Publication White Paper:
Forensic Engineering Analysis of Safety Shooting Glasses

David Danaher, P.E.

Background

Currently there are voluntary standards in place that outline the impact resistance of safety glasses.  Although the standards set forth by the American National Standards Institute (ANSI) outline testing that can be used to calculate a minimum level of energy which the lens should absorb, independent testing has shown that the performance of modern safety glasses exceed the minimum requirements of the absorbed energy. 

 

ANSI standard Z87.11, titled Occupational and Educational Personal Eye and Face Protection Devices, was developed to provide minimum requirements for personal eye protection.  The standard outlines several aspects of eye protection. Four tests were specified;

1) High Mass Impact, Drop Ball Impact, High Velocity Impact, and Penetration Test.  The High Mass Impact test states that spectacles shall be capable of resisting impact from a pointed object weighing 17.6 ounces dropped from a height of 50 inches. 

2) The High Velocity Impact test states that the spectacles shall resist an impact from a 0.25 inch diameter steel ball traveling 150 feet per second. 

3) The Drop Ball Impact test states that spectacles shall be capable of resisting impact from a 1 inch diameter steel ball dropped from a height of 50 inches. 

4) The Penetration Test states that spectacles shall be capable of resisting impact from a weighted projectile weighing 1.56 ounces dropped from a height of 50 inches.

Based on the specifications outlined in the above standard, the kinetic energy for each of the tests was calculated.  The kinetic energy for the High Mass Impact test was 4.44 ft-lbs, the High Velocity Impact test was 0.80 ft-lbs, the Drop Ball Impact test was 0.61 ft-lbs, and the Penetration Test was 0.41 ft-lbs.

 

For the purpose of this investigation the primary focus is test #2: the high velocity impact resistance of the lenses.

The standard and the tests are minimum guidelines for the manufacturing of eye protection glasses and do not limit the manufacturers to design and build safety glasses which exceed the standard.  To determine the actual amount of kinetic energy safety shooting glasses could absorb without penetration, this engineer performed a series of tests on a variety of safety glasses.

Testing Method

Testing was performed in order to determine the level of kinetic energy which the lens of safety shooting glasses could absorb without a projectile penetrating the lens.  Seventeen pairs of ANSI Z87.1 certified safety shooting glasses were randomly selected from several manufacturers and mounted to a testing fixture. Each lens (left and right) was impacted once for a total of 34 impacts.  The velocity of each shot was measured and recorded with a chronograph, and each projectile was weighed prior to the discharge. 

 

The 17 selected glasses were manufactured by Remington, Radians, and Red Head, marketed as safety shooting glasses.  The glasses varied in design from a solid one piece construction, a frame with a one piece lens, to a frame with a two piece lens.  Three of each model was tested except for the Radians brand due to availability. The glasses also varied in price from relatively expensive to the relatively inexpensive. 

 

 

Safety glasses prior to testing.

Testing Results

After each test the glasses were documented and examined to determine if the projectile had penetrated the lens, and the kinetic energy was then calculated.  The muzzle velocity on four of the tests was either not recorded due to an error, or the velocity was significantly lower than the other tests, most likely due to the fit of the projectile in the barrel.  Therefore, these four tests were considered outliers and were not used in the average velocity calculation.

 

The testing shows that the adjusted average velocity of the projectile was approximately 920 fps with an average weight of the projectile of approximately 0.44 grams.  The testing also shows that the average kinetic energy absorbed was 12.75 ft-lbs.  As shown in the above table, all of the glasses tested were able to withstand the impact with no penetration of any of the lenses.  Figure 3 below shows an example of the result of the testing on the safety glasses manufactured by Red Head.

 

 

 

 

 

Example of impact testing on the safety glasses

(upper photograph shows front view,

lower photograph shows top view).

 

In all of the tests performed on the variety of safety glasses, results showed that none of the lenses were penetrated by the projectile.  Comparing the testing to the highest level of energy in the ANSI standard shows that the glasses withstood impacts which were 2.5 times higher in kinetic energy than the standard requires.  To give context to the level of energy the safety glasses were subjected to during testing; figure 4 shows a penny that was tested using the same level of kinetic energy.

 

 

 

 

 

 

 

 

 

 

 

Example of impact testing on a penny.

Conclusion

 

The safety glasses tested absorbed more kinetic energy than the calculated kinetic energy based upon the requirements outlined in ANSI Z87.1.  The use of safety glasses greatly reduces the likelihood of an eye injury even with projectiles that have a kinetic energy greater than that calculated from the ANSI standard.

 

Although the glasses tested exceed the threshold of absorbed energy specified by the standard, there is nothing preventing a manufacturer to produce a product that only meets the minimum requirement. 

 

 

References

 

  1.  ANSI Z87.1-2003, Occupational and Educational Personal Eye and Face Protection Devices, American Society of Safety Engineers, 1800 East Oakton Street, Des Plaines, Illinois 60018

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