Visual acuity test simulator

A visual acuity test measures the smallest letters each eye can read at a standard distance and records it as a fraction such as 20/40.

Parts you use

• Snellen chart: rows of letters that get smaller down the chart. Each row is labeled with the distance at which standard vision reads it.
• Occluder: a solid paddle that covers the eye not being tested.
• Pinhole occluder: a paddle with one or more small holes. It cuts blur from a glasses problem (refractive error).

Steps

1. Set up the lane. Seat the patient at the chart distance the chart is made for, usually 20 feet (6 meters), or a mirror or projector system calibrated for a shorter room. Light the chart well.
2. Cover one eye. Cover the eye you are not testing with an occluder (a solid paddle with no holes), not the patient's fingers. By convention, often the right eye (OD) is tested first; some clinics start with the worse eye.
3. Read from the top. Ask the patient to read each line left to right, starting with large letters. No squinting: squinting works like a pinhole and gives a better score than the patient's normal vision.
4. Find the smallest line. Keep going until the patient misses most letters on a line. In this simulator, the last line with more than half read right is the acuity line (this simulator's own convention; follow your clinic's rule).
5. Record partial lines. Write 20 over that line's number. Subtract letters missed on it and add letters read on the next line, for example 20/40-2 or 20/50+1.
6. Test both eyes together. If your clinic records it, test both eyes together (OU, both eyes open) after each eye alone.
7. Pinhole if reduced. If vision is reduced, repeat with a pinhole occluder (a disk with small holes). Record it as PH, for example PH 20/25.
8. Test the other eye. Switch the occluder and repeat for the left eye (OS). Note sc (without correction) or cc (with correction).

Recording examples

• Read all of 20/40, nothing on 20/30: 20/40.
• Read 20/40 with two missed: 20/40-2.
• Read all of 20/50 and one letter on 20/40: 20/50+1.
• Read all of 20/40, missed two on 20/30, one right on 20/25: 20/30-2+1.

This simulator's own convention counts a line as read when more than half its letters are right. Clinics differ on this rule, so follow your own clinic's policy.

FAQ

What does 20/40 vision mean?

At 20 feet the patient reads letters that a person with standard vision can read from 40 feet. The top number is the test distance; the bottom number is the line read.

How do you write a partial line, like 20/40-2?

Write the smallest line the patient read with most letters right, then subtract the letters missed on that line. 20/40-2 means the 20/40 line was read with two letters missed.

What does 20/50+1 mean?

The patient read the whole 20/50 line and got one letter right on the next smaller line, 20/40.

What does a pinhole test show?

It helps sort out why vision is blurred. Blur from a refractive error (a glasses problem) is the kind a pinhole can mostly clear, because it blocks the angled light rays. A refractive cause is likely when the line read through the pinhole beats the line read without it. If the score stays the same, something else, such as a cloudy lens, may be involved, and the doctor needs to know.

What if the patient cannot read the big E?

Move the patient closer and change the top number to the new distance, for example 10/200 at 10 feet. If that fails, test counting fingers (CF), then hand motion (HM), then light perception (LP). No light perception is recorded as NLP.

What do OD, OS, sc and cc mean?

OD is the right eye and OS is the left eye. sc means without correction (no glasses or contacts) and cc means with correction.

Can the patient squint or use a hand to cover the eye?

No. The goal is to record how the patient normally sees, and squinting would spoil that: the narrowed lid gap acts as a crude pinhole and can make the score look better than it really is. A hand leaves gaps to peek through; use a solid occluder.

What about patients who cannot read letters?

Use a chart they can answer, such as a tumbling E, Landolt C, numbers or pictures for young children.

Studying for the COA exam? See the COA certification guide. More clinic simulators: applanation tonometer, swinging flashlight test, lensometer and keratometer.