0 | 100% |
| 1 | 10 |
| 2 | 1 |
| 3 | 0.1 |
| 4 | 0.01 |
| 5 | 0.001 |
| 6 | 0.0001 |
As an example, a Continuous Wave (CW) laser operating at 532nm requires an ocular MPE of 2.55mW/cm2 for eye safety. Assuming that the laser’s output power is a constant 5W, the OD requirement of the LPE would be 3.708. Expressed as internal transmittance, an OD of 3.708 is equivalent to 1.959e-4, or decimally as: 0.0001959. Thus LPE possessing an OD of 3.708 would diminish the 5W of 532nm green laser light to 0.0009795 watts (0.9795mW) – effectively reducing a Class 4 laser to a Class 1 laser – thus incapable of producing damage to any structure of the eye. With this brief outline of the development of the laser and its safety considerations, Kentek invites you to explore the various LASER HELPERS sections which follow.