A Buyers Guide | Optical Density and its Relation to Wavelength
Wavelength: Wavelength is expressed in nanometers (nm), micrometers (µm), or towards radio waves are measured in centimeters (cm) or meters (m). There are typically two wavelengths associated with a laser:
- The aiming beam wavelength (the aiming beam is "eye safe")
- The operating wavelength - the operating beam is the one you need protection from.
Please note: Multiple‑wavelength systems may require a custom filter or multiple eyewear.
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, (low wavelength then visible light), while other lasers emit infrared light (higher wavelength then visible light). It is important to know what the wavelength of your laser is - different lasers require different amounts of protection as outlined in ANSI.
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 on how different wavelengths have different biological impacts. Optical Density is a measurement of how much light is absorbed by the lens of the particular wavelength that is being measured.
Below is an example for how to read the short hand notation for optical density and the attenuation of energy passing through the given filter.
Please note, each wavelength will have a different optical density. For simplicity, it is stated as a range of wavelengths.
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.