High-speed particle testing of eyewear
There are many environments where it is important to protect the eyes and face against optical hazards or flying fragments.
Eyewear can offer protection against lasers or intense light sources (known as optical hazards) as well as physical threats, such as airborne dust particles or fragments of material (such as swarf ejected from metalworking machinery). In the latter case, it is important to provide employees with protection against high energy impacts.
Requirements for protective eyewear are described in EN 166:2001, with optical test methods defined in EN 167:2001, and non-optical in EN 168:2001. Eyewear claiming to provide protection against high-speed particles needs to conform to all of the mandatory requirements of EN 166, as well as the high-speed particle tests described. EN 168:2001 clause 9 also describes the test method for testing the resistance of eyewear to high-speed particles. This test is simple in principle – the eyewear is placed on an appropriate head form, and a 6mm diameter steel ball is propelled at the eyewear, at a controlled speed, at a specified point on the eyewear.
SATRA uses a compressed-air propulsion system to achieve this, accelerating the ball along a horizontal barrel toward the head form. The speed of the ball is measured using a sensor mounted just before the exit point of the barrel, based on the assumption that frictional/air resistance losses between the end section of the barrel and the point of impact with the eyewear are negligible (the release point from the barrel being in close proximity to the eyewear). The acceleration of the ball, and therefore the speed of impact, can be varied by adjusting the pressure of the air in the chamber before release.
Typical speeds required for impact are:
- 45m/s (101mph) for low energy impact
- 120m/s (268mph) for medium energy impact
- 190m/s (425mph) for high energy impact.
At a typical projectile mass of 0.86g, this equates to kinetic energies of 0.87, 6.2 and 15.5 joules respectively. These energies may not appear to be high, but with a maximum contact area of only a few square millimetres and a typical deceleration time of less than 10 microseconds, the pressure exerted can be huge (more than 1,000 bar), certainly enough to penetrate eyes or flesh. It should be noted that spectacles are only tested at the lowest energy, goggles may be tested at the lowest or medium energy, and only full-face shields can claim protection against high energy impacts.
To ensure that protective eyewear will not fracture or deform and make contact with the eyes or flesh, it is impacted at four points during testing – frontal points and lateral points of both eyes. It is then inspected for damage or contact with the head form. Contact is checked using carbon paper placed behind the eyewear, which will show a mark if impacted.
Eyewear claiming protection against high-speed particle impacts must also meet the mandatory optical requirements for all protective eyewear, including field of vision, transmittance of light, diffusion of light (although there is a reduced requirement for this type of eyewear), and spherical, astigmatic and prismatic refractive powers.
The eyewear must also meet the requirements for lateral protection. Here a probe is used to check that the eyewear prevents ingress of matter from the sides of the face.
The oculars are required to withstand an impact of a 22mm ball (43g mass) at a speed of 5.1m/s (this test is usually carried out using a vertical drop test) whilst fulfilling the same requirements as those in the high-speed particle impact test.
Further information on SATRA's PPE certification and testing services is available at www.satra.com/ppe
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