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Calculating Protective Eyewear Scale Numbers

Laser Safety Calculations

This page provides an explanation of how to calculate the required scale numbers and protection ratings for laser safety eyewear in accordance with BS EN 207 & 208. Optical density (OD) alone is insufficient to determine the appropriate protection levels (scale numbers) for laser safety eyewear. OD simply tells you how much or how little light is transmitted through a filter at the laser wavelength. It doesn't tell you, for instance, that your 100W CO2 laser will burn a hole through your plastic laser safety glasses before you know what is happening. EN 207 and 208 require that you also take account of the damage threshold of the filter and frame of the eyewear.

Please note that the protection rating you require applies both to the filter and frame of the eyewear. The protection rating of the eyewear as a whole will be the lower of that of the filter and frame. This is why the same LASERVISION filter material can be awarded different scale numbers depending upon which frame it is fitted into.

IMPORTANT NOTE: the following is intended for general guidance only and cannot be considered to be comprehensive. Additionally, Pro-Lite cannot be held liable for errors or omissions contained in these guidance notes. We recommend that you seek the advice of your laser safety officer. Alternatively, please use our Enquiry Form to submit details of your laser to Pro-Lite and we will gladly provide you with a formal recommendation.

EN 207 Full Protection Eyewear

EN 207 applies to full protection safety glasses. For low power, visible lasers, alignment glasses can be used that allow you to view the diffuse beam spot. Refer to the EN 208 section below for further details. The protection rating is termed the Scale Number in the jargon of EN 207. This is the 'Lx" number, where x is an integer from 1 to 10 corresonding to the maximum power density or energy density that the filter or frame can withstand for 10 seconds or 100 pulses.

Calculating the Scale Number for a Continuous Wave (CW) Laser

You will need to know the following parameters: your laser wavelength in nanometers (nm); the laser power in Watts (W); and the smallest beam diameter in meters (m) that you can foreseeably be exposed to.

The steps necessary to calculate the scale number for a CW laser are as follows:

  1. Calculate the area of the laser beam. The area is the product of pi times the square of the radius of the beam.
  2. Now calculate the power density of the beam by dividing the power by the area of the beam.
  3. Next, refer to the table of protection levels in EN 207 (click here).
  4. Select the column corresponding to the wavelength range of your laser, either 180-315nm, >315-1400nm or >1400-10000nm.
  5. Then select the column in that wavelength range for the laser type D. D means continuous wave or CW.
  6. Finally, choose the first scale number (Lx) which is equal to or higher than the power density of your laser.

As a worked example, take the case of a continuous wave Nd:YAG laser that produces 50W power at 1064nm in a 5mm diameter beam. The beam area is pi * [2.5 x 10-3]2 m2, which equals 1.96 x 10-5 m2. The power density is 50 / 1.96 x 10-5 or 2.55 x 106 W/m2. With reference to the table of protection levels given in EN 207, 2.55 x 106 W/m2 falls between the scale numbers L5 and L6. You choose L6 as the lowest scale number that provides the required protection level.

The full specification for the laser safety eyewear necessary for this CW Nd:YAG laser is: D 1064 L6 where D means continuous wave, 1064 is the laser wavelength in nm and L6 is the EN 207 scale number for eyewear suitable for use at up to 107 W/m2. Please note that the optical density of the eyewear is implicit in the scale number and for this example, the OD will be at least 6.

Calculating the Scale Number for a Pulsed Laser

You will need to know the following parameters: your laser wavelength in nanometers (nm); the average laser power in Watts (W); the energy per pulse in Joules (J); the pulse repetition frquency in Hertz (Hz); the pulse duration in seconds (s); and the smallest beam diameter in meters (m) that you can foreseeably be exposed to.

The protection rating required for a pulsed laser requires two calculations. You must first perform the calculation for the equivalent CW laser based on the average power emitted by the laser. You must then perform a second calculation for the case of the pulsed output.

  1. Calculate the area of the laser beam. The area is the product of pi times the square of the radius of the beam.
  2. Now calculate the average power of the beam. This is the energy per pulse multiplied by the pulse repetition frequency.
  3. Next, calculate the average power density of the beam by dividing the average power by the area of the beam.
  4. Next, refer to the table of protection levels in EN 207 (click here).
  5. Select the column corresponding to the wavelength range of your laser, either 180-315nm, >315-1400nm or >1400-10000nm.
  6. Then select the column in that wavelength range for the laser type D.
  7. Now choose the first scale number (Lx) which is equal to or higher than the average power density of your laser. This is the end of the first of the two calculations for a pulsed laser.
     
  8. For the second part of the calculation , take the beam area you calculated in step 1 above and calculate the energy density of the beam by dividing the pulse energy by the area of the beam.
  9. If your laser wavelength falls between 400 to 1400nm, you need to calculate a corrected energy density by multiplying the energy density from the previous step by the factor N0.25 where N is the total number of pulses emitted in 10 seconds. The total number of pulses in 10 seconds in the pulse repetition rate in Hertz (Hz) multiplied by 10.
  10. Next, refer to the table of protection levels in EN 207 (click here).
  11. Select the column corresponding to the wavelength range of your laser, either 180-315nm, >315-1400nm or >1400-10000nm.
  12. Then select the column in that wavelength range for the laser type corresponding to the pulse duration of your laser, either I (for long pulsed), R for giant pulsed (Q-switched) and M for modelocked.
  13. Finally, choose the first scale number (Lx) which is equal to or higher than the energy density of your laser.

Please note that the above calculation does not apply to the case of a modelocked laser having a pulse duration of less than 1ns, and having a wavelength that is not within the wavelength range from 315 to 1400nm. For lasers with pulse durations of < 1ns and with wavelengths shorter than 315nm or longer than 1400nm, you must perform a calculation based upon the peak power density from the laser.

As a worked example, consider a pulsed Nd:YAG laser that produces 250mJ per pulse at 1064nm at a pulse repetition frequency of 10Hz. The duration of each pulse is 7ns and the accessible beam diameter is 7mm. The first step is to calculate the average power density of the laser. The beam area is pi * [3.5 x 10-3]2 m2, which equals 3.85 x 10-5 m2. The energy per pulse is 250mJ and at a pulse repetition frequency of 10Hz, the average power is 250 x 10-3 * 10, which equals 2.5 Watts. The average power density is 2.5 / 3.85 x 10-5 or 6.49 x 104 W/m2. With reference to the table of protection levels given in EN 207, 6.49 x 104 W/m2 falls between the scale numbers L3 and L4. You choose L4 as the lowest scale number that provides the required protection level for the average power of this laser.

Next, we must perform the second part of the calculation. The energy density of the laser is 
[250 x 10-3] / [3.85 x 10-5] or 6.49 x 103 J/m2. As the wavelength of this laser falls within the 400-1400nm band, we must now calculate the corrected enrgy density. At a pulse repetition frequency of 10Hz, the factor N (number of pulses emitted in 10 seconds) is 10*10 equals 100. N0.25 equals 3.16 , so the corrected energy density is [6.49 x 103] * 3.16, which equals 2.05 x 104 J/m2. With reference to the table of protection levels given in EN 207 for a 7ns laser at 1064nm, 2.05 x 104 J/m2 falls between the scale numbers L6 and L7. You choose L7 as the lowest scale number that provides the required protection level for the pulse energy of this laser.

The full specification for the laser safety eyewear necessary for this pulsed Nd:YAG laser is: D 1064 L4 and R 1064 L7. Although the OD is not stated explicitly in this specification, the optical density of the filter will be at least 7.

EN 208 Alignment Protection Eyewear

Alignment filters provide partial blocking of low power, visible wavelength (400-700nm) laser beams. This allows the wearer to safely view the visible beam spot for laser beam alignment. Alignment filters are assigned an R rating in accordance with BS EN 208. The principle of alignment protection filters is that they attenuate the power of the laser such that if the beam were to accidentially hit the filter directly, the only light that would be transmitted would be equivalent to a class 2 exposure, less than 1mW for a continuous wave laser. Class 2 lasers are those low power, visible devices for which the eye's blink reflex affords sufficient protection

Alignment glasses are not intended for direct viewing of the laser beam, rather for diffuse, indirect viewing (i.e. reflections from a scattering surface). Alignment glasses must also withstand a direct hit from the laser for which they are chosen for at least 10 seconds or 100 pulses under standardised conditions

Calculating the Scale Number for Alignment Protection Eyewear for CW Lasers

You will need to know the following parameters: your laser wavelength in nanometers (nm); the laser power in Watts (W); and the beam diameter in meters (m).

The steps necessary to calculate the scale number for alignment protection eyewear for use with CW lasers (or pulsed lasers with a pulse duration of greater than 0.2 milliseconds) are as follows:

  1. Check that your laser is CW (or pulsed but with a pulse duration of > 0.2ms) and has a wavelength between 400-700nm.
  2. For a beam with a diameter of 7mm diameter of less, use the power of the laser for the next step.
  3. For a beam with a diameter of greater than 7mm, calculate that proportion of the beam power that will fit through a 7mm aperture (this is based upon the ratio of beam areas).
  4. Next, refer to the table of protection levels in EN 208 (click here).
  5. Select the column corresponding to a CW laser.
  6. Finally, choose the first scale number (Rx) which is equal to or higher than the power (or power through the 7mm aperture) for your laser.

As a worked example, consider a frequency doubled Nd:YAG laser that produces a CW power of 5 Watts at 532nm in a beam of 2mm diameter. As the beam size is less than 7mm, we determine the scale number required for the full 5 Watt output. With reference to the table of scale numbers given in EN 208, 5W falls between R3 and R4, so you must choose R4 as the lowest scale number that provides the required protection level for the power of this laser. The numeral 4 indicates an optical density of 4, which in turn means that should the full 5W from this laser hit the alignment protection eyewear, the power that would be transmitted through the filter would be 5 * 1/10,000 which equals 5 x 10-4 Watts or 0.5mW. This power is below the maximum permissible exposure (MPE) level for a class 2 laser which means that the eye would be protected by its inherent blink reflex. At the same time, one could still see the (green) beam spot of this laser beam as it scatters from a diffusing surface.

The full specification for the laser safety eyewear necessary for this CW frquency doubled Nd:YAG laser is 532 10W R4 where 532 is the laser wavelength in nm, 10W is the maximum laser power that the eyewear should be used with at this wavelength in Watts and R4 is the EN 208 scale number for eyewear suitable for use at up to 10W. Please note that the optical density of the eyewear is implicit in the scale number and for this example, the OD will be at least 4.

Calculating the Scale Number for Alignment Protection Eyewear for Pulsed Lasers

You will need to know the following parameters: your laser wavelength in nanometers (nm); the laser energy in Joules (J); the pulse repetition frequency in Hertz (Hz); the pulse duration in seconds (s); and the beam diameter in meters (m).

The steps necessary to calculate the scale number for alignment protection eyewear for use with pulsed lasers (those with a pulse duration of greater than 1ns and up to 0.2ms) are as follows:

  1. Check that your laser is pulsed with a pulse duration of greater than 1ns and no greater than 0.2ms and has a wavelength between 400-700nm.
  2. For a beam with a diameter of 7mm diameter of less, use the energy of the laser for the next step.
  3. For a beam with a diameter of greater than 7mm, calculate that proportion of the beam energy that will fit through a 7mm aperture (this is based upon the ratio of beam areas).
  4. Multiply the pulse energy (or energy adjusted for a 7mm aperture) by the factor N0.25, where N is the number of pulses emitted in a 10 second period. The total number of pulses in 10 seconds in the pulse repetition rate in Hertz (Hz) multiplied by 10.
  5. Next, refer to the table of protection levels in EN 208 (click here).
  6. Select the column corresponding to a pulsed laser.
  7. Finally, choose the first scale number (Rx) which is equal to or higher than the energy (or energy through the 7mm aperture) for your laser.

As a worked example, consider a frequency doubled Nd:YAG laser that produces pulses of 0.5mJ at 532nm in a beam of 2mm diameter with a pulse duration of 100ns and a pulse repetition frquency of 1kHz. As the beam size is less than 7mm, we determine the scale number required for the full 0.5mJ output. The corrected energy is [5 x 10-4] * [1,000 * 10]0.25 which equals 5 x 10-3 Joules. With reference to the table of scale numbers given in EN 208, 5mJ (5 x 10-3 J) falls between R4 and R5, so you must choose R5 as the lowest scale number that provides the required alignment protection level for the energy from this laser. The numeral 5 indicates an optical density of 5, which in turn means that should the full beam from this laser hit the alignment protection eyewear, the energy that would be transmitted through the filter would be 5 x 10-3 * 1/100,000 which equals 5 x 10-8 Joules or 5nJ. This energy is below the maximum permissible exposure (MPE) level for a class 2 laser which means that the eye would be protected by its inherent blink reflex. At the same time, one could still see the (green) beam spot of this laser beam as it scatters from a diffusing surface.

The full specification for the laser safety eyewear necessary for this pulsed frequency doubled Nd:YAG laser is 532  2 x 10-4J  R5 where 532 is the laser wavelength in nm, 2 x 10-4J is the maximum laser energy that the eyewear should be used with at this wavelength and R5 is the EN 208 scale number for eyewear suitable for use at up to this energy. Please note that the optical density of the eyewear is implicit in the scale number and for this example, the OD will be at least 5.

If you need further help in choosing your eyewear, Click here to jump to our enquiry form. Alternatively, you can download EYEPRO Software which will calculate the required protection levels based upon your laser parameters. EYEPRO will also recommend available filters and frames from the extensive LASERVISION range (see: Knowledge Base). If you don't see what you need, please enquire as we can probably offer a custom filter. 

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