SIGMA LASER STENT AND TUBE CUTTER
B-4
990-401
The “M” Classes **
As discussed below in the M Classes Table, a laser product is classified as the "non-M" class when
both
Conditions 1 and 2 are satisfied
, i.e. the measured power is below the AEL (and therefore below the
MPE for the eye) even when the measurement requirements reflect the possible use of optical
instruments.
If one of the power values measured according to Condition 1 or Condition 2 is larger than the AEL, the
product can no longer be in that "non-M" category, i.e. it can not be Class 1 or Class 2. It can however
be in the Class-M category when the power or energy measured with a 7 mm aperture at a distance of 10
cm from the apparent source is below the AEL. The measurement condition of 7 mm aperture at a
distance of 10 cm from the apparent source
ensures
that the exposure of the naked eye to radiation from
a Class-M product is below the MPE for the eye, i.e. as long as no optical instrument is used, Class 1M
is as safe as Class 1 (and Class 2 as safe as Class 2M).
As the MPE can potentially be
exceeded considerably
for the Classes 1M and 2M when exposure occurs
with optical instruments, the maximum power or energy collected with optical instruments (i.e. the
power or energy as measured under Condition 1 and 2) is limited to the AEL of Class 3B. Consequently
in terms of the power of a product when measured with a large aperture (5 cm at 2 m) or at close
distance (7 mm at 14 mm), this power can be larger for a Class 1M or Class 2M laser product as the
power which is allowed for Class 3R, for instance, for a HeNe laser product (632.8 nm) with a 10 mW
collimated beam with a diameter of 4 cm (1/e points) at the exit aperture and 5 cm at a distance of 2 m
from the laser, the power measured through a 7 mm pupil will be about 0.3 mW, hence the product
would be assigned Class 1M (the AEL for Class 1M is 0.39 mW for a point source), however, when
measured with a 5 cm aperture. all of the 10 mW would be collected and the measured power would
exceed the AEL for Class 3R. The product is to be classified as Class 1M, as this is the smallest AEL
which is not exceeded. The irradiance is at least a factor of 3 below the MPE for 0.25 second exposure
duration and is also below the MPE for an exposure duration longer than 10 seconds.
The concept outlined here facilitates a practical risk management and also makes the setting of
restrictions on the use of a given category of products simpler and more flexible. For instance when the
exposure with optical instruments is unlikely, the use of Class 1M and Class 2M will represent a
correspondingly small risk for injury. For the practical use of the Class-M category in terms of user
controls and restrictions it is important to define which one of the two measurement conditions was not
fulfilled, i.e. if the AEL was exceeded for measurement Condition 1, user controls should be considered
when it is anticipated that exposure through telescopes can occur. Generally, collimated laser beams
with a large diameter fall in that category and an example could be the use of a range finder on a
military training ground where it is “reasonably foreseeable” that exposure with large binoculars occurs.
By way of comparison, the use of a Class 1M laser speed meter when used for traffic control, where
exposure with binoculars is unlikely, might be acceptable in terms of associated risk, i.e. probability for
injury. On the other hand, when a product is assigned to the Class-M category because it failed
Condition 2, it is usually safe to be exposed to radiation from the product through binoculars or
telescopes, while an exposure with an eye loupe or magnifier would increase the hazard. An example for
the general category of products which could fail Condition 2 are LED and various fiber optic based
laser sources.
**
The “M” Classes portion is an excerpt from
Review of Current Topics in Laser Safety
By Karl
Schulmeister, Austrian Research Centers Seibersdorf
