Calc Function

    • Calcs that help predict probability of a diseaseDiagnosis
    • Subcategory of 'Diagnosis' designed to be very sensitiveRule Out
    • Disease is diagnosed: prognosticate to guide treatmentPrognosis
    • Numerical inputs and outputsFormula
    • Med treatment and moreTreatment
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    Chief Complaint


    Organ System


    Patent Pending

    Visual Acuity Testing (Snellen Chart)

    Assess binocular and monocular visual acuity.


    This calculator is intended as a convenient screener for visual acuity to be used on a mobile device and should not be used as a replacement for in-office testing. Evidence for smartphone apps for Snellen visual acuity is limited, and currently no app has been found to be accurate to within at least one line of formal visual acuity testing (and further validation is required).


    • Ensure proper room lighting and set phone brightness to 100%.
    • Hold the screen 4 feet (1.2 m) from the patient (approximately the end of a standard hospital bed if patient is sitting upright).
    • Test each eye independently. The patient should completely cover the opposite eye.
    • See Pearls/Pitfalls for further instructions.
    When to Use
    Why Use

    To assess binocular and monocular visual acuity in cooperative patients.

    • Patients over the age of 40 may have presbyopia and often require reading glasses; thus, their near-vision test results may be unreliable if the test is administered without reading lenses.

    • Alternate charts exist for illiterate patients or young children, but have limited studies evaluating their use in smartphones.

    • While the Snellen chart is the most widely used method of testing visual acuity in clinical practice, it is not without its pitfalls, including difficulty with standardization of the chart, as well as test-retest reliability.

    Instructions for Use:

    • Ensure proper room lighting and set phone brightness to 100%.

    • Hold the screen 4 feet (1.2 m) from the patient (approximately the end of a standard hospital bed if patient is sitting upright).

    • Test each eye independently. The patient should completely cover the opposite eye.
    • If the patient is unable to read any of the letters on the chart, the person checking vision should hold up fingers at varying distances and assess whether the patient can count them; this is recorded as counting fingers (CF) vision (e.g. CF at 4 feet, 1.2 m).

    • If the patient cannot count fingers, the person checking vision should move his/her hand across the patient’s field of vision to assess whether the patient can see this; this is recorded as hand motion (HM) vision.

    • Finally, if the patient is unable to see hand movements, a penlight should be swept across the patient’s eye to assess whether the patient can tell when the light is pointed at the eye. If they are able to identify when the light is pointed at the eye, this is recorded as light perception (LP) vision. If they are unable to identify the light, this is recorded as no light perception (NLP) vision.

    Provides a portable and quick method to assess visual acuity.

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    Next Steps
    Creator Insights


    • Visual acuity worse than 20/25 should be evaluated by a licensed eye professional to determine whether corrective lenses or other treatments may be necessary.

    • Visual acuity is not a measure of a patient’s prescription. This is a separate value that needs to be measured in an office setting.


    Any patient with sudden changes in visual acuity from baseline or new obscurations in vision requires immediate referral to an ophthalmologist for dilated fundus examination and further testing.

    Evidence Appraisal

    • There are two main charts used to test visual acuity – Snellen charts, which use a geometric scale, and logMAR charts, which use a logarithmic scale.

    • While both have been widely studied, Snellen charts are more widely used in clinical practice and logMAR charts are used more often in the context of research studies given the ease of statistical analysis.

    • Data looking at eleven different smartphone applications found that the accuracy of the applications was limited (none were within one line of the true Snellen visual acuity) (Perera 2015).

    • One study suggested that the visual acuity on smart phone apps may not be completely accurate, but the results are reproducible allowing the app to track change from baseline (Phung 2016).

    • There has been one study validating the use of smartphone charts in measuring visual acuity; however, this validation was application-specific and thus more studies need to be conducted to elucidate the true validity of these charts (Bastawrous 2015).

    Dr. Herman Snellen

    About the Creator

    Herman Snellen, MD, (d. 1908) was an ophthalmology professor at Utrecht University in the Netherlands. He was also the director of the Netherlands Hospital for Eye Patients. Dr. Snellen’s research interests included astigmatism, glaucoma, and correction of visual acuity using eyeglasses and ophthalmological surgery.

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    About the Creator
    Dr. Herman Snellen
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