A basic vision screening requires surprisingly little equipment: an eye chart, something to cover one eye, a way to measure distance, and a well-lit room. Whether you’re screening children at a school, employees at a workplace, or setting up a simple check at home, the process follows the same core steps. Here’s how to do it right.
What You Need Before You Start
Gather these items before your first screening:
- A distance eye chart. The Snellen letter chart is the most common. For children under five or anyone who can’t read letters, use Lea Symbols (simple pictures designed to detect vision problems as accurately as letter charts) or a tumbling E chart, where the person points in the direction the letter E faces.
- An occluder. This is anything that fully blocks one eye without pressing on it. A commercial paddle occluder works best, but a paper cup or opaque card will do. Avoid letting people use their hand, since fingers spread apart and allow peeking.
- A measuring tape. You need to set an exact testing distance. For distance acuity, that’s 20 feet (6 meters). For near vision, it’s 16 inches (40 cm).
- Good, even lighting. The chart should be uniformly lit with no glare or shadows falling across it. A brightly lit room with overhead fluorescent or LED lighting typically works. Avoid placing the chart directly opposite a window, which creates glare. Clinical settings aim for at least 10 to 30 foot-candles of illumination on the chart surface.
If you don’t have a full 20-foot room, you can mount the chart on a wall and place a mirror at 10 feet. The person reads the chart’s reflection, which optically simulates the full 20-foot distance. Many clinics use this trick in small exam rooms.
How to Test Distance Visual Acuity
Distance acuity is the centerpiece of any vision screening. It measures how clearly a person sees objects far away and produces the familiar “20/20” result.
Position the person exactly 20 feet from the chart, seated or standing. Have them wear their glasses or contact lenses if they normally use them for distance vision. Cover one eye with the occluder. Start with the eye the person considers their weaker eye, so you test it before fatigue sets in. Ask them to read the letters starting from the top row and working downward.
The smallest line where they correctly identify more than half the letters is their visual acuity for that eye. Record it as the fraction printed next to that line. If they read the entire 20/40 line and also get one letter on the 20/30 line, you’d record “20/40+1.” If they miss two letters on the 20/40 line but can’t go further, record “20/40-2.” This notation gives a more precise picture than rounding.
Switch the occluder to the other eye and repeat. Then, if your protocol calls for it, test both eyes together without the occluder.
Testing Young Children
Children under about four years old often can’t reliably read letters, and traditional picture charts like Allen figures tend to miss amblyopia (lazy eye), the most important condition screening is meant to catch at this age. Lea Symbols, a set of four simple shapes (circle, square, house, apple) designed to detect vision differences as sensitively as letter charts, are the preferred alternative.
Before testing, let the child hold a practice card with the symbols and name or point to each one. This confirms they understand the task. Then test at 10 feet instead of 20 if needed, adjusting the recorded acuity accordingly.
Expect some difficulty with very young children. Studies of Lea Symbols in pediatric settings found that only about 31% of children under 48 months could complete the single-symbol distance test, compared to 93% of children over age four. If a child can’t cooperate with a chart-based test, instrument-based photoscreeners (handheld devices that photograph the eye’s light reflex) are an effective alternative for ages one through three. For children older than four, letter-based charts in a logMAR progression work well, with Lea Symbols reserved for those who can’t manage letters.
Passing and Failing Thresholds
The acuity level that triggers a referral depends on age. The American Association for Pediatric Ophthalmology and Strabismus recommends these thresholds:
- Ages 3 to 3 years 11 months: The child should correctly identify most symbols on the 20/50 line. Failing to do so warrants referral.
- Ages 4 to 4 years 11 months: The threshold tightens to the 20/40 line.
- Age 5 and older: Children should read at least the 20/32 line with each eye. Refer any child who cannot.
For adults in a workplace or community screening, 20/40 is the most commonly used pass/fail cutoff, since it’s the legal driving standard in most U.S. states. A two-line difference between the two eyes (for example, 20/20 in one eye and 20/40 in the other) also warrants referral, even if both eyes technically pass individually.
How to Screen Near Vision
Near vision screening checks the ability to see fine detail up close, which matters for reading, desk work, and screen use. Use a handheld near vision card, such as a Rosenbaum or Sloan card. These look like miniature eye charts with rows of shrinking letters or numbers.
Have the person hold the card exactly 16 inches from their eyes. Many cards come with an attached cord cut to that length so neither you nor the person has to guess the distance. Test each eye separately with the occluder, just as you did for distance. The person reads the smallest line they can, and you record the result. For preliterate children or non-English speakers, HOTV cards (using only the letters H, O, T, and V, which the child matches to a key card) work at the same 16-inch distance.
Checking for Depth Perception
Stereoacuity tests measure whether both eyes work together to perceive depth. The most common screening tool is the Stereo Fly Test (also called the Titmus test), a booklet of images viewed through polarized glasses.
The person puts on the polarized glasses and looks at a large image of a fly. If their depth perception is intact, the fly’s wings appear to rise off the page, and most people instinctively pull back or try to pinch the wings. The booklet then presents a series of circles and animal shapes at increasingly subtle depth levels. Each set corresponds to a value measured in seconds of arc, a unit of angular difference. Lower numbers mean finer depth perception. Normal stereoacuity is generally 40 seconds of arc or better.
A person who can’t see the fly popping out at all likely has very poor binocular vision and should be referred. This test is especially useful in children, since poor stereoacuity can signal strabismus (eye misalignment) or amblyopia that might not show up on a chart test alone.
Color Vision Screening
Color vision is not part of every routine screening, but it matters for certain occupations (pilots, electricians, rail workers) and is sometimes included in school screenings. The most widely used tool is an Ishihara plate book, a set of dotted circles where numbers or paths are visible only to people with normal color vision.
Hold the plates about 30 inches from the person under daylight-balanced lighting (not yellow incandescent light, which distorts colors). The person identifies the number or traces the path on each plate. Missing several plates suggests red-green color deficiency, the most common type. Smartphone-based color vision apps are beginning to show promise. One validation study found that a phone-based version of the Farnsworth D-15 color arrangement test had 100% sensitivity and 99.5% specificity compared to the standard physical test. However, results can vary by phone model and screen calibration, so physical tests remain the standard for formal screening.
Common Mistakes That Skew Results
Small errors in setup can produce inaccurate results. The most frequent problems are easy to avoid once you know them.
Incorrect distance is the biggest one. Even a foot or two off from the marked 20-foot line changes the acuity score. Tape a line on the floor and have the person’s chair placed so their eyes, not their toes, sit at the line. For near vision, let the cord do the measuring rather than eyeballing 16 inches.
Peeking around the occluder is especially common with children. Watch for head tilting or the occluder drifting away from the face. If you suspect peeking, the score is unreliable. Some screeners use adhesive eye patches for young children to eliminate the problem entirely.
Memorization can also inflate results. If you’re screening a line of students who can overhear each other, use a chart with multiple versions or randomized letter sequences. Similarly, avoid coaching. If someone hesitates on a letter, don’t prompt them. Wait a few seconds, then move on and record the line above as their result.
Finally, keep in mind that a screening is not a diagnosis. It identifies people who need a full eye examination, not what’s wrong with their eyes. Anyone who fails should be referred to an optometrist or ophthalmologist for comprehensive testing.