Laser Safety Guide - Basics
Laser radiation, like all light, consists of electromagnetic radiation. Electromagnetic radiation travels in waves like sound and is produced by the movement of charged particles. In contrast to sound, electromagnetic radiation does not need a medium in which to travel. Some examples of electromagnetic radiation are the radiation in the form of warmth, x-rays and -rays that emerge from radioactive decomposition and radiation artificially generated by radio transmitters. In fact, electromagnetic radiation is found as natural phenomenon in almost all areas of daily life.
When the electromagnetic radiation is within the range visible to the human eye, between 380 and 780 nm (nm = nanometer = one billionth of a meter), it is called visible light. This range is called the visible spectrum. When all wavelengths in the visible spectrum are emitted simultaneously, it is perceived as white light.
When white light falls on an optically dispersive element such as a prism or birefringent filter you can see the visible spectrum due to refraction. It starts at the short wave as the color violet, turning to blue, green, then yellow and goes to the long wave, which appears as red. Beyond the long wave (red) of the spectrum is the near and far infrared range. Below the shortwave range (blue) is the ultraviolet range. Lasers are sometimes thought to emit radiation only in the visible portion of the electromagnetic spectrum; however, this is not exclusively true. The term 'light' refers to a specific range of the electromagnetic spectrum between 150 nm up to 11000 nm, i.e. from UV-'light' up to far infrared 'light.'
Why Laser Safety
The 'light' from powerful lasers can be concentrated to power densities (power per area or watts/cm2) that are high enough to evaporate tissue, metal or ceramics. Because our eyes are much more sensitive to light they are at increased risk. In fact, it is possible to cause irreversible ocular injury from just one accidental exposure to a direct or reflected laser beam even at lower power output levels.
What makes lasers dangerous compared to conventional light sources?
The main danger from hazardous exposure to laser light is due to their 'spatial coherence.'
This means that the wave traits of the laser beam have:
A fixed relation to time and space (coherent).
Are all of the same wavelength (monochromatic).
Can travel over great distances as a nearly parallel beam (collimated).
All of this means that the power or energy that impacts an area such as the eye is independent of the distance to the radiation source.
Imagine a laser pointer with a beam spot that remains about the same size over great distances. If you compare a thermal source of radiation like a light bulb, with a laser you will observe several differences. The light bulb emits light over a very broad spectrum of wavelengths with no specific direction of dispersion. A physicist would say that the bulb produces incoherent light.
When comparing a light bulb with a laser, both emitting 1 W optical power, the power of the bulb that may reach the eye decreases with distance because the bulb radiates in all directions.
By comparison a visible 1 watt collimated laser beam at a distance of 1 meter or more could fully enter the eye resulting in a retinal irradiance that increases by a factor of 100,000 due to the refractive and focusing properties of the eye (this assumes a normally dilated pupil diameter of 7 mm i.e. eyes adapted to darkness). The quantity of light that can hit the eye is not the only danger. While the bulb creates an image on the retina of approximately 100 µm, the laser light, which can be much more easily focused, may be reduced to a spot of just a few micrometers (~ 10 µm) in diameter.
Therefore, the light quantity that hits the eye is concentrated on a much smaller spot. The power density (power per area or watts/cm2) resulting from this concentration maybe sufficiently high, that any tissue in the focus will be heated and very quickly destroyed.
Since the fovea (responsible for sharp central vision and located on the retina) also has a size of just a few micrometers, it is possible to lose one's eyesight by one single laser pulse.