Volume 79, Issue 8 , Pages 455-463, August 2008
Optical quality and impact resistance comparisons of 2 football helmet faceshields
Article Outline
Abstract
Background
Currently there is no standard that specifically addresses the optical and impact performance of football protective faceshields. This study compared the impact resistance and optical quality between 2 popular football faceshields. Testing was performed only on new faceshields.
Methods
To test impact resistance, baseballs were propelled at the faceshields with velocities up to 66.4 m/sec. Structural integrity was evaluated after each impact. Ten visors from each of 2 companies underwent a single impact at various velocities. Two visors from each company were impacted 3 times to evaluate the effects of repeated blows. Additional visors were conditioned to −10°C and impacted once. Additionally, prismatic power, refractive power, haze, visible light, and ultraviolet (UV) transmittance, and optical distortion were measured to evaluate optical quality. All testing was done with faceshields mounted to facemask and, when appropriate, to a helmet.
Results
None of these new faceshields fractured even with impact velocities up to 66.4 m/sec. With regard to optical quality, both protectors met the optical requirements for the standards of faceshields for selected sports (ASTM F803-2003).
Conclusions
Both faceshields tested should protect football players from anticipated impacts while providing adequate optical quality for satisfactory visual performance.
Keywords: Eye injuries, Sports, Football, Eye protection, Face shield
Sports-related injuries affect participants of all ages. Annually, there are an estimated 3.7 million sports-related injuries that present to U.S. emergency departments.1 Injuries to the eyes comprise approximately 1% of this total. In 2000 there were an estimated 40,000 sports-related eye injuries that were treated in U.S. emergency rooms.2 It is believed that the total number of eye injuries is perhaps 2.5 times this total when patients examined in clinics and private offices are included.3, 4 Greater than 70% of the sports-related eye injuries occurred in individuals younger than 25 years.5, 6
In 1993, a national report by Prevent Blindness America reported eye injury statistics associated with different sports. Behind basketball, baseball, pool sports, and racket sports, football was the fifth greatest contributor to sports-related eye injuries with almost 2,200 estimated eye injuries in patients younger than 25 years of age.7 Using these data, the sports safety committees of the American Academy of Pediatrics and the American Academy of Ophthalmology issued a report recommending that football helmets be equipped with a polycarbonate faceshield for face and eye protection.8
Eye injuries in organized football are relatively rare events. The eye injury to Orlando Brown of the Cleveland Browns on December 19, 1999, is one notable exception.9 The injury occurred when a referee threw a penalty flag (with sewn-in weights) in Brown's direction. The flag entered Brown's facemask and hit him directly in his right eye, resulting in recurrent intraocular bleeding. Although this injury was an anomaly caused from a thrown referee flag, it serves to illustrate the fragility of the eye and the importance of vision to our everyday lives. Although the game of football can be brutal with high-energy collisions, the eyes and face are relatively protected with the facemask and helmet worn in organized events.
The National Collegiate Athletic Association (NCAA) monitors injuries in numerous collegiate sports and provides injury statistics for practice and game participation. Because of the low numbers involved, eye, nose, and face injuries are not even listed in their summaries of football injuries.10 In contrast, eye and face injuries account for 19.7% of injuries in field hockey.11
Football eye injury statistics are provided, however, in the database for the National Electronic Injury Surveillance System (NEISS), which is overseen by the Consumer Product Safety Commission. It documents injuries and illnesses associated with consumer products that result in an emergency room visit. Injuries are documented at representative emergency rooms across the country, and a statistical estimate is then generated to provide overall national injury data. Summary data with sample patient histories are provided online by the NEISS.12 It is unknown how many of the documented eye injuries associated with football are from organized league activities or from “street” ball; however, histories are provided to help sort out the eye injury mechanism for each event. All 30 case histories from each of the last 5 years of data (2002 through 2006) were reviewed for this study. The “cause” of most of these eye injuries could be gleaned from the case histories. The summary of the injury mechanisms is provided in Table 1, Table 2. Nearly one third of the injuries were from the football itself, whereas nearly one fifth were from fingers to the eyes.12
Table 1. Football-associated injuries NEISS database 2002–2006: Summary of 150 case histories
| Percent | Injury |
|---|---|
| 31.33 | Hit with football |
| 18.00 | Finger |
| 7.33 | Debris/foreign body |
| 4.00 | Conjunctivitis |
| 4.00 | Elbow |
| 3.33 | Helmet |
| 2.67 | Ran into object |
| 2.00 | Knee |
| 5.33 | Abrasion, shoulder, player, equipment |
| 1.33 | Keys, cheering |
| 20.67 | Unknown |
Table 2. Football-associated injuries NEISS database 2002–2006: Age ranges of injured as percentage of total injured
| Year | Estimated number of injuries | Age range of injured | |||
|---|---|---|---|---|---|
| 2 to 14 years | 15 to 24 years | 25 to 44 years | 45 years and older | ||
| 2002 | 2087 | 55.4% | 29.2% | 12.3% | 3.1% |
| 2003 | 1967 | 49.2% | 41.5% | 7.7% | 1.5% |
| 2004 | 1768 | 63.9% | 26.4% | 8.3% | 1.4% |
| 2005 | 1641 | 53.0% | 36.9% | 9.2% | 0.0% |
| 2006 | 1965 | 54.6% | 36.4% | 9.1% | 0.0% |
| 5-year total | 9428 | 55.6% | 33.9% | 9.4% | 1.2% |
Although it is not known in how many of the NEISS eye injury events a football helmet with facemask was worn, it is logical to assume that rarely would an eye be impacted by a football if a helmet with facemask were properly worn. This is also true for many of the other “high-energy” impact injuries listed. Therefore, providing eye protection from finger pokes or small projectiles (e.g., field of play debris) would be the primary purpose of a faceshield for football. Also, for maximum protection, the faceshield should be of a nonfracture material to avoid eye injuries from the faceshield itself. Therefore, faceshields should be tough enough to survive foreseeable impacts.
There currently are no industry standards or regulations regarding the performance of faceshields for football. Sports faceshields or eye protectors for skiing, hockey, baseball, racquet sports, and various other sports have optical and impact resistance requirements set by the American Society for Testing and Materials (ASTM). Industrial faceshields are required to meet standards set by the American National Standards Institute (ANSI). ANSI and the ASTM are voluntary standards development organizations that incorporate manufacturers, professional groups, consumers, and other interested parties in an attempt to develop consensus standards for products and procedures.
It was the goal of this study to determine the impact resistance and optical quality for the 2 most popular collegiate and National Football League football faceshields, Oakley (Foothills Ranch, California) and Nike (Beaverton, Oregon), using the methods outlined by ANSI and the ASTM for other protective eye equipment. It is hoped this research may help determine appropriate industry standards for football helmet faceshields.
Methods and materials
Oakley and Nike each donated 40 standard, clear football faceshields for testing. For impact testing, the faceshields were mounted on a standard collegiate Riddell (Elyria, Ohio), size large, football helmet (model no. VSR-4) with a Riddell Z-2EG facemask (see Figure 1A). The opening for vision between the “bars” on the facemask was approximately 3 inches (vertical) × 6 inches (horizontal). For optical testing, the faceshields were mounted on an isolated facemask (see Figure 1B). This ensured the faceshield was flexed as normally worn and allowed for optical testing through the faceshield in the “as worn” position without having to sacrifice a helmet by cutting projection holes through the rear of the helmet.
Figure 1.
A, Helmet with facemask and faceshield. B, Faceshield mounted onto facemask.
The thicknesses of the faceshields were measured near the location of the normal visual axis. These measures were 2.83 mm for the Oakley shield and 2.36 mm for the Nike shield. Normal optical analysis was performed on 3 representative samples of each faceshield. These specific faceshields were not impacted before optical testing. The optical tests run were: (1) prismatic power (horizontal and vertical), (2) refractive power, (3) haze, (4) UVA and UVB transmittance, and (5) optical distortion.
These tests were accomplished using standardized protocols as specified in ANSI Z87.1-200313 for occupational eye and face protection and ASTM F803-200314 for eye protection in selected sports. All optical analyses were performed in a standard laboratory atmosphere (temperature and humidity) unless otherwise noted in this report.
For optical testing, the faceshields were mounted to the isolated facemasks without the helmet and were positioned so that the second horizontal bar from the bottom (see Figure 1A) was at a downward angle of 20°. The faceshields were oriented so that the measuring line of sight passed through the vertical midpoint between the innermost bars of the masks (i.e., vertically centered through the wide opening).
To measure impact resistance, the faceshields were mounted onto the facemasks and the helmet. The helmets were placed onto a standard headform and secured with a helmet chin strap. Precise guidelines for the methods of impacting the shields are described by ASTM F803 regulations.14 A standard major league baseball was used as the projectile for impact (weight, 5 ounces; mass, 0.145 kg). A baseball was chosen for the testing because it was felt to approximate the hardness and curvature of a player's fist or shoe. The baseball was shot from an air cannon as described in ASTM F803.
We surmised that these items (fist and shoe) are likely the items of highest potential impact energy to the faceshield that could fit through a typical opening in the facemask. Although a helmet-to-facemask impact with moving players may also be of high energy,15 it is hard to imagine this impact contacting the eyes or orbital areas. As long as it survives the impact, the facemask itself should protect the eyes from any impact with large objects of large diameter (e.g., helmet, knee, football).
The air pressure setting for the cannon was calibrated with subsequent ball velocity before impact testing to be able to set the approximate baseball velocity for each impact run. The exact ball velocity of each impact, however, was measured with an optical timing device. Initially, all faceshields were impacted only once. Representative impacts were videotaped from the side to determine the distance that the helmet and faceshield were driven backward from the baseball impact. This measure was used to determine the approximate impact force.
A ZEST procedure16, 17 was designed to determine the baseball velocity at which 50% of each shield type reached failure points, including cracks, fractures, or broken clips/bindings. The ZEST procedure has been shown to require fewer impacts than the classical “Bruceton” method18 for impact testing to reach an acceptable estimate of the impact resistance of optical products. The Bruceton method requires many impacts at several different projectile velocities. The ZEST procedure concentrates most of the impacts just above and just below the “anticipated” mean fracture velocity. The “anticipated” mean fracture velocity is recalculated after each impact using the projectile velocity and whether the faceshield fractured or survived the impact. The initial impact velocity for testing was from testing 2 sample shields. These shields, one from each manufacturer, had been used by college players and had certainly sustained multiple impacts as well as having had substantial exposure to solar radiation. Both shields fractured at a test velocity of 38 m/sec (124 ft/sec). The initial test velocity for the ZEST procedure was chosen to be 34 m/sec because it was just slightly below the sample fracture velocity. (It should be noted, however, that no new shield of either type was fractured during testing even at the highest impact velocity of which the air cannon was initially capable [54 m/sec].) Supplemental testing was done to determine the effects of multiple impacts and of low temperature.
Two shields of each type were impacted a total of 3 times at the initial maximum velocity setting. Additionally, 3 shields of each type were preconditioned to −10°C (14°F) and subsequently impacted, again at the initial maximum velocity setting.
As all faceshields survived the initial maximum velocity, components for the air cannon were modified to increase the maximum safe operating air pressure. These changes provided an increase in baseball velocity to 66.4 m/sec (217.8 ft/sec). Three shields of each type were subsequently impacted at this velocity.
Results
Optical testing
Three representative samples of each faceshield were tested according to standardized optical performance protocols. The specific tests run were (1) prismatic power; (2) refractive power; (3) haze; (4) UVA, UVB, and visible light transmission; and (5) optical distortion. The results of testing are shown in Table 3. Although no standard currently exists for football faceshields, standards do exist for industrial safety faceshields (Z87.1-2003) and faceshields for selected sports (ASTM F 803-2003). The optical requirements for faceshields as specified by these standards are shown in Table 4. Included for comparison are the standard values for Z87.1-2003 plano spectacles.
Table 3. Optical measurements
| Prismatic power (prism diopters) | ||||
|---|---|---|---|---|
| Prismatic power | Vertical imbalance | Horizontal imbalance | ||
| Left ocular | Right ocular | |||
| Oakley visors | ||||
 1 | 0.10 | 0.03 | 0.00 | 0.03BI |
| 2 | 0.05 | 0.05 | 0.00 | 0.00 |
| 3 | 0.08 | 0.08 | 0.00 | 0.00 |
| Nike visors | ||||
| 1 | 0.06 | 0.08 | 0.00 | 0.16BI |
| 2 | 0.03 | 0.08 | 0.00 | 0.13BI |
| 3 | 0.06 | 0.09 | 0.00 | 0.17BI |
| Refractive power (diopters) | ||||||
|---|---|---|---|---|---|---|
| Left ocular | Right ocular | |||||
| Meridional power | Astigmatism | Meridional power | Astigmatism | |||
| Oakley visors | ||||||
| 1 | 0.05 | 0.01 | 0.04 | 0.04 | 0.01 | 0.03 |
| 2 | 0.04 | 0.01 | 0.03 | 0.04 | 0.01 | 0.03 |
| 3 | 0.04 | 0.01 | 0.03 | 0.04 | 0.00 | 0.04 |
| Nike visors | ||||||
| 1 | 0.04 | 0.01 | 0.03 | 0.04 | 0.02 | 0.02 |
| 2 | 0.06 | 0.03 | 0.03 | 0.06 | 0.03 | 0.03 |
| 3 | 0.04 | 0.00 | 0.04 | 0.04 | 0.01 | 0.03 |
| Haze | ||
|---|---|---|
| Location | Measured value (%) | |
| Oakley visors | ||
| 1 | Left ocular | 0.30 |
| 2 | Right ocular | 0.36 |
| 3 | Left ocular | 0.26 |
| Nike visors | ||
| 1 | Left ocular | 0.37 |
| 2 | Right ocular | 0.40 |
| 3 | Left ocular | 0.42 |
| Luminous transmittance | |||
|---|---|---|---|
| Left ocular (%) | Right ocular (%) | Left/right | |
| Oakley visors | |||
| 1 | 88.0 | 88.0 | 1.00 |
| 2 | 88.0 | 88.0 | 1.00 |
| 3 | 8.0 | 88.0 | 1.00 |
| Nike visors | |||
| 1 | 91.2 | 91.2 | 1.00 |
| 2 | 91.1 | 91.0 | 1.00 |
| 3 | 91.1 | 91.0 | 1.00 |
| Ultraviolet transmittance | ||
|---|---|---|
| UV-B 290 to 315 nm (%) | UV-A 315 to 380 nm (%) | |
| Oakley visors | ||
| 1 | 0.00028 | 0.0034 |
| 2 | 0.00032 | 0.0039 |
| 3 | 0.00031 | 0.0039 |
| Nike visors | ||
| 1 | 0.00017 | 0.0084 |
| 2 | 0.00017 | 0.0084 |
| 3 | 0.00017 | 0.0075 |
Table 4. Optical requirements from Eye Safety Standards
| ASTM 803-2003 | ANSI Z87.1-2003 | ||
|---|---|---|---|
| Sport faceshield | Faceshield | Plano spectacles | |
| Refractive power | +0.06 to −0.18 D. | No requirement | +0.06 to −0.06 D. |
| Astigmatism | ≤0.12 D. | No requirement | ≤0.06 D. |
| Prismatic power | ≤0.50 pd | ≤0.37 pd | ≤0.50 pd |
| Prismatic imbalance | ≤0.25 pd Vert and BI | ≤0.37 pd Vert | ≤0.50 pd Vert |
| ≤0.50 pd BO | 0.12 pd BI to 0.75 pd BO | 0.25 pd BI to 0.50 pd BO | |
| Haze | ≤3% | ≤3% | ≤3% |
| Luminous transmittance | ≥85% | ≥85% | ≥85% |
The induced refractive power and astigmatism were minimal and essentially equal for the 2 types of faceshields. All values for both shield types met the standard values for sports faceshields (ASTM 803-2003) and even met those for plano spectacles as specified by ANSI Z87.1-2003.
Prismatic powerThe overall prismatic power values for each protector type met the faceshield requirements for both the ASTM F803-2003 and the more strict ANSI Z87.1-2003 standards. Measured prismatic power was less than 0.12 prism diopters for all measurements. Additionally, neither faceshield type showed any vertical prismatic imbalance; however, the Nike faceshields showed a mild base-in imbalance, which failed to meet the ANSI Z87.1-2003 standard, but did meet the requirements of the ASTM F803-2003 standard.
HazeNeither the Oakley nor Nike faceshield types had greater than 0.5% haze in any shield tested. The ANSI Z87.1-2003 standard for clear lenses and the ASTM 803-2003 standard both require haze to not exceed 3% in protective face wear. Both faceshield types were well under this limit.
Luminous and UV transmittanceThe Oakley shield had a consistent luminous transmittance of 88% for each of the 3 shields tested. The Nike shields varied slightly but had an average luminous transmittance of 91%. Both these values were above the clear lens requirements of ASTM 803-2003 (85%) and ANSI Z87.1-2003 (85%).
Both faceshield types had extremely low UV transmittance. All UVA and UVB transmittance measures were well less than 0.1%.
Optical distortionOptical distortion is assessed by a visual inspection of a fine ruling target. Figures 2A and B show representative findings for the respective shields. The fringe lines through the Oakley shields appeared to have crisp smooth edges and little or no areas of striation. Overall, they were relatively straight and parallel. There was a slight orange peel appearance to the entire view through the Oakley shield. The fringe lines of the Nike shields did appear to have jagged edges and some areas of striation, but overall, they too, were relatively straight and parallel.
Figure 2.
A, Image of ruling as viewed through Oakley Faceshield. B, Image of ruling as viewed through Nike Faceshield.
Impact-resistance testing
It was the original goal of the study to determine an impact velocity at which 50% of each type of shield reached a failure point. There were 16 shields involved in the initial testing phase. None of the shields was cracked or fractured at baseball impact velocities up to 54 m/sec (177 ft/sec). This was the highest speed attainable without modifications being made to the standard setup for the air cannon. The faceshield mounting clip was cracked on one Oakley shield at the maximum impact velocity; however, the cracking of the mounting clip did not impact the face or eyes of the headform.
To test the faceshields under more harsh conditions, 6 faceshields (3 of each type) were cooled to −10°C for 1 hour and impact tested. Again, there were no failure points reached by either shield type with impact velocities reaching 54 m/sec. Four additional shields underwent repeated impact testing with multiple impacts (3 impacts to each of the 4 shields) with mean projectile velocity of 53.3 m/sec. Again, none of these shields reached failure points with any of the impacts.
Modifications were subsequently made to the air cannon to allow greater internal air pressure to increase the velocity of the baseball to greater than 66 m/sec (range, 62 to 66.4 m/sec). Six shields were subjected to testing at these higher speeds. None of these 6 shields fractured, although all 3 Oakley shields had cracks in the retention clips for these high-velocity impacts.
Impact forceThe videotape of several impacts of the faceshields was viewed to determine the distance of helmet and faceshield deflection. Videotaping was not done for the impacts made after the modification of the air cannon; therefore, the maximum impact velocity that was videotaped was 54 m/sec. At this velocity, the approximate movement of the center of the faceshield was 11.5 cm (headform movement + faceshield flexure). This distance was used to calculate representative impact forces for the projected baseballs. The series of equations used to calculate this value is shown below. The impact velocity (V1) is used along with the maximum deflection, s, to solve for the acceleration, a, of the baseball as it impacts the shield and is deflected. Mass of the baseball multiplied by its acceleration provides an estimate of the impact force.
V2 = velocity at maximum deflection = 0
a = acceleration of baseball
s = maximum deflection of faceshield = 0.115 m
m = baseball mass = 0.145 kg
a = baseball acceleration as it impacts the shield Using this technique, the impact force for the baseballs onto the faceshields at 54 m/sec was calculated to be approximately 1,800 Newtons.
Although faceshield displacement was not measured for the impact with the baseball velocity of 66.4 m/sec, the force associated with this impact can be calculated using several assumptions. If one assumes that the faceshield displacement with the 66.4 m/sec impact is identical to that with the 54 m/sec impact (i.e., 11.5 cm), then the new impact force would increase as the square of the increase in velocity. That is, the new impact force (F) would be equal to:
Inspection of impacted shields was also performed. The shields of both types often showed small “smudge” marks at the point of baseball impact. Additionally, the Nike shields showed subtle vertical striations in the lateral portions of each shield. The striations were located directly above the attachment positions of the shield to the facemask and appeared to be caused from flexure of the mask at these lateral locations. The striations were slightly more noticeable as the actual impact velocities increased. They appeared to be subtle variations in the surface coatings on the shields, as the integrity of the polycarbonate material remained intact. The Oakley shields did not show these vertical striations after impact.
Discussion
Loss of vision from an eye injury is most often preventable. It is estimated that 90% of all eye injuries could be avoided by wearing the proper protective eyewear.5, 6, 7, 19, 20 Although the number of eye injuries in football appear to be fewer in number than with other sports12 (e.g., baseball and basketball), the injury to football player Orlando Brown in 19999 illustrates the potential for catastrophic consequences if proper precautions are not taken.
The NEISS statistics report that a high percentage of football eye injuries are sustained from contact with the football. Although a properly worn helmet with facemask will certainly help prevent these eye injuries, eye contact is possible during street games or during practice when a helmet is not worn. In these situations, the wearing of an athletic eye protector is warranted, especially for “at-risk” individuals (e.g., high myopes or functionally 1-eyed individuals). Although there is no eye protector currently available designed to protect the eyes from football injury outside the helmet, ASTM F803-200314 protectors for racquet sports possess the general safety characteristics for sports not found in dress eyewear. These devices are designed and tested to protect the eyes from impacts from balls hit at high speeds and from the racquets themselves. When properly fit, the frames are large enough to transfer impact energy to the large bones of the face and away from soft orbital tissues. Additionally, a tough polycarbonate lens is typically provided to help limit all eye contact.
For objects small enough to hit the face through a football facemask, a kicking motion with a shoe and a punching motion with a hand/fist are sources of substantially high energy. In this study, a baseball was chosen as the tested projectile to approximate the hardness and general diameter of these objects (shoe and fist). Our testing showed that both faceshields survived even the highest impact velocities that our equipment was able to generate. At the higher velocities tested (up to 218 ft/sec), the impact force was calculated to be approximately 2,500 Newtons. Several studies have measured the impact force for the kicking motion. Two studies were reported using physically fit young adult soccer players. The studies reported maximum impact forces of 2,439 Newtons21 and 1,025 Newtons.22 From these reported values, we feel justified in concluding that both faceshields are sufficiently tough to withstand the maximum foreseeable impact forces sustained while playing football.
In terms of optical quality, there was minimal difference between the 2 shield types. Both faceshields met all optical requirements for the F803-2003 standard; however, the Nike faceshield failed to meet the more strict prismatic imbalance requirement for the ANSI Z87.1-2003 faceshield. There is greater latitude in prismatic imbalance within the ASTM faceshield standard for selected sports as that faceshield is manufactured with a greater shield thickness to ensure it survives anticipated higher-energy impacts. Additionally, it should be noted that horizontal prismatic imbalance is highly dependent on the stress placed on the faceshield. If the faceshield curvature does not match the exact curvature of the facemask to which it is mounted, lateral prism can be introduced as the faceshield is flexed during the attachment procedure. Although both shields passed all optical requirements for faceshields for selected sports, the Nike shield had slightly greater overall luminous transmittance, whereas the Oakley shield showed less horizontal prismatic imbalance and less optical distortion.
These findings support a conclusion that, when properly worn, the faceshields from either manufacturer should provide both the optical and safety characteristics necessary for elimination of most eye injuries sustained during organized football activities. Of concern, however, is that the “used” faceshields from both manufacturers that were initially tested to provide pilot impact data failed at impact velocities well below the maximum impact velocities that were survived using “new” faceshields. Thus, the faceshields appear to have a decreasing impact resistance with age, possibly caused by material breakdown from multiple impacts, from chemical changes from extended exposure to solar radiation, or from some unknown cause. Future studies should address these issues to determine a useful faceshield life. After a faceshield is used for its “useful life,” the faceshield should then be replaced.
Conclusions
Study results show that both the Nike and the Oakley football faceshields meet the optical requirements currently specified for faceshields for selected sports (ASTM F803-2003). Also, both faceshield types, when new, provide impact protection sufficient to withstand a force equal to a direct kick to the faceshield. Because this is projected to be the highest foreseeable force encountered during typical football play, increased use of helmet-mounted football faceshields should reduce the number of eye injuries for participants of all ages. Additionally, because the 1-year-old faceshields used initially in this study failed at impact levels well below that found for the new faceshields, the impact resistance of the football faceshields should be investigated after the protectors are exposed to representative environmental conditions.
Conflict of interest
None of the authors have any financial arrangement with the companies listed in this report or with any competitor company. All impact and optical testing was performed at ICS Laboratories, Inc., 1072 Industrial Parkway North, Brunswick, Ohio 44212.
Acknowledgment
The authors thank Louis Van Hoose (football equipment manager) and Rob Lachey (head equipment manager) from The Ohio State University for providing faceshields and football helmets necessary for the completion of this study.
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PII: S1529-1839(08)00287-X
doi:10.1016/j.optm.2008.02.010
© 2008 American Optometric Association. Published by Elsevier Inc. All rights reserved.
Volume 79, Issue 8 , Pages 455-463, August 2008
