Visual Acuity Testing in Canada vs the US: Which Notation and Device Route Applies?

Quick facts

• A visual acuity test measures the smallest optotype a person can correctly resolve at a stated distance. The distance is part of the result, not a detail of the test.
• Notation: a Canadian chart is usually marked in metres (6/6) and a chart printed for the United States is usually marked in feet (20/20). The two lines are the same threshold.
• United States: 21 CFR 886.1150 names a visual acuity chart as Class I (general controls) and exempts it from premarket notification subject to the limitations in 21 CFR 886.9. The same section carries the product code for a Digital Visual Acuity Test.
• Canada: the Medical Devices Regulations, SOR/98-282 contain no named acuity entry. A printed chart is a non-invasive device under Rule 7(1) of Schedule 1; the class of a self-illuminating unit turns on whether the unit is an active device.
• Optotype standard: ISO 8596:2017 specifies Landolt ring optotypes and a method for measuring distance acuity under photopic conditions for certification or licensing, and states that it is not intended as a standard for clinical measurement.
• The three things a buyer usually conflates are separate: the notation (6/6 vs 20/20), the test distance (6 m vs 20 ft) and the device route (named section vs classification rule).

A visual acuity test is the first number in almost every eye record, and it is the one most often copied between systems without its distance. The result only means something if the test distance and the notation are known, and the two countries express the same threshold in different units. This article sets out what the test measures, how the notation converts, how each country classifies the chart or screen used to record it, and what an eye care practice should keep on file.

What is a visual acuity test actually measuring?

Visual acuity is a threshold measurement. The examiner presents optotypes of decreasing size at a fixed distance and records the smallest line the patient reads correctly. Two quantities are recorded, not one: the distance at which the test was performed, and the size of the optotype resolved at that distance.

That is why the notation is written as a fraction. In metric notation the numerator is the test distance in metres and the denominator is the distance at which a person with a standard reference acuity would resolve the same optotype. A result written as 6/6 means the patient resolved, at 6 metres, an optotype that the reference eye resolves at 6 metres. A result written as 6/12 means the patient needs to be at 6 metres to resolve what the reference eye resolves at 12 metres.

The American notation works identically with feet. A result written as 20/20 means the patient resolved at 20 feet what the reference eye resolves at 20 feet.

Neither notation is a unit of size. Both are ratios, and the ratio is dimensionless. That is the fact most often lost when a chart printed in one convention is read by a practitioner trained in the other.

How does visual acuity notation differ between Canada and the United States?

The threshold is identical; the printed number is not. The conversion is exact for the standard lines because 20 feet is 6.096 metres and the clinical convention treats 6 metres and 20 feet as the same test distance.

Metric notation (6 m)	Imperial notation (20 ft)	logMAR	What the line represents
6/6	20/20	0.0	Reference threshold
6/7.5	20/25	0.10	One line below reference
6/9	20/30	0.18	Two lines below reference
6/12	20/40	0.30	Common licensing cut-off for driving
6/18	20/60	0.48	Reduced acuity
6/24	20/80	0.60	Reduced acuity
6/60	20/200	1.00	Legal blindness threshold in many jurisdictions

The table is the whole conversion. A chart marked 6/9 and a chart marked 20/30 are asking the patient to resolve the same angular size, at a test distance the two conventions regard as equivalent.

There is a second difference that is not about units. A metric chart is normally read at 6 metres and an imperial chart at 20 feet, and 20 feet is shorter. In a room that is tight, the shorter distance is the practical reason an American chart is chosen, and the notation follows the wall rather than the other way round. Where a mirror is used to halve the lane length, the stated distance is the optical path length, not the room length.

How do Canada and the United States classify the chart or screen used for the test?

The United States writes a named entry. Section 886.1150(a) describes a visual acuity chart as a device that is a chart, such as a Snellen chart with block letters or other symbols in graduated sizes, intended to test visual acuity. Section 886.1150(b) places it in Class I (general controls), exempts it from premarket notification subject to the limitations in § 886.9, and exempts it from the quality system regulation in part 820 with the exception of the record-keeping requirements in § 820.180 and the complaint file requirements in § 820.198.

Canada reaches the class by rule. Two definitions from the Regulations do the work. An invasive device is a medical device that is intended to come into contact with the surface of the eye or penetrate the body, either through a body orifice or through the body surface. An active device is a medical device that depends for its operation on a source of energy other than energy generated by the human body or gravity, and a device that transmits or withdraws energy or a substance without substantially altering it is not an active device.

Item	Canada	United States
Named entry for the chart	None in Schedule 1	21 CFR 886.1150
Printed wall chart	Class I by Rule 7(1) (other non-invasive devices)	Class I, stated in the section
Self-illuminating or screen-based test	Classified by rule; the device is active, so the rule that captures it depends on what it does with energy	Covered by the same section under the Digital Visual Acuity Test product code
Premarket notification	No exemption list, because there is no named entry	Exempt subject to § 886.9
Marketing status in Canada	Class determines the licensing route	Class plus exemption determine the route
Where the class can move	Rules 7(2)(a) and 7(2)(b) can lift a non-invasive device to Class II	Not within the section; a different intended use points elsewhere

The interesting entry in that table is the third row. A printed chart is plainly non-invasive and lands in Class I under Rule 7(1). A powered unit that displays optotypes is an active device, because it depends on a source of energy and alters what the patient sees rather than merely transmitting energy. Once the device is active, the Canadian class follows from what the device does: an active device that supplies energy for the purpose of imaging or monitoring physiological processes is Class II under Rule 10(1), and any other active device is Class I under Rule 12.

That split does not exist in the American code, because the same section covers both the printed chart and the digital test. The American buyer therefore looks up one number; the Canadian buyer has to ask what the unit does before the rule can be applied.

Which standard governs the optotype itself?

The optotype is specified separately from the device classification, and it is worth separating the two because a licensing test and a clinic test are governed differently.

ISO 8596:2017 specifies a range of Landolt ring optotypes and a method for measuring distance visual acuity under photopic conditions for the purposes of certification or licensing. The scope then says plainly that the document is neither intended as a standard for clinical measurements nor for the certification of blindness or partial sight, and that other optotypes used for clinical investigations are described in an annex for information.

That sentence is the one a practice should hold on to. A chart bought for a licensing test and a chart bought for everyday clinical work are not required to use the same optotype set, and a supplier who quotes a standard number for both has not read the scope.

The recognised clinical optotypes are not all equivalent. Snellen block letters are familiar and fast, but they carry letter-recognition effects: some letters are harder to resolve than others at the same angular size, so a line is not a uniform difficulty. The tumbling E and the Landolt ring remove the letter-recognition variable because the task is to identify an orientation rather than a shape, which is why they are used where the patient cannot read a Latin alphabet. Picture optotypes, including the LH and Lea symbol sets, are used with young children and are calibrated against the same angular sizes.

The practical consequence for a lane that holds more than one chart card is that the result and the chart must be recorded together. A 6/9 on a letter chart and a 6/9 on a Landolt ring chart are not interchangeable values even though the notation is identical, because the task differs.

What should an eye care practice record for each acuity test?

Six items, and most of them come from the chart's own paperwork rather than from the practice.

• The chart identifier. Manufacturer, model and the optotype set, because the optotype set is part of the measurement.
• The test distance used. Recorded in the notation itself, and worth confirming against the lane marking rather than the room.
• The notation convention. Metric or imperial, stated once so that a result copied into another system is not silently reinterpreted.
• The lighting condition. Acuity is specified under photopic conditions, so the chart illumination is part of the test condition, not a room preference.
• The device status. In Canada, the class and licensing position for a powered unit; in the United States, whether the form sits inside the § 886.9 exemption or required a premarket notification.
• The cleaning instruction. A chart touched by a patient's hand, or a unit touched between patients, needs a stated cleaning method from the maker.

Which consumables does an eye lane restock alongside its charts?

An examination lane restocks barriers, chart cards, paper and cleaning supplies on the same cycle as its instruments. For practices buying at case level, the wholesale ordering page sets out account and case terms, and the B2B wholesale collection lists the lines held for institutional buyers. The Snellen and Amsler chart set covers the acuity and field cards kept at the lane, the exam table paper roll covers the couch between patients, and a practice that wants the monitoring and consumable lines together can start with the sterilization monitoring collection or the biological indicator 5-pack trial.

Sources

1. 21 CFR 886.1150, visual acuity chart
2. 21 CFR 886.9, limitations of exemptions
3. 21 CFR 886.4750, ophthalmic eye shield
4. 21 CFR 886.1570, ophthalmoscope
5. 21 CFR 886.1930, tonometer and accessories
6. Medical Devices Regulations, SOR/98-282, full text and Schedule 1
7. openFDA device classification API
8. FDA device classification database
9. ISO 8596:2017, visual acuity testing, optotypes
10. ISO 8597:1994, method of correlating optotypes
11. ISO 8255-1:2017, microscope cover glasses
12. College of Optometrists of Ontario, standards of practice
13. Canadian Association of Optometrists, eye health library
14. World Health Organization, blindness and visual impairment
15. Ontario Ministry of Health, what OHIP covers
16. Canadian Centre for Occupational Health and Safety, OSH answers
17. US OSHA bloodborne pathogens standard, 1910.1030
18. Public Health Ontario, infectious diseases
19. ISO 10993-1:2018, biological evaluation of medical devices
20. Health Canada, medical devices

Related reading

• Eye Charts in Canada vs the US: Which Optotype Standard Applies
• Near Vision Reading Cards in Canada vs the US
• Comprehensive Eye Check-Ups in Canada vs the US: Which Exam Interval Applies

CliniEco Medical supplies eye care, laboratory and clinic consumables of the kind described in this article. MDEL #35334. This article is written for clinic, procurement and infection-control professionals and is not clinical guidance; acuity testing technique and the interpretation of a result belong to the treating professional and to the provincial requirements that apply to the practice.

Frequently Asked Questions

Is 6/6 the same as 20/20?

Yes. Both express the same threshold. The metric form writes the test distance in metres, so 6/6 means the patient resolved at 6 metres an optotype that a reference eye resolves at 6 metres. The imperial form writes the same ratio in feet, so 20/20 means the patient resolved at 20 feet what the reference eye resolves at 20 feet. The clinical convention treats 6 metres and 20 feet as the same test distance.

What class is a visual acuity chart in the United States?

Class I, general controls, under 21 CFR 886.1150. The section exempts the device from premarket notification subject to the limitations in 21 CFR 886.9, and exempts it from the quality system regulation in part 820 except for the record-keeping requirements in 820.180 and the complaint file requirements in 820.198.

Does Canada have a named entry for a visual acuity chart?

No. Schedule 1 to the Medical Devices Regulations classifies devices by rule rather than by catalogue entry. A printed chart is a non-invasive device and falls under Rule 7(1) as Class I. A powered unit that displays optotypes depends on a source of energy, so it is an active device and the applicable active-device rule has to be applied instead.

Does ISO 8596 apply to a clinic acuity chart?

Not in the way a supplier usually implies. ISO 8596:2017 specifies Landolt ring optotypes and a method for measuring distance visual acuity under photopic conditions for certification or licensing, and its scope states that it is not intended as a standard for clinical measurement. A chart bought for everyday clinical work is not required by that document to use the same optotype set as a licensing chart.

Why does the test distance change the notation?

Because the notation is a ratio, and the numerator is the distance at which the test was performed. A room that is too short for 6 metres may use 4 metres with a chart printed for 4 metres, and the notation changes to match. When a mirror is used, the distance recorded is the optical path length through the mirror, not the length of the room.

What should be recorded with an acuity result?

The chart identifier and optotype set, the test distance, the notation convention, the lighting condition, the device class or licensing position, and the maker's cleaning instruction. The optotype set matters because a letter chart and a Landolt ring chart at the same notation do not pose the same task.