Buyers Guide | What is OD (Optical Density) Laser Safety?
Optical Density ("OD") in regards to laser safety is a measurement of how much light is attenuated[i] by the lens of the particular wavelength that is being measured. The required optical density 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 short hand 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 what laser safety protection is needed.
There are typically two wavelengths associated with a laser:
- The aiming beam wavelength - the aiming beam is typically "eye safe" | depicted in the image below as red
- The operating beam wavelength - the operating beam requires proper laser safety protection | depicted in the image below as green
|Operating Beam Laser vs Aiming Beam Laser (Typically Eye-Safe)|
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 directly - including the aiming beam (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 protection is for diffused viewing only. Proper laser safety is to never look directly at a laser beam; if you are hit with any laser in the eye, look away immediately.
If the alignment beam exceeds the MPE, of course utilize proper alignment eyewear (where the MPE threshold is not exceeded factoring appropriate exposure duration). The aiming beam needs to be able to be seen to align the laser correctly (that is the point of an aiming beam), and therefore attenuating the laser to a point it cannot be seen is an issue. Laser safety alignment eyewear should be used if possible and may be needed. 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 between 380 nm (violet) and 740 nm (red). Other times, individuals need protection from Ultra-Violet light, (lower wavelength than visible light), while other lasers emit infrared light (higher wavelengths than visible light on the magnetic spectrum). It is important to know what the wavelength of your laser is - different lasers require different amounts of protection as outlined in ANSI.
[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.