Due to the unique characteristics of laser radiation (i.e. coherent, collimated and monochromatic) there is increased danger to the eyes. Therefore special optical filters that transmit 'normal' light but block laser light should be used.

Since laser light has a specific wavelength dependent on the laser active medium that emits light, protective filters that match the wavelength and power of the specific source of laser radiation are needed.

Optical Density (OD)

Optical density is the attenuation of light that passes through an optical filter. The higher the OD value the higher the attenuation. The mathematic expression of optical density is the logarithm to the base ten of the reciprocal of the transmittance and is given by the following equation:

D = -log10 T

The optical density is a measure that indicates how many decimal places the transmission shifts at the required wavelength.

Selecting Laser Protective Eyewear

Laser Safety Officers (LSOs) should consider the actual working environment, viewing conditions and beam delivery systems when determining the most appropriate protective equipment needed to reduce potentially hazardous exposures to laser light. Laser safety eye protection options include spectacles, goggles, eye safety filters, full face shields, etc. Regardless of the specific type of eye protection system required to minimize potential exposures to levels below applicable MPEs the following types of light attenuation materials are generally used in their construction:

Absorptive

Reflective

Absorptive filters used in the construction of safety eyewear products are typically made with either polycarbonate or glass materials, where light absorption at a given wavelength is a function of material thickness. This may be calculated in terms of OD by:

OD = -log10 Pd · lT() (D2/D1)

Where:

Pd = Reflection factor

I,()= Transmittance at thickness D1

D2 = Desired material thickness (mm)

D1 = Thickness of material for known internal transmittance (mm)

Polycarbonate protective eyewear provides an excellent low cost, light weight safety solution but may have slightly lower OD's and visible light transmission (VLT) values than filter glass or coated filters.

Absorbing glass filters generally provide higher optical densities, better VLT values but at a slightly higher cost per pair when compared to polycarbonate safety eyewear.

Thin-film coated filters provide precise protection from specific wavelengths and can have excellent VLT values. This is done by applying physical vapor deposition (PVD) technology of dielectric materials upon various substrates. Precise (¼ wavelength thickness) vapor deposition of alternate HI-LO index of refraction dielectric coatings is applied directly to absorptive polycarbonate or glass filter materials. This provides laser protection by reflective as well as absorptive properties.

By capitalizing on the desirable absorptive and reflective characteristics of various filters laser protective eyewear manufactures can bond or laminate two or more filters together using special UV-cured optical adhesive lamination technology. This extends the spectral protection range of a single type of protective eyewear to cover a broader range of laser wavelengths.

Spectral scans of filters are completed over an entire range of wavelengths from which optical density data is generated and compared with published OD specifications to verify the absorptance or reflective properties. Filters will always exceed posted specifications but optical densities of wavelengths may vary slightly due to manufacturing processes.

The LSO shall review all eyewear and approve it to be used with a laser.

Regardless of the type of protective filters used in laser safety eyewear the LSO should routinely examine them for integrity, proper labeling and storage. Protective eyewear exhibiting gaps between the filter and frame, delamination or other visible signs of damage should not be used; rather they must be repaired or replaced.