Laser Physics & Classification

Laser light is electromagnetic radiation. So are radio waves, microwaves, infrared, ultraviolet and X-rays. They are the same kind of energy. Only the wavelength differs.

Wavelength is the distance between wave peaks. We measure it in nanometres. A nanometre is one billionth of a metre.

Our eyes see only a narrow band. The Standard covers lasers from 380 to 780 nanometres. [cl. 1]

Wavelength sets the colour. Violet is about 380 to 450 nanometres. Blue is 450 to 495. Green is 495 to 570. Yellow is 570 to 590. Orange is 590 to 620. Red is 620 to 780.

Just outside this band are ultraviolet and infrared. You cannot see them. They can still injure.

A spotlight and a laser both make visible light. A laser has three special properties.

First, one wavelength. A laser makes one colour only. A red laser at 638 nanometres gives 638 and nothing else. So exposure limits can be matched to the exact wavelength.

Second, coherence. The light waves move in step. Coherent light can be focused to a tiny spot. A one-watt laser can injure an eye. A sixty-watt light bulb cannot.

Third, a narrow beam. A laser beam spreads very slowly. It can stay hazardous several hundred metres from the projector. [cl. 4.2]

Together, these make a few watts of light dangerous. A tiny, focused spot can injure the retina in a fraction of a second.

Laser classes come from IEC 60825-1. A higher class number means a greater hazard. [cl. 3.20]

There are eight classes. The class is on the label of every laser product.

Class 1 is safe in normal use. Class 1M is safe unless you look through binoculars or a magnifier. Class 1C is only for devices that touch the skin.

Class 2 is a visible laser up to 1 milliwatt. Your blink reflex protects you. Class 2M is the same, but unsafe with optical aids.

Class 3R is up to 5 milliwatts of visible light. Looking straight into it can still be hazardous.

Class 3B is up to 500 milliwatts. A brief direct hit can injure an eye.

Class 4 is above 500 milliwatts. It can injure from the direct beam and, in some cases, from reflections. It can burn skin and start fires. [cl. 4.1]

These limits are for continuous visible beams. Most laser shows use Class 3B or Class 4. These beams can also damage video projectors and cameras. [cl. 4.1]

Know what is inside a projector and you will know how it can fail.

Laser sources make the light. Most show projectors use separate red, green and blue sources.

A combiner merges the colours into one beam. Each colour is dimmed or brightened on its own. That is how the projector makes any colour.

Two small mirrors on fast motors steer the beam. They are called galvanometers, or galvos. One moves the beam left and right. The other moves it up and down.

The beam leaves through the aperture. [cl. 3.4]

The galvos matter most for safety. If they stop, the beam stops moving. A still beam is far more hazardous than a moving one. Module 5 covers this.

Five numbers describe how hazardous a beam is.

Wavelength sets the colour and the exposure limit that applies.

Power is measured in watts. It is the energy the beam carries each second. A five-watt beam delivers five joules every second.

Beam diameter is how wide the beam is. At the projector it is a few millimetres.

Divergence is measured in milliradians. It is how fast the beam widens. One milliradian adds one millimetre of width for each metre travelled.

Irradiance is power divided by area, in watts per square metre. This is what harms tissue. Eye exposure is judged over a 7 millimetre circle, about the size of the pupil. [cl. 9.5]

A beam with low divergence stays tight. It stays hazardous further away. Try the explorer below.

The eye is the organ most at risk from a laser.

The cornea and lens focus light onto the retina. A laser beam focuses to a tiny spot there. The light becomes up to 100,000 times stronger.

Thermal injury is a burn. The retina absorbs the energy and heats. Even a very brief exposure can do it, such as a scanned beam passing the eye. It does not hurt. [cl. 4.2]

Photochemical injury is a chemical reaction. Ultraviolet light and visible light from 400 to 600 nanometres cause it. Blue light below 500 nanometres is the main concern. It builds up over time. [cl. 4.2]

Thermomechanical injury is a shockwave in the tissue. Very short, high-peak pulses cause it, such as Q-switched lasers. [cl. 4.2]

Even below the exposure limit, a bright beam can cause flash blindness. Vision is impaired for a short time. [cl. 4.2]

The retina has no pain sensors. A person may not know they are injured.

Skin can be injured too. But the beam must be much stronger than for the eye. [cl. 4.3]

Skin has no focusing lens. The light is not concentrated.

Too much visible or infrared light causes pain, reddening or blisters. Class 4 beams can do this. [cl. 4.1, 4.3]

Ultraviolet light harms skin like sunburn. [cl. 4.3]

Class 4 beams can also start fires. Fabric, paper and dark plastics are at risk. [cl. 4.1]

When bright light hits your eyes, you blink and turn away. This is the aversion response.

The Standard assumes it limits an accidental exposure to 0.25 seconds. [cl. 3.1]

That is enough for Class 2 lasers. It is not enough for Class 3B or Class 4.

Reason one is speed. A scanned beam crosses the eye in a few tens of microseconds. Damage can happen before any reaction. [cl. 9.3.2]

Reason two is behaviour. Spectators may keep watching a show on purpose. Alcohol and drugs slow the reflex. [cl. 3.1]

Reason three is invisible light. A beam you cannot see gives no flash to blink at.

Most projectors mix red, green and blue to make colour.

Blue is often 445 nanometres. Below 500 nanometres, cumulative photochemical injury matters. [cl. 4.2]

Green is often 520 nanometres. The eye is most sensitive to green. A green beam looks brighter than a red beam of the same power.

Red is often 638 nanometres. It looks dimmer than green. So never judge how dangerous a beam is by how bright it looks.

Older green sources at 532 nanometres can leak invisible infrared. Check the filter.

Each colour adds to the total exposure. Know the wavelengths in your projector.