A Snellen chart looks the same on both sides of the border and produces two different-looking numbers. The same patient with the same vision can leave one clinic with "6/6" on the record and another with "20/20," and a referral that copies one figure into a form designed for the other creates a record that is ambiguous the moment someone asks what it means.
That is not a clinical disagreement. It is a notation convention, and the two countries write their federal vision requirements in different orders — one metric first, one imperial first. For a Canadian optometry practice, ophthalmology office or school screening programme, understanding the convention is the difference between a chart that is stocked correctly and a referral that gets sent back.
What is a Snellen chart, and how is it printed?
A Snellen chart is a printed set of optotypes — letters, or symbols for patients who cannot read letters — arranged in lines that get smaller down the chart. Each line corresponds to a visual acuity value, and the value is a fraction: the test distance over the distance at which the letter would subtend five minutes of arc to a standard observer.
The chart is not one product. It is a family, and the right member depends on the patient.
| Chart family | Optotype | Typical test distance | Where it fits |
|---|---|---|---|
| Snellen letters | Latin letters | 6 m or 20 ft | Adults and reading children |
| Tumbling E | Rotating E | 6 m or 3 m | Patients who cannot name letters |
| Landolt C | Ring with gap | 5 m | Standardised testing, research |
| Lea symbols | Simple shapes | 3 m | Pre-literate children |
| logMAR or ETDRS | Letters, equal difficulty per line | 4 m | Clinical trials, low vision |
The distinction that matters for a clinic is between a chart used for a quick screening and a chart used to produce a comparable, repeatable measurement. A logMAR chart gives one optotype per line at each size, so the score is a number rather than a line name, and it behaves differently from a Snellen chart when acuity is very poor.
How do Canadian and US records write the same acuity?
This is the part that cross-border referrals trip over. Both countries use the Snellen fraction, but the two federal examples below show how each writes it.
In Canada, the Canadian Aviation Regulations (SOR/96-433) put the metric figure first and the imperial equivalent in brackets. Section 804.26 requires a person who assesses runway visibility to "have a distant visual acuity with or without correction of 6/12 (20/40) or better in each eye separately and binocular visual acuity of 6/9 (20/30) or better."
In the United States, 14 CFR 67.103 writes the standard in feet with no metric equivalent at all. For a first-class airman medical certificate it requires "distant visual acuity of 20/20 or better in each eye separately, with or without corrective lenses," and "near vision of 20/40 or better, Snellen equivalent, at 16 inches in each eye separately."
The same pattern holds in the commercial driving standard. 49 CFR 391.41 requires "distant visual acuity of at least 20/40 (Snellen) in each eye without corrective lenses," a "field of vision of at least 70° in the horizontal meridian in each eye," and "the ability to recognize the colors of traffic signals and devices showing standard red, green, and amber."
| Item | Canada | United States |
|---|---|---|
| Fraction written as | 6/6 (metric first) | 20/20 (imperial first) |
| Test distance | 6 metres | 20 feet |
| Near vision | Expressed in metric where specified | "20/40 or better, Snellen equivalent, at 16 inches" |
| Regulatory example | Canadian Aviation Regulations, s. 804.26 | 14 CFR 67.103; 49 CFR 391.41 |
| Record convention | Line value plus which eye, with or without correction | Line value plus which eye, with or without correction |
| Conversion | 6/6 equals 20/20; 6/12 equals 20/40; 6/9 equals 20/30 | Same ratios, different units |
Because 6 metres is very close to 20 feet, the fractions are numerically equivalent: 6/6 and 20/20 describe the same acuity, and 6/12 and 20/40 describe the same acuity. The mistake is not the maths — it is dropping the notation or mixing the two systems in one field, so that "6/40" or "20/12" appears on a record where neither means anything.
Which chart should an eye clinic keep on the shelf?
Three charts cover most of a general practice: a standard distance chart for adults, a symbol chart for children and non-readers, and a near card. A practice that sees children for school screening adds a matching symbol chart at the screening distance, because comparing a 3-metre symbol chart with a 6-metre letter chart is not a comparison at all.
A few rules keep the measurements honest:
- Test distance is part of the result. A Snellen fraction is only valid at the distance printed on the chart. Moving a 6-metre chart to 3 metres changes the value unless the chart is designed for it.
- Lighting and contrast are part of the result. A chart printed on grey stock or read in a dim corridor gives a lower score than the same patient would get in a properly lit lane.
- The chart is a shared surface. Between patients, the chart, the occluder and the chair arm are the things hands and faces touch. Cleaning them belongs in the between-patient routine alongside hand hygiene.
Which chart is right for a child who cannot read letters?
Use a symbol chart, and use the same one across visits. A tumbling E, a Landolt C and a Lea symbol chart are not interchangeable in a screening record, because a child who has learned one symbol set performs differently on another. Matching the chart to the child's age and to the programme the result is being reported into is the practical rule.
A second habit matters more than the chart choice: record which chart was used. "6/9" with no chart named tells the next clinician very little, while "6/9, Lea symbols at 3 m" is a complete observation.
What does a vision testing station need besides the chart?
The chart is the visible part of the station. The rest is the consumables that make the test reproducible and clean: examination gloves for handling occluders and for any contact with the patient's face, individually wrapped swabs for cleaning shared surfaces, exam table paper for the lane, and tape or a card holder to keep the chart at eye level.
Two consumables run out first in a screening week: gloves and exam table paper. Both are case-quantity items, and both are the reason a screening day stops early.
Do the two countries regulate the chart itself the same way?
Neither country treats a printed acuity chart as a high-risk device, and the regulatory hooks that do exist are about what touches the eye rather than what the patient reads. In Canada, the Medical Devices Regulations (SOR/98-282) classify devices that come into contact with the surface of the eye as Class II under Schedule 1 Rule 2(1), which is the hook that catches contact lenses and lens care products rather than wall charts. In the United States, 21 CFR part 886 classifies ophthalmic devices by intended use, and even a prescription spectacle lens sits at Class I and is exempt from premarket notification.
The practical consequence for a clinic is that the paperwork burden lands on the products that contact the eye, and the burden for the chart itself lands on infection control. The College of Optometrists of Ontario publishes infection control expectations for members, and the same principle applies to a chart station in any setting: shared surfaces get a defined cleaning step, and single-patient items are not reused.
Sources
- Canadian Aviation Regulations, SOR/96-433 (s. 804.26, distant visual acuity in metric notation)
- 14 CFR 67.103, Eye standards for a first-class airman medical certificate
- 49 CFR 391.41, Physical qualifications for drivers
- Medical Devices Regulations, SOR/98-282 (Schedule 1, Rule 2)
- 21 CFR part 886, Ophthalmic devices
- 21 CFR 886.5844, Prescription spectacle lens
- FDA, Medical device classification database
- FDA, Premarket notification 510(k) database
- FDA, Device registration and listing database
- American Academy of Ophthalmology, Amblyopia
- American Academy of Ophthalmology, How to take care of contact lenses
- American Optometric Association, Color vision deficiency
- College of Optometrists of Ontario, Infection control
- College of Optometrists of Ontario, Standards of practice
- Canadian Association of Optometrists, Eye health library
- WHO, Blindness and visual impairment fact sheet
- WHO, World report on vision
Ordering for a clinic, optical practice or care home? Wholesale and multi-site ordering covers account setup and case pricing, and the B2B wholesale collection lists the lines stocked for institutional buyers. Sites that also run an in-house sterilizer can start with the biological indicator 5-pack trial.
Related product: Related product: USB heated eye mask with three temperature settings (graphene heating element)
Related reading
- Eye Clinic Consumables: Gloves, Swabs and Tape for Optometry
- Heated Eye Masks for Digital Eye Strain: 3-Temperature Options
- Audiology Clinics: Between-Patient Hygiene for Hearing Test Rooms
Frequently Asked Questions
Is 6/6 the same as 20/20?
Yes. Six metres and twenty feet are close enough that the two fractions describe the same acuity, and 6/12 and 20/40 are also equivalent. The numbers differ because one system is metric and the other is imperial, not because the measurement differs.
Why do Canadian records use 6/6 and US records use 20/20?
It is a notation convention, and the federal instruments follow it. The Canadian Aviation Regulations write visual acuity as "6/12 (20/40)" with the metric value first, while 14 CFR 67.103 writes US aviation eye standards in feet with no metric equivalent.
What is the difference between a Snellen chart and a logMAR chart?
A Snellen chart uses a fixed number of optotypes per line with decreasing size, and the score is a line value. A logMAR chart uses an equal number of optotypes on every line and scores by letter, which gives a repeatable number at low acuity levels.
Can a clinic use a 3-metre chart at 6 metres?
No, not without changing the result. A Snellen fraction is only valid at the distance the chart was designed for, so a chart printed for 3 metres read at 6 metres produces a fraction that does not mean what it says.
Which chart should be used with a child who cannot name letters?
Use a symbol chart such as a tumbling E, a Landolt C or a Lea symbols chart, and record which one was used. Different symbol sets are not interchangeable in a screening record, because children perform differently on a symbol set they have not seen before.
Should the chart be cleaned between patients?
The chart and occluder are shared surfaces that patients touch and stand close to, so cleaning them belongs in the between-patient routine alongside hand hygiene. Single-patient items such as an occluder card should not be reused.
Does a printed eye chart need a medical device licence in Canada?
A printed chart is not the kind of product the Medical Devices Regulations catch. The Class II hook under Schedule 1 Rule 2(1) turns on contact with the surface of the eye, which is why it applies to contact lenses and lens care products rather than to a wall chart.
CliniEco Medical supplies examination gloves, exam table paper, cotton swabs, gauze and the wider clinic consumable lines described in this article. Health Canada MDEL #35334. This article is written for clinic, optical and procurement professionals and is not clinical guidance; chart selection, testing distance and interpretation belong to the treating eye care professional.
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