Visual Acuity Standards

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Author: Michael Tipton
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Visual Acuity Standards

  1. 1. Visual acuity standards for Beach Lifeguards Mike Tipton, *Emily Scarpello, Tara Reilly, **James McGill University of Portsmouth, UK *Aston University, UK **Southampton General Hospital, UK
  2. 2. Background <ul><li>The ability of lifeguards to rescue individuals is predicated on their ability to identify those in trouble </li></ul><ul><li>This is dependent on: </li></ul><ul><ul><li>Surveillance techniques </li></ul></ul><ul><ul><li>Visual Acuity </li></ul></ul><ul><li>This project considered visual acuity requirements only </li></ul>
  3. 3. Visual Acuity <ul><li>Often measured with the Snellen chart </li></ul><ul><ul><li>&quot;Normal&quot; visual acuity is recorded as 6/6 (metres) vision; at 6m the test subject should be able to see the same as a &quot;normal&quot; person with good eyesight </li></ul></ul><ul><ul><li>6/12 means that a test subject can see the same at 6m as a &quot;normal&quot; person with good eyesight can see at 12m </li></ul></ul><ul><li>The maximum acuity of the human eye without visual aids is generally thought to be about 6/4 </li></ul><ul><li>The ability of the eye to resolve a spatial pattern separated by a visual angle of one minute of arc </li></ul>
  4. 4. Visual Acuity <ul><li>Alternative method, record the minimum angle of resolution (MAR) </li></ul><ul><ul><li>Relates to the resolution required to resolve the elements of a letter. 6/6 equates to a MAR of one minute of arc, 6/12 equates to two minutes of arc and so on </li></ul></ul><ul><li>LogMAR is the log10 of the MAR (geometric to linear sequence) </li></ul>Table 1. The relationship between the different acuity scales (NB. to convert logMAR to Snellen: [Inverse log x 6]) 0.301 2 0.5 6/12 0 1 1 6/6 LogMAR MAR Decimal Snellen
  5. 5. Visual Acuity <ul><li>The charts measure just one aspect of visual capability: the ability to resolve small high contrast letters </li></ul><ul><li>This relates well to the ability to read text (resolve static images from distance) </li></ul><ul><li>It may correlate less well with the ability to see a person in difficulties in the water </li></ul><ul><ul><li>this task involves the ability to locate/detect dynamic objects of various sizes in a range of contrasts </li></ul></ul>
  6. 6. Background <ul><li>Current recommended standard for UK beach lifeguards: at worst, 6/12 in one eye, uncorrected </li></ul><ul><li>The use of correction (spectacles) is currently not recommended </li></ul><ul><ul><li>Restriction of the visual field; lost or damaged when removed </li></ul></ul><ul><li>Calculations </li></ul><ul><ul><li>To see a 25cm diameter head at 300m should require a visual acuity of 6/17 </li></ul></ul><ul><ul><li>To discriminate the features of a head at this distance is likely to require 6/3, beyond the capability of most individuals </li></ul></ul><ul><li>Binoculars </li></ul><ul><ul><li>A beach lifeguard must be able to locate/detect something in the water with the naked eye as a cue to instigate further investigation with binoculars </li></ul></ul>
  7. 7. Background <ul><li>We hypothesised that due to location and detection factors, visual acuity would have to be better than 6/17 </li></ul>
  8. 8. Methodology <ul><li>21 BLG (16 male, 5 female, all under 35 years) gave their consent to participate </li></ul><ul><li>Eye tests to ensure normal vision (mean (range) visual acuity 6/4.8 [6/3.8-6/5]) </li></ul><ul><li>Subjects undertook a series of tests on beaches at Bournemouth (N=9) and Westward Ho! (N=12) </li></ul><ul><li>Vision was blurred (using spherical lenses placed within a trial frame) to approximately 6/70 </li></ul><ul><li>Visual acuity was improved every minute by 0.25 dioptre increments until they could identify the target to the point at which they would investigate it further using binoculars </li></ul>
  9. 9. Methodology <ul><li>The subjects looked out to sea or across a wet beach </li></ul><ul><li>Targets: humans immersed to different depths or Head-sized buoys up to 300m out to sea </li></ul><ul><li>The tests were performed on the same day, in good weather, uniform lighting conditions and a sea state of 0-1 (calm) </li></ul>
  10. 10. Methodology <ul><li>Vision was blurred (using spherical lenses placed within a trial frame) to approximately 6/70 </li></ul><ul><li>Visual acuity was improved every minute by 0.25 dioptre increments until they could identify the target to the point at which they would investigate it further using binoculars </li></ul>
  11. 12. Results from Bournemouth (B) and Westward Ho! (W) 6/44 0.87 (0.15) (vi) W: identify a buoy on the wet beach at 100m 6/18 0.48 (0.13) (v) W: identify a buoy on the wet beach at 200m 6/10 0.23 (0.19) (iv) W: identify a buoy on the wet beach at 300m 6/36 0.78 (0.25) (iii) W: identify a buoy at 100m out to sea (n=8) 6/11 0.28 (0.18) (ii) W: identify a buoy at 200m out to sea (n=8) 6/43 0.86 (0.23) (iv) B: locate and identify a human head within a 100m radius out to sea (volunteer immersed to neck) 6/33 0.74 (0.15) (iii) B: identify a human immersed to the waist at 300m in the sea 6/13 0.33 (0.16) (ii) B: identify an arm waving at 300m in the sea (volunteer immersed to neck) 6/7 0.06 (0.17) (ii) W: locate and identify a human head within a 300m radius out to sea (volunteer immersed to neck) 6/8 0.11(0.06) (i) B: locate and identify a human head within a 300m radius out to sea (volunteer immersed to neck) Snellen LogMAR Mean (SD) Condition
  12. 13. Results from Bournemouth (B) and Westward Ho! (W) 6/44 0.87 (0.15) (vi) W: identify a buoy on the wet beach at 100m 6/18 0.48 (0.13) (v) W: identify a buoy on the wet beach at 200m 6/10 0.23 (0.19) (iv) W: identify a buoy on the wet beach at 300m 6/36 0.78 (0.25) (iii) W: identify a buoy at 100m out to sea (n=8) 6/11 0.28 (0.18) (ii) W: identify a buoy at 200m out to sea (n=8) 6/43 0.86 (0.23) (iv) B: locate and identify a human head within a 100m radius out to sea (volunteer immersed to neck) 6/33 0.74 (0.15) (iii) B: identify a human immersed to the waist at 300m in the sea 6/13 0.33 (0.16) (ii) B: identify an arm waving at 300m in the sea (volunteer immersed to neck) 6/7 0.06 (0.17) (ii) W: locate and identify a human head within a 300m radius out to sea (volunteer immersed to neck) 6/8 0.11(0.06) (i) B: locate and identify a human head within a 300m radius out to sea (volunteer immersed to neck) Snellen LogMAR Mean (SD) Condition
  13. 14. Results from Bournemouth (B) and Westward Ho! (W) 6/44 0.87 (0.15) (vi) W: identify a buoy on the wet beach at 100m 6/18 0.48 (0.13) (v) W: identify a buoy on the wet beach at 200m 6/10 0.23 (0.19) (iv) W: identify a buoy on the wet beach at 300m 6/36 0.78 (0.25) (iii) W: identify a buoy at 100m out to sea (n=8) 6/11 0.28 (0.18) (ii) W: identify a buoy at 200m out to sea (n=8) 6/43 0.86 (0.23) (iv) B: locate and identify a human head within a 100m radius out to sea (volunteer immersed to neck) 6/33 0.74 (0.15) (iii) B: identify a human immersed to the waist at 300m in the sea 6/13 0.33 (0.16) (ii) B: identify an arm waving at 300m in the sea (volunteer immersed to neck) 6/7 0.06 (0.17) (ii) W: locate and identify a human head within a 300m radius out to sea (volunteer immersed to neck) 6/8 0.11(0.06) (i) B: locate and identify a human head within a 300m radius out to sea (volunteer immersed to neck) Snellen LogMAR Mean (SD) Condition
  14. 15. Discussion <ul><li>Variability between subjects: </li></ul><ul><ul><li>Differing ability of individuals to adjust to the lenses </li></ul></ul><ul><ul><li>Central processing required to search, see and identify a head in the sea </li></ul></ul><ul><li>This also explains why the visual acuity required to see a human head at 300m in the sea was 6/7, rather than the theoretical figure of 6/17 </li></ul>
  15. 16. Recommendations <ul><li>BLG should have visual acuity of 6/7 or better </li></ul><ul><li>As this will exclude some individuals, consideration could be given to allowing BLG to wear glasses </li></ul><ul><li>It seems logical to base the requirement for uncorrected eyesight on what the BLG must see when they have removed their glasses and are moving towards a casualty </li></ul><ul><ul><li>A BLG should have uncorrected vision, in their worst eye, that is at least equivalent to that required to see a head from 200m-300m distance, or an arm waving from 300m = 6/14 on average. </li></ul></ul><ul><li>In terms of the Snellen chart, it is recommended that the corrected vision for BLG be 6/9 and the unaided acuity be no worse than 6/18 in either eye. </li></ul>
  16. 17. Acknowledgements <ul><li>Thanks to: the volunteers; Dr Frank Eperjesi (Aston University); Adam Wooler, Peter Dawes, Chris Lewis and the RNLI who funded the project. </li></ul>References <ul><li>Crick, F (1994) The Astonishing Hypothesis. Touchstone Books. </li></ul><ul><li>Tipton MJ, Newton P & Jaki, P (2004) Consideration and recommendations concerning eyesight standards for RNLI beach lifeguards. University of Portsmouth Report. </li></ul><ul><li>Tipton M, Scarpello E & Reilly T (2006) Eyesight standards for Royal National Lifeboat Beach Lifeguards. University of Portsmouth Report. </li></ul><ul><li>Tipton M, Scarpello E, Reilly T & McGill J (2007) Visual acuity standards for beach lifeguards. British Journal of Ophthalmology, in press . </li></ul>
  17. 19. Visual Acuity <ul><li>The charts measure just one aspect of visual capability, the ability to resolve small high contrast letters </li></ul><ul><li>Whilst this relates well to the ability to read text (resolve static images from distance), it may correlate less well with the ability to see a person in difficulties in the water; this task involves the ability to locate/detect dynamic objects of various sizes in a range of contrasts </li></ul><ul><li>Additionally, a person with 6/6 (logMAR 0) vision does not necessarily have &quot;perfect&quot; vision; those with 6/6 vision may suffer from other visual problems such as colour-blindness, inability to focus on nearby objects (hypermetropia [long-sightedness]) or difficulty tracking moving objects. This project dealt only with visual acuity. </li></ul>
  18. 20. Background <ul><li>The “patrolled area” of the beach lifeguards of the RNLI stretches 300m out to sea </li></ul><ul><li>Geometric calculations to determined the angle subtended by a human head at this distance suggest a visual acuity requirement of 6/17 </li></ul><ul><li>However, we hypothesised that due to location and detection factors, visual acuity would have to be better than this. </li></ul>
  19. 21. Results from Bournemouth (B) and Westward Ho! (W) 6/44 0.87 (0.15) (vi) W: identify a buoy on the wet beach at 100m 6/18 0.48 (0.13) (v) W: identify a buoy on the wet beach at 200m 6/10 0.23 (0.19) (iv) W: identify a buoy on the wet beach at 300m 6/36 0.78 (0.25) (iii) W: identify a buoy at 100m out to sea (n=8) 6/11 0.28 (0.18) (ii) W: identify a buoy at 200m out to sea (n=8) 6/43 0.86 (0.23) (iv) B: locate and identify a human head within a 100m radius out to sea (volunteer immersed to neck) 6/33 0.74 (0.15) (iii) B: identify a human immersed to the waist at 300m in the sea 6/13 0.33 (0.16) (ii) B: identify an arm waving at 300m in the sea (volunteer immersed to neck) 6/7 0.06 (0.17) (ii) W: locate and identify a human head within a 300m radius out to sea (volunteer immersed to neck) 6/8 0.11(0.06) (i) B: locate and identify a human head within a 300m radius out to sea (volunteer immersed to neck) Snellen LogMAR Mean (SD) Condition
  20. 22. Recommendations <ul><li>BLG should have visual acuity of 6/7 or better </li></ul><ul><li>As this will exclude some individuals, consideration could be given to allowing BLG to wear glasses </li></ul><ul><li>It seems logical to base the requirement for uncorrected eyesight on what the BLG must see when they have removed their glasses and are moving towards a casualty </li></ul><ul><ul><li>To maintain sight of a casualty requires less VA than that required to locate/detect them in the first place, thus a BLG should have uncorrected vision, in their worst eye, that is at least equivalent to that required to see a head from 200m-300m distance, or an arm waving from 300m. The average for these activities is 6/14. </li></ul></ul><ul><li>In terms of the Snellen chart, it is recommended that the corrected vision for BLG be 6/9 and the unaided acuity be no worse than 6/18 in either eye. </li></ul>

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