An Overview of Laser Safety Glasses Specifications

Enabling you to choose the correct product and maximise your safety, this article explains the most common terms, specifications, and standards surrounding laser safety eyewear using examples from RS Components’ catalogue. In just a few minutes, you can have a more complete understanding of Visible Light Transmission (VLT), Optical Density (OD), EN Standards, LB Ratings, and RB Scales. Some miscellaneous tips and recommendations relating to laser safety conclude the article.

1. Visible Light Transmission (VLT, %).

It is the proportion of visible light, ranging between approximately 400 nm and 700 nm, transmitted by the filter. The example spectrophotometer data in Fig. 1, taken from the datasheet for RS Stock No. (238-8159) , includes the VLT in a red box. It is useful for gauging how dark your surroundings will appear when using this filter in ambient light conditions.

Fig. 1. Spectrophotometer data for RS Stock No. (238-8159) .

In general, filters with a VLT of approximately 10% or less will allow very little visible light to reach your retina, and your surroundings will appear relatively dark. As your vision may be impaired due to a lack of brightness when using such eyewear, you may wish to increase your work area ambient lighting to mitigate physical accidents. Whilst allowing more visible light to pass, filters with a VLT between 10 % and 20 % is significant and might still impair your vision, and care must be taken. Filters with a VLT value greater than 20% is not a cause for concern and, due to the tint on the filter, may only affect colour recognition rather than the brightness of your surroundings.

2. Optical Density (OD).

Defined as OD=log_10⁡〖1/T〗, where T is a value between 0 and 1, Optical Density is an alternative way of quantifying the spectral transmittance of a filter across different wavelength ranges. OD values at several wavelength ranges are embossed onto the filter for reference.

For example, RS Stock No. (697-3531) has an OD of greater than 3 between 615 nm and 720 nm (visible red light). Its spectrophotometer data is included in Fig. 2. This filter transmits less than 0.1 % of optical power in that wavelength range, and this effectively “blocks out” the light. The same filter has an OD of 0.12 at 520 nm (visible green light), transmitting 75.9 % of optical power in that wavelength range.

Fig. 2. Spectrophotometer data for RS Stock No. (697-3531) .

As another example, RS Stock No. (697-3529) has an OD of greater than 1.5 between 630 nm and 700 nm (visible red). Its spectrophotometer data is included in Fig. 3. This filter transmits less than 3.2 % of optical power in that wavelength range. If you are using a laser emitting sufficient optical power in this wavelength range, this filter allows enough light to reach your retina to perform visible alignment tasks.

Fig. 3. Spectrophotometer data for RS Stock No. (697-3529) .

3. EN207 and LB Ratings.

EN207 is a laser safety testing standard that ensures the eyewear meets its specified OD values, but it also considers the damage threshold of the eyewear. The standard uses LB ratings that are typically embossed onto the filter alongside the OD values. LB ratings look like those in Fig. 4.

Fig. 4. Example markings on laser safety eyewear RS Stock No. (127-1569) .

Blue box: OD values; Yellow arrows: EN207 LB ratings; Red arrows: EN208 RB ratings.

The LB ratings for the eyewear in Fig. 4, indicated with yellow arrows, are given as

190-315 D LB7 + IR LB4 >315-400 DIRM LB5.

This notation can be broken down, as follows:

• 190-315, >315-400: the wavelength range in nm
• DIRM: these letters refer to the laser operating mode as below
  
  • D: continuous wave (CW) lasers
  • I: long pulse lasers (~μs)
  • R: Q-switched lasers (~ns)
  • M: femtosecond lasers (~fs)
• LB4/5/7: the filter has an OD greater than the LB number. For example, the eyewear in Fig. 1 has an OD > 7 and effectively transmits less than 1/10000000 of CW power for 190-315 nm radiation. However, the LB number also refers to the maximum power or energy density the eyewear can be exposed to before damage occurs, which is wavelength and operating mode (DIRM)-specific and must be looked up in the EN207 documentation.

4. EN208 and RB Scales.

This standard applies to eyewear suitable for aligning visible laser beams. Rather than attenuating radiation in a specified wavelength range almost entirely (~ OD > 5), this eyewear is designed to effectively reduce a laser’s optical power to within the eye-safe threshold whilst still transmitting enough light to enable beam alignment tasks. EN208 uses RB scales, not to be confused with EN207’s LB ratings, that allow the user to quantify eyewear’s damage threshold and OD.

For example, in Fig. 4 the EN208 RB scales assigned to this eyewear are

.01W 2*10E-6J 532 + 630-<636 RB1, and

.1W 2*10E-5J 636-640 RB2.

The first element of each line represents the maximum CW power in Watts. The second element represents the maximum pulse energy in Joules. These are the damage thresholds. The next element represents the wavelength(s) the damage thresholds apply to. The final element simply clarifies the RB rating corresponding to these maximum values, and also indicates the OD, i.e., RB1 corresponds with an OD between 1 and 2, and RB2 with an OD between 2 and 3, etc.

Misc. notes:

• Laser safety eyewear offers protection against accidental exposure to stray or diffused reflection of laser beam energy for a maximum exposure of 5 seconds.
• Filters have passed environmental testing with extended ranges of +65 to 160°F.
• Store laser safety eyewear in its protective case when not in use.
• Store laser safety eyewear in areas not exceeding 26°C (80°F).
• Discard laser safety eyewear if damaged or faded, or if scratches impair your vision.
• Clean with a mild detergent or any over the counter lens/sunglasses cleaner (not alcohol) and wipe with non-abrasive cotton cloth.
• A laser safety officer (LSO) internal to your organisation will help you choose appropriate eyewear. Given thorough details of your application, some laser suppliers can also suggest what eyewear is needed.