How to Calculate Optical Density for Laser Safety Glasses for Maximum Protection

JUL 04

How to Calculate Optical Density for Laser Safety Glasses for Maximum Protection

By cosmick

Laser light is a beam of coherent light optically amplified by emissions of electromagnetic radiation. It has a wide range of practical applications and uses in modern life.laserbeam

Lasers are Dangerous to Eyes

However, because laser beams are so tightly focused, they are quite dangerous. Even a small amount of laser light can cause permanent eye injury, burning the retina, and therefore government regulations control exposure. In the United States, that’s the American National Standards Institute (ANSI) Z136.1. The Occupational Health and Safety Administration has jurisdiction over workplaces.

Experts classify laser emitting devices according to the wavelength and power of the beams they generate, and, consequently on how much danger they pose to users.

Most of the danger from laser light comes from the heat, but at some wavelengths, there are also dangerous photo-chemical effects. With high-powered lasers, even reflected diffuse light can damage the eyes. However, most of the danger from laser beams comes from its coherence, concentrating an intense amount of energy upon a tiny spot of the retina, destroying photoreceptor cells.

Infrared lasers pose a particular hazard. Because they cannot be seen, they do not trigger the eye’s blink reflex.

Powerful lasers in the 400-1400 nm range penetrate the eyeball and heat the retina. At other wavelengths, the cornea and lens absorb the energy, leading to cataracts or burns.

The retina has no pain receptors, so exposed workers often don’t even know they’ve been injured until they detect the resulting vision problems.

Laser Classification System

Class 1

Safe for normal use.

Class 1M

Safe for normal use, but not if the light passes through magnifying lenses such as microscopes and telescopes.

Class 2

Safe because the human blink reflex blocks excess exposure. This applies to some laser pointers.

Class 3R

Safe if handled properly.

Class 3B

These beams are harmful when the human eye is directly exposed, but diffuse reflections do not cause damage. Regulations require operators directly exposed to wear protective glasses.

Class 4

This covers lasers more powerful and dangerous than the Class 3B. Class 4 lasers can cause permanent eye injury through either direct exposure or reflected light. They can also burn the skin and cause fires. Most lasers used in industry, for medical applications and for scientific research are Class 4.

Laser Wavelengths

Just like ordinary light, laser beams come in all colors. They measure that in nanometers (nm), the light’s wavelength.

The wavelength affects calculating the Optical Density. Also, laser operators must wear the glasses specifically for that laser’s wavelength. Laser glasses for another machine, with a beam of a different wavelength, are not safe even if their OD is just as high.

There are three main ranges of light wavelengths:

* Ultraviolet – 100-400 nm

* Visible – 440-750 nm

* Infrared – 750nm to 1mm

The visible range of laser light goes from violet to red just like a rainbow. And laser safety lenses will usually appear of that color, but you should never take that for granted. Also, check the specifications printed in the frames.

Maximum Permissible Exposure (MPE)

This is the highest energy per unit of surface area or power of a laser which is safe, as measured at the cornea, assuming a worst-case scenario. ANSI Z136.1 explains how to calculate MPEs. OSHA bases it on the radiation’s wavelength, the pulse duration, the amount of time its beams remains on the retina, and the size of the image that hits the retina.

OSHA Requires Wearing Protective Eyewear When Using Class 3B and 4 Lasers

They classify glasses and goggles by its Optical Density (OD). That is the base-0 logarithm of the attenuation factor by how much the filter reduces the beam’s power. Required eyewear must reduce the OD to power below the maximum permissible exposure.

For example, glasses with OD 2 reduce the laser beam by a factor of 100 (10 to the 2nd power).

You must also make certain the lenses and frames of the glasses are physically strong enough to take a direct hit from the laser without breaking. Therefore, you must also take into account the damage threshold.

The OD and light wavelengths the glasses protect against are usually pad printed on the frames.

How to Calculate the Required Optical Density

Disclaimer: This is for educational purposes only. For a definitive calculation of the OD required by your laser equipment, consult a Certified Laser Safety Officer or professionals at a laser safety glasses company.

This is an online calculator from the Laser Institute of America that can simplify the estimating process, and it is not definitive either.

What is Your Laser’s Pulse Mode?

* Constant wave

* Single-pulse

* Repetitive pulse

Other Variables

You also need to know your laser’s:

* Wavelength – in nanometers (nm)

* Average power output – in Watts (W)

* Pulse width – for single and repetitive pulse lasers – in seconds (s)

* Energy per pulse – in Joules (J)

* Pulse repetition frequency – for repetitive pulse lasers – in Hertz (Hz)

Consult your laser’s label or manual. The label is on a metal plate attached to the machine. If you can’t find the plate or owner’s manual, ask the manufacturer.

OSHA on Lasers and Optical Density

The Occupational Safety and Health Administration goes into great detail in the chapter on Control Measures and Safety Programs.

OSHA gives the OD equation as OD = log10 H0/MPE

And provides an example to demonstrate.

It also provides a table of ODs for different types of lasers. It depends on the exposure duration, which relates to the pulse repetition frequency.

The laser safety experts Phillips Safety Products stand ready to help you decide to calculate which laser glasses or goggles will provide the eye safety you require. All Phillips Safety laser glasses meet or exceed the requirements of ANSI Z136.1. We give free consultations and Optical Density calculations. Contact us today and we can calculate the optical density for your laser safety glass.

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