Applied Optics Laser Safety: Use of Neutral Density Filters

Our laser code of practice states that all class 3b and 4 beams should be attenuated to 3a or below wherever possible (preferably to class 2 power if possible).

You should normally use neutral density filters to achieve this and  the filter should be installed at the laser aperture before any other optical components.

For class 4 lasers the high powers mean that filters based on absorption often cannot be used and special techniques need to be used.

For class 3b lasers use neutral density filters and check the output power with a power meter to ensure you are operating in the safe regime.

Neutral density filters which are specified as % transmission are trivial to use, however most neutral density filters are specified in optical density (OD) or Bels (the attenuation in decibels is the OD*10).

Power transmission factor is:

Power transmission factor = 10^-OD, where OD is the number of optical densities of the filter. For instance 0.3 OD is 10^-0.3=0.5 so the power is attenuated by one half, 0.3 ODs is 3dB of optical power attenuation.

Optical densities OD	Fraction of power transmitted
0.1	79%
0.3	50%
0.5	32%
1.0	10%
1.5	3%
2.0	1%
2.5	0.3%
3.0	0.1%

Since the scale is logarithmic cascaded neutral density filters give an net optical density equal to the addition of the individual ODs, the transmission is equal to the product of  the individual transmissions.

For a 20mW HeNe an ND filter of 0.6OD or greater gives a class 3a beam, an ND filter of 1.3 or greater  gives a class 2 beam.

Everyone must be completely familiar with this scale and understand its implications for the safety of their experiment.

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Location of this document: http://optics.eee.nottingham.ac.uk/optics/safety/documents/neutral_density_filters.php