Quick facts
- A 6/6 (20/20) letter subtends five minutes of arc, which is about 8.7 mm at a six-metre lane and about 8.9 mm at twenty feet; that number is the one a ruler check verifies.
- The United States files a visual acuity chart under 21 CFR 886.1150 as a Class I device, exempt from premarket notification subject to 886.9, product code HOX.
- Canada has no chart-specific classification entry to read; the operative provincial rule is infection control, and Ontario's provincial guidance classifies eye equipment that contacts mucous membranes as critical.
- Laminate haze and print fade reduce contrast before the print looks damaged, so a chart can pass a visual glance and still fail a measured check.
- Three failures account for most bad charts: solvent cleaning, sunlight or UV yellowing, and a reprint whose line spacing does not match the scale printed on it.
- Section text, classes and product codes were read on 2026-10-21 from the Cornell LII copy of the CFR and the FDA classification database.
A printed chart is the least expensive instrument in an eye lane and the one nobody recalibrates. It stops being accurate in three ways that are measurable: the letters are no longer the size the scale claims, the contrast between letter and background has dropped, or the surface scatters light because the laminate has hazed. The first is caught with a rigid rule, the second and third with a lamp and a printed reference card. This article sets out the arithmetic behind letter size, the checks that catch each failure, a replacement rule that survives an inspection, and the two regulatory routes. The printed Snellen and Amsler chart set sold for Canadian lanes is used as the working example throughout, and the regulatory reading comes from the current CFR text and the provincial guidance cited at the end.
What makes a printed chart drift out of specification?
A chart is a geometric instrument. Every line on it is defined by an angle subtended at a stated distance, and the printing carries that angle as a physical height. A 6/6 letter subtends five minutes of arc: at six metres that is 6000 mm multiplied by the tangent of five minutes, or 8.73 mm. At twenty feet the same angle gives 8.87 mm. Print the same chart at a slightly different reduction, or reproduce it on a copier that stretches one axis, and every line is wrong by the same proportion while still looking like a chart.
The second drift is contrast. Chart print is black on white with a defined stroke width; laminate adds a second optical surface. Ultraviolet light, repeated alcohol or quaternary wiping and abrasion from a paper occluder all reduce the modulation between letter and background. The eye tolerates that loss well, which is why the change is missed: a patient reads the line, so nobody questions the chart.
The third drift is mechanical. A chart that lives on a wall is knocked, re-hung or moved to a second lane, and both the mounting height and the viewing distance change; the scale printed on the chart is true only at the distance it was designed for. The lane calibration article covers distance and light; this article covers the object itself.
How do you check letter size without a calibration service?
You measure it. The check takes about two minutes per chart and needs one rigid steel rule, one soft tape for the lane, and the specification sheet that came with the chart. Record the measured height of the 6/6 line and of one line in the middle of the chart; if the two disagree with each other by more than the rounding of the print, the chart is a reprint that does not hold its scale.
Then check the two conditions that make the angle true. Lane distance runs from the chart surface to the patient's corneal plane, not to the back of the chair, and the 6/6 line sits at eye level within the tolerance your protocol states. A chart hung eight centimetres high changes the angle for the upper lines, which is why the failure appears as an inconsistency between lines rather than as a uniform error.
The optotype itself is standardised. ISO 8596:2017 sets out the standard optotype and the way it is presented, which is what makes a result transferable between two clinics. A chart that uses a house symbol, a stretched letter, or a stroke width that does not match the optotype it claims to follow cannot be compared with a chart that does.
What does routine cleaning do to a laminated chart?
Laminated print survives hand hygiene but not solvent. Alcohol and quaternary ammonium wipes attack the plasticiser in the film, and the surface goes from clear to a fine white haze that scatters light. The effect is cumulative: a chart wiped twice a day for a year has had roughly five hundred cleaning events, and the contrast loss at that point is visible under a desk lamp even though the print is intact.
The workable routine separates two jobs. Dust and fingerprints come off with a damp cloth and a neutral detergent, dried lint-free. Disinfection between patients follows the product label and the provincial guidance rather than a general wipe. Ontario's provincial guidance for cleaning, disinfection and sterilization places eye equipment that contacts mucous membranes in the critical category, and equipment that only contacts intact skin in the non-critical one, which is why the chart and the occluder are handled as separate items with separate rules in the cleaning-frequency article.
Cotton roll and gauze, not paper towel, is what the chart surface should see: paper towel is abrasive enough to leave a visible polish mark on laminate after a few hundred passes.
Gloves matter too: a wipe made with a glove that has just handled a soiled occluder moves contamination across the chart, so the glove change belongs to the cleaning step.
Which failures are checkable, and what does each one look like?
The table below converts the three drifts into checks with a pass condition, a fail trigger and a record field. It is written so that a practice manager who is not an optician can run it, and so that an inspector can read the result six months later.
| Check | How | Fail trigger | Record |
|---|---|---|---|
| 6/6 letter height | Rigid rule, flat against the chart | Outside the height on the specification sheet | Measured mm, date, initials |
| Line-to-line consistency | Compare a middle line with 6/6 | Ratio drift larger than print rounding | Pass or fail |
| Contrast | Desk lamp at 45 degrees, look across the surface | Visible haze or grey background | Haze yes or no |
| Lane distance | Soft tape, chart surface to corneal plane | Distance differs from the scale distance | Measured cm |
| Mounting | Spirit level, 6/6 line at eye level | Tilt or height outside protocol | Pass or fail |
| Surface damage | Fingertip across the 6/6 line | Scratch, crease or lifted laminate | Photo reference |
When should a chart be replaced instead of cleaned?
Replace when a measured check fails, not when the chart looks tired. The practical triggers are a 6/6 measurement outside the specification sheet, a middle line that no longer agrees with the 6/6 line, haze you can see across the surface under a lamp, or a crease that crosses the 6/6 line. A chart that fails the size check cannot be rescued by cleaning, and a chart that fails the contrast check cannot be rescued by anything.
Two failures deserve early replacement because they mislead quietly: a scanned reprint can carry a little stretch and stay legible, and a chart at the wrong height varies with posture.
The record is the part that survives staff turnover. One line per check - identifier, measured value, initials, date - shows a defensible process, and where two lanes hold the same model the identifier has to separate them.
What changes on the American side of the border?
In the United States a visual acuity chart is a device with its own regulation. 21 CFR 886.1150 identifies it as a chart such as a Snellen chart with block letters or other symbols in graduated sizes, intended to test visual acuity, and classifies it as Class I, exempt from premarket notification subject to §886.9, with product code HOX in the FDA classification database. That is a lighter route than most ophthalmic instruments take: the refractometer and the perimeter are also Class I, but the ophthalmic camera is Class II with special controls.
In Canada the same chart is a medical device under the Medical Devices Regulations, and the manufacturer holds the licence that puts it on the market; the clinic's own obligation sits in infection control and in the record it keeps, not in a chart-specific class. That asymmetry is the practical difference for a buyer: an American purchaser reads a class and a product code, and a Canadian purchaser reads the label, the instructions and the provincial cleaning rule.
| Item | Canada | United States |
|---|---|---|
| Instrument of record | Medical Devices Regulations, SOR/98-282; provincial infection-control guidance | 21 CFR 886.1150; 21 CFR 886.9 exemption conditions |
| Class | No chart-specific class to read | Class I (general controls) |
| Premarket route | Device licence held by the manufacturer | Exempt from premarket notification, subject to 886.9 |
| Product code | n/a | HOX |
| Clinic obligation | Clean and disinfect per provincial rule; keep the record | Follow the labelling; keep the record your accreditor asks for |
| Standard behind the optotype | ISO 8596:2017 | ISO 8596:2017 as a recognised consensus standard |
How should the check be written into the practice record?
Write one procedure with four lines: what is measured, with what, how often, and who signs. The interval is a practice decision, but the trigger for an unscheduled check is not: any chart that moves lane, any chart that comes back from cleaning with a changed surface, and any chart involved in an examination that gave an unexpected result.
Keep the specification sheet with the record. A measurement without the stated height means nothing, and the sheet is the only document that carries that height for the chart in the room.
Everything a lane needs to keep the check honest sits in one catalogue section: the printed chart, the occluder, the pinhole and the rigid rules used for measurement. Order through the wholesale programme if you are fitting out more than one lane and want case pricing on an account, or become a distributor if you resell to clinics. The printed chart set is the reference item this article measured, and the hospital-grade consumables used beside it are listed under the eye care department page.
Sources
- 21 CFR 886.1150 - Visual acuity chart (Cornell LII, read 2026-10-21)
- 21 CFR 886.9 - Limitations of exemptions (Cornell LII, read 2026-10-21)
- ISO 8596:2017 - Ophthalmic optics, visual acuity testing, standard optotype and its presentation
- Provincial guidance for cleaning, disinfection and sterilization in health care settings (Public Health Ontario, PDF)
- CDC guideline for disinfection and sterilization in healthcare facilities - summary of recommendations
- Medical Devices Regulations, SOR/98-282 (Justice Laws)
- Health Canada - medical devices portal (federal device programme)
- American Academy of Ophthalmology - eye exam overview
- American Academy of Ophthalmology - infection control in the eye clinic
- FDA product classification database - search by regulation number
- openFDA device classification API (class and product code lookup)
- CCOHS - OSH Answers index (Canadian occupational health reference)
If the check sheet is the part of this article you want on the wall, the printable compliance log pack holds a blank inspection form you can fill in by hand and file beside the chart.
For a lane with several rooms, the sterilization log generator on the same page builds a dated sheet per room, which is the quickest way to give each chart its own identifier in the record.
If your province asks for something the form does not cover, ask a compliance specialist: the reply is a written answer specific to your equipment and province, with the regulation or standard it rests on cited.
Related reading
- Eye chart in Canada and the US: which optotype standard applies
- Calibrating a vision test lane: distance, light and chart size
- Snellen chart and eye test charts: which scale is used
CliniEco Medical is a Canadian medical device importer with MDEL #35334. Nothing in this article is clinical advice for a specific patient, and no product in it is presented as a substitute for the instructions supplied with a device.
Frequently Asked Questions
How often should a printed eye chart be replaced?
There is no fixed interval in any standard. Replace on a measured failure: a 6/6 letter height outside the specification sheet, a middle line that no longer agrees with the 6/6 line, visible laminate haze, or a crease across the 6/6 line. Practices that wipe charts daily usually reach a contrast failure before a size failure.
What is the correct letter height for 6/6 at six metres?
About 8.7 mm. The 6/6 letter subtends five minutes of arc, and 6000 mm multiplied by the tangent of five minutes gives 8.73 mm. At twenty feet the same angle gives 8.87 mm, which is why an American chart and a metric chart are not interchangeable at the wrong distance.
Can a laminated chart be disinfected with alcohol wipes?
Alcohol attacks the film and produces a haze that reduces contrast long before the print fails. Use a neutral detergent for soil and the disinfectant named on its label for the shared-surface job, then dry with a lint-free cloth. Check the surface under a lamp at 45 degrees after the first month of any new routine.
Is a printed chart a regulated device in Canada?
It is a medical device under the Medical Devices Regulations, and the manufacturer holds the licence that allows it to be sold. Canada does not publish a chart-specific classification entry the way the United States does at 21 CFR 886.1150, so the clinic's own obligations are the labelling, the provincial cleaning rule and its record.
Do I need a calibration service for a chart?
No. A rigid rule, a soft tape and the specification sheet cover the size and distance checks. A calibration service earns its fee on instruments with an output - a tonometer, a lensmeter, a perimeter - not on printed paper.
What is the difference between a Class I chart in the US and the Canadian route?
The American route names the device and exempts it from premarket notification subject to 21 CFR 886.9, with product code HOX. The Canadian route puts the licence on the manufacturer and leaves the clinic with labelling, cleaning and record duties. The clinical performance of the chart is the same on both sides.
Does the occluder need the same check as the chart?
It needs a different one. The occluder is a contact surface, so its check is cleanliness and integrity, and a cracked or opaque paddle changes the examination without changing any number on the chart. Keep the two checks on one form so neither is skipped.
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