Snellen Chart

Interactive distance visual-acuity chart, scored letter by letter as the patient reads.

For educational purposes only. A teaching and screening aid, not a medical device; it does not replace a standard clinical examination.

How to use

  • Have the patient view the screen at the distance a screen is normally used from: 60 cm (about arm’s length) for a computer, 40 cm (a normal reading distance) for a phone or tablet. Every letter in Test patient is true size at that distance; it can be changed under Chart settings (40, 50, 60 or 70 cm).
  • Choose SC (without glasses), CC (with their glasses) or PH (pinhole) at the top of the chart; each eye has a separate score for each. At 40–60 cm a presbyopic patient needs their reading or near correction for CC. Occlude the other eye fully (an occluder or the palm, not fingers), and test the right eye first, then the left, then both if needed.
  • Press Test patient: the whole chart, true size, centred on a full screen, with the examiner’s column on the right (eye and correction, acuity so far, distance, Exit). Tap an acuity in the chart’s right margin to highlight the line to read; start at a line the patient reads easily and ask them to read each letter aloud from left to right. Encourage guessing.
  • Score the highlighted line with the numbered buttons in the examiner column — one per letter, tap once for correct, again for incorrect — or ✓✓ for a whole line read correctly. The small lines are too small to tap on the chart itself, and no scoring mark is shown on the chart, where the patient would see it. Move down until most of a line is missed.
  • If the patient cannot read the top line, record counting fingers, hand motion, light perception or no light perception with CF, HM, LP or NLP under the chart (CLD, MM, PL, PPL in French). It replaces the chart result for that eye; marking a letter clears it.
  • Switch eye with OD / OS under the chart, or with the eye selector above it (which also offers OU). Each eye has its own letters and marks, and keeps its result until reset.

How it is scored

  • Letter by letter (logMAR chart): every letter is worth 0.02 logMAR. With the top line at 1.0 (20/200), logMAR = 1.1 − 0.02 × letters read, and the ETDRS letter score = 85 − 50 × logMAR. Lines above the first one scored, and any line skipped above a line that was passed, are credited as read; unanswered letters on a scored line count as not read.
  • Line notation: the result line is the smallest line on which more than half of the letters were read. It is written with the letters missed on that line and the letters read below it — for example 20/25 −1 +2.
  • The Snellen layout has uneven steps between lines, so a letter is not worth a fixed amount there and it is scored by line only.
  • Snellen, metric and decimal values of a letter-by-letter score are conversions of the logMAR value (20 × 10^logMAR), rounded and marked as approximate.

Limitations

  • Letters are drawn at true size using an estimate of this screen’s pixel size, from the type of device (phone, tablet, Mac, other). That is typically within about 15 %, a fraction of a line, but it is not a measured size: treat the result as a screening acuity, keep the browser at 100 % zoom, and confirm with a standard chart.
  • This is an acuity at the distance the screen is viewed from (40–60 cm), not a 6 m distance acuity: it needs the patient’s near or reading correction if presbyopic, and can differ from a distance acuity (for example in myopia, or with an accommodative problem). Confirm a distance acuity on a standard 6 m or 3 m chart.
  • At these distances the smallest lines are under a millimetre tall. A line whose strokes would be thinner than one screen pixel cannot be drawn faithfully, so it is not shown (“Too small for this screen at this distance”) — for example 20/10 on a laptop at 60 cm, and 20/15 and 20/10 on an iPad at 40 cm. Drawn lines measured within about 2 % of true size (within one screen pixel on the smallest).
  • A patient who moves closer or further changes the letters’ angular size: 10 cm closer at 60 cm makes every letter about 20 % larger. Keep the distance steady.
  • Letters are drawn on the published 5 × 5 construction — the classic Snellen letters (C D E F L O P T Z) on the Snellen chart, Sloan letters on the logMAR chart — not from a licensed font, so equal legibility of every letter is approximate.
  • Results are kept only in this browser tab and are lost on reload. No name or identifier is collected.

References

  • Bailey IL, Lovie JE. New design principles for visual acuity letter charts. Am J Optom Physiol Opt 1976;53:740-745.
  • Ferris FL 3rd, Kassoff A, Bresnick GH, Bailey I. New visual acuity charts for clinical research. Am J Ophthalmol 1982;94:91-96.
  • Ferris FL 3rd, Bailey I. Standardizing the measurement of visual acuity for clinical research studies: guidelines from the Eye Care Technology Forum. Ophthalmology 1996;103:181-182.
  • Sloan LL. New test charts for the measurement of visual acuity at far and near distances. Am J Ophthalmol 1959;48:807-813.
  • International Council of Ophthalmology. Visual Acuity Measurement Standard. 1984.
  • Kaiser PK. Prospective evaluation of visual acuity assessment: a comparison of Snellen versus ETDRS charts in clinical practice. Trans Am Ophthalmol Soc 2009;107:311-324.
  • Bach M. The Freiburg Visual Acuity Test — automatic measurement of visual acuity. Optom Vis Sci 1996;73:49-53.

For educational purposes only. The Clinical Exam tools are part of an educational platform. They are screening and teaching aids, not certified medical devices: a result depends on the screen (letter sizes are estimated for the type of device), the lighting and the testing distance. They are not a diagnosis, and any abnormal or doubtful result should be confirmed with a standard clinical test.

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