Page 197 - PowerPoint 演示文稿

Safety Lenses 187

Table 12.4: Grades of Impact resistance BS EN 166
Symbol Level of Impact resistance Test Ball Speed Best material can achieve

F Low energy 45 m/s All types
B Medium energy 120 m/s Goggles & Face shields.
A High energy 190 m/s Face shields
S Increased robustness - - Toughened glass

Most BS EN standards use the specifications in BS EN 166. The old BS
standards tend to use different specifications. BS EN 166 was first issued in
1994 and superseded BS 2092. It is now more than 10years old and was last
amended in 2002. The standards there in are the most important to an
optometrist.
A typical safety spectacle has the following lens markings:

5- 3.1 “AOS” 1 F
The first digit, i.e., 5 is the code number. This specifies the wavelength
of the radiation that the filter offers protection against (see table 12.1).
The second 3.1 is the shade number. The shade number signifies the
tint density. The shade number ranges from 1.1 to 4.1. Welding filters are
often specified with shade number only. 1.1 corresponds to the light
transmission factor of 80% which is maximum permissible tint for night
driving. 3 correspond to an light transmission factor of 17.8 to 8.5% - the
maximum permissible tint for driving in day light.
The optical class is signified by the number 1 (see table 12.2). The optical
class indicates the measure of optical tolerance to which the lenses have
been manufactured. Prescription safety eyewear needs to be checked to
the same standards as normal spectacles and can be marked only 1 or 2.
Class 3 is only to be used in eye protectors that are not to be used for long
use.
F on the oculars signifies the impact resistance (see table 12.3). There
are two tests for increased robustness:
1. The oculars have to withstand the impact of a 22 mm diameter steel
ball weighing 43 gm traveling at 5.1 m/s.
2. The eye protectors must withstand the impact of a 6 mm diameter steel
ball weighing 0.86 g traveling at 12 m/s.
The letter ‘T’, for example, FT after impact resistance shows that the eye
protectors will be effective at higher temperature also.