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Visual Acuity Standards
 

Visual Acuity Standards

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Author: Michael Tipton

Author: Michael Tipton
(03-26)

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    Visual Acuity Standards Visual Acuity Standards Presentation Transcript

    • 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
    • Background
      • The ability of lifeguards to rescue individuals is predicated on their ability to identify those in trouble
      • This is dependent on:
        • Surveillance techniques
        • Visual Acuity
      • This project considered visual acuity requirements only
    • Visual Acuity
      • Often measured with the Snellen chart
        • "Normal" visual acuity is recorded as 6/6 (metres) vision; at 6m the test subject should be able to see the same as a "normal" person with good eyesight
        • 6/12 means that a test subject can see the same at 6m as a "normal" person with good eyesight can see at 12m
      • The maximum acuity of the human eye without visual aids is generally thought to be about 6/4
      • The ability of the eye to resolve a spatial pattern separated by a visual angle of one minute of arc
    • Visual Acuity
      • Alternative method, record the minimum angle of resolution (MAR)
        • 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
      • LogMAR is the log10 of the MAR (geometric to linear sequence)
      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
    • Visual Acuity
      • The charts measure just one aspect of visual capability: the ability to resolve small high contrast letters
      • 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
    • Background
      • Current recommended standard for UK beach lifeguards: at worst, 6/12 in one eye, uncorrected
      • The use of correction (spectacles) is currently not recommended
        • Restriction of the visual field; lost or damaged when removed
      • Calculations
        • To see a 25cm diameter head at 300m should require a visual acuity of 6/17
        • To discriminate the features of a head at this distance is likely to require 6/3, beyond the capability of most individuals
      • Binoculars
        • 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
    • Background
      • We hypothesised that due to location and detection factors, visual acuity would have to be better than 6/17
    • Methodology
      • 21 BLG (16 male, 5 female, all under 35 years) gave their consent to participate
      • Eye tests to ensure normal vision (mean (range) visual acuity 6/4.8 [6/3.8-6/5])
      • Subjects undertook a series of tests on beaches at Bournemouth (N=9) and Westward Ho! (N=12)
      • Vision was blurred (using spherical lenses placed within a trial frame) to approximately 6/70
      • 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
    • Methodology
      • The subjects looked out to sea or across a wet beach
      • Targets: humans immersed to different depths or Head-sized buoys up to 300m out to sea
      • The tests were performed on the same day, in good weather, uniform lighting conditions and a sea state of 0-1 (calm)
    • Methodology
      • Vision was blurred (using spherical lenses placed within a trial frame) to approximately 6/70
      • 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
    •  
    • 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
    • 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
    • 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
    • Discussion
      • Variability between subjects:
        • Differing ability of individuals to adjust to the lenses
        • Central processing required to search, see and identify a head in the sea
      • 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
    • Recommendations
      • BLG should have visual acuity of 6/7 or better
      • As this will exclude some individuals, consideration could be given to allowing BLG to wear glasses
      • 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
        • 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.
      • 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.
    • Acknowledgements
      • Thanks to: the volunteers; Dr Frank Eperjesi (Aston University); Adam Wooler, Peter Dawes, Chris Lewis and the RNLI who funded the project.
      References
      • Crick, F (1994) The Astonishing Hypothesis. Touchstone Books.
      • Tipton MJ, Newton P & Jaki, P (2004) Consideration and recommendations concerning eyesight standards for RNLI beach lifeguards. University of Portsmouth Report.
      • Tipton M, Scarpello E & Reilly T (2006) Eyesight standards for Royal National Lifeboat Beach Lifeguards. University of Portsmouth Report.
      • Tipton M, Scarpello E, Reilly T & McGill J (2007) Visual acuity standards for beach lifeguards. British Journal of Ophthalmology, in press .
    •  
    • Visual Acuity
      • The charts measure just one aspect of visual capability, the ability to resolve small high contrast letters
      • 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
      • Additionally, a person with 6/6 (logMAR 0) vision does not necessarily have "perfect" 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.
    • Background
      • The “patrolled area” of the beach lifeguards of the RNLI stretches 300m out to sea
      • Geometric calculations to determined the angle subtended by a human head at this distance suggest a visual acuity requirement of 6/17
      • However, we hypothesised that due to location and detection factors, visual acuity would have to be better than this.
    • 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
    • Recommendations
      • BLG should have visual acuity of 6/7 or better
      • As this will exclude some individuals, consideration could be given to allowing BLG to wear glasses
      • 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
        • 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.
      • 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.