What is OD for laser protection?

Buyers Guide |  What is OD (Optical Density) Laser Safety?

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Optical Density (OD) in laser safety is the amount of light attenuated[i] by the lens of the particular wavelength that is being measured. The required OD determined by the World Laser Safety in the USA found in ANSI Z136.1 are dependent upon several factors, including the wavelength of the laser. The standards were created through research based on how different wavelengths have different biological impacts.

Below is an example for how to read the shorthand notation for optical density and the attenuation of energy passing through a given filter.

Please note, each wavelength will have a different optical density. For simplicity, it is stated as a range of wavelengths.

Wavelength is the first parameter needed to begin assessing the type of laser safety protection needed.

There are typically two wavelengths associated with a laser:

1. The aiming beam wavelength - Typically "eye safe" | depicted in the image below as red.
2. The operating beam wavelength - the operating beam requires proper laser safety protection | depicted in the image below as green.

The aiming beam is to aid the user in positioning the operating laser beam. Generally, the aiming beam will be low powered and typically red. Do not look directly at any laser including the aiming beam. A common use case for an aiming beam is a laser pointer. For more information on laser safety with an operating beam vs an aiming beam, click here.

If you are in doubt whether or not your aiming beam is eye-safe for diffused viewing, please check with your laser safety officer or call us before making any assumptions. Recently our team has come across wavelengths utilized for aiming beams that are inappropriate without proper laser safety protection.

Never look at any laser (including the aiming beam) directly. Stand behind the beam while viewing it, and do not bring your eyes near the axis during alignment or any other operation with lasers. All laser safety PPE is for unintentional direct and diffused viewing only. If you are hit with any laser in the eye, look away immediately.

If the alignment beam exceeds the MPE, use proper alignment eyewear (where the MPE threshold is not exceeded factoring appropriate exposure duration). Attenuating the laser to a point it cannot be seen is an issue. So, make sure to align the laser correctly. Laser safety alignment eyewear should be used at all times. Always be safe when dealing with lasers.

Having proper personnel with appropriate training and standard operating procedures during the alignment process are proper laser safety amongst several other laser safety controls.

Many lasers are within the range of the visible light spectrum of 380 nm (violet) and 740 nm (red). Individuals also need protection from lasers which emit Ultra-Violet light, (lower wavelength than visible light) and infrared light (higher wavelengths than visible light on the magnetic spectrum). It is important to know the wavelength of the laser you&#;re working with - different lasers require different degrees of protection as outlined in ANSI.

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[i] Outside of the context of laser safety, Optical Density more commonly refers to the refractive index, and the term &#;Absorbance&#; is used in lieu of what we are referring to here as Optical Density.

Question and Answer Surrounding the Topic

Question: If my glasses have an optical density of 7+ between 190-385 nm, does that mean they provide that protection to wavelengths beneath 190, such as 110 nm?

Answer: No, this is not the correct way to view laser safety, protecting from a lower or higher wavelength outside the specified range for a laser safety product is not a safe way to shop.

Question: Can you provide me with laser safety glasses or goggles that protect from every laser?

Answer: No, by blocking the entire visible light spectrum, you would ensure that no visible light would be passing through the glasses and therefore would have no visibility. Although materials do block multiple wavelengths, blocking all wavelengths of visible light would defeat the purpose of having glasses - because you would not be able to see anything out of them.

However, some glasses offer protection from multiple wavelengths. Furthermore, another way to combat dealing with multiple lasers may be to wear multiple laser safety glasses.

Once you have this information, you can use this following formula to calculate the required optical density: 

Optical Density = Log (Maximum Power Density Output/ Maximum Permissible Exposure).

*Where Maximum power density output and Maximum permissible exposure are both expressed in the same units of joules/cm2 or watts/cm2

*The Maximum Permissible Exposure (MPE) is determined by the American National Standards Institute (ANSI) and is based on the wavelength of the laser. You can find the MPE for your laser by referring to the ANSI Z136.1 (Table 5b ) standard.

Or, an easier method, is to use the Laser Institute of America&#;s free OD Calculator to determine the required OD. 

*Please note that if you have a pulsed laser, instead of continuous wave, you need to know the energy per pulse, pulse width, pulse repetition for the calculator

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.

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