A laser can be considered as a highly collimated source of extremely intense electromagnetic radiation that is defined by three characteristics: monochromatic, directional and coherent. Due to the temporal and spatial coherence of the laser beam it can be considered as a point source of phenomenal brightness that can easily exceed the brightness of the sun. Lasers are a more directional light source than any other common fixture as stage lights or a follow-spot. The higher the optical output power of the laser, the greater the potential hazard. 1. Eye hazards The major danger of laser beams is hazards from beams entering the eye since this is the organ most sensitive to light. The simplest way to explain this is to say, "just as a magnifying glass can be used to focus the sun and burn wood, the lens in the human eye focuses the laser beam into a tiny spot than can burn the retina". Lasers in the visible and near infrared range of the spectrum have the greatest potential for retinal injury, as the cornea and lens are transparent to these wavelengths and the lens can thus focus the laser energy onto the retina. The maximum transmission by the cornea and lens, and the maximum absorption of laser energy in the retina occurs in the range from 550 nm to 400 nm. Argon and YAG lasers operate in this range clearly making them the most hazardous lasers. Laser damage and retinal burns can occur when a laser beam enters the human eye. Lasers are a 'point source' of light much like the sun thus the eye focuses on infinity when viewing lasers especially in a darkend space where no other light sources are present. Laser beams are almost parallel thus the lens of the human eye will focus them down to a small spot. A laser beam with low divergence entering the eye can be focused down to a spot 10 to 20 microns in diameter. 2. Skin hazards Exposure of the skin to high power laser beams (1 or more watts) can cause burns. At the under five watt level, the heat from the laser beam will cause a flinch reaction before any serious damage occurs. The sensation is similar to touching any hot object , you tend to pull your hand away or drop it before any major damage occurs. With higher power lasers, a burn can occur even though the flinch reaction may rapidly pull the affected skin out of the beam. Thses burns can be quite painful as the affected skin can be cooked and forms a hard lesion that takes ages to heal. 3. Electrical hazards Most medium and high power lasers operate on 220 or even higher AC voltages; draw lots of current and frequently use multi-phase electrical connections. The power supply (exciter) for the laser typically doubles or even triples the line voltages before feeding them to the laser head where lethal voltages can be present. An average of two people per year die from laser electrocution. If it becomes necessary to open the power supply to diagnose or correct problems, you should insure that the main power has been disconnected at the breaker or disconnect switch. With the cabinet of the laser head or exciter open, terminals carrying lethal voltages may be exposed. In some systems large electrolytic capacitors are used for smoothing DC voltages. Even with 'bleed' resistors these can take several times to discharge fully and should be treated with caution and possibly discharged manually before working on the system. Even small HeNe lasers of the type found in school science labs operate at very high voltages. While the power supplies typically don't deliver enough amperage to kill you, they will deliver a very nasty jolt that could cause heart palpitations. Before connecting the laser to the power supply, insure that the AC power is turned off.