Eye Charts — Digital, Backlit and Traditional

Eye charts as a core diagnostic tool

Eye charts are a fundamental diagnostic instrument in every ophthalmology practice, allowing precise assessment of visual acuity and detection of refractive errors. Since Dutch ophthalmologist Herman Snellen introduced the first charts in 1862, vision-testing technology has evolved considerably. Modern clinics can choose from a range of solutions — classic cardboard charts, backlit LED panels and advanced digital charts with unique point-illumination.

Traditional Snellen charts

Traditional Snellen charts are made from cardboard or heavy paper and follow the classic Snellen layout. They consist of 11–12 rows of optotypes that decrease in size; each row is marked by a fractional value indicating visual acuity. Standard dimensions are 31.0 × 45.5 cm, and the construction is based on the classical 5×5 grid developed by Snellen.

Types of optotypes in traditional charts

Traditional eye charts are available in several variants tailored to different patient groups:

• Letter chart — contains Latin letters (A, C, E, G, L, N, P, R, T, V, Z, B, D, F, H, K, O, S, U, Y) arranged in decreasing size.
• Number chart — uses Arabic numerals 0–9 for patients who have difficulty recognizing letters.
• Picture chart — designed for children; contains simple drawings such as a mushroom, duck, or hand.
• Landolt rings — rings with gaps in various positions; particularly useful for patients unfamiliar with alphabets.
• Snellen hooks / Tumbling E — the letter “E” oriented in different directions; used for testing children and illiterate patients.

Limitations of traditional charts

Cardboard or coated-paper charts have several limitations that affect the quality and durability of vision testing. The main problem is fading — prolonged exposure to light causes gradual bleaching of optotypes, which can reduce contrast and lead to inaccurate results.

Warping and mechanical deformation are another important limitation. Cardboard construction is sensitive to moisture, temperature changes and mechanical damage, which can cause surface undulation and optotype distortion. Such deformations directly impair the precision of acuity testing.

Glare and reflections from glossy or photographic paper surfaces can significantly hinder optotype reading, especially under strong room illumination (for example, the PN-EN 12464-1:2022-01 standard recommends 500 lx in the optotype reading area). In contrast to traditional charts, optotypes on the OKO chart are coated with an anti-reflective finish that eliminates unwanted reflections and ensures optimal contrast during testing.

Finally, traditional charts require periodic replacement due to normal wear, which generates additional operating costs and can cause gaps in diagnostic availability.

Backlit LED charts

Backlit LED charts represent a significant technological advance in vision diagnostics, combining the advantages of traditional charts with modern illumination capabilities. Medical devices using LED technology provide uniform backlighting of optotypes directly from the rear panel, eliminating problems associated with glare and reflections typical of paper charts. Compared with traditional charts, LED backlighting offers better contrast control and stable lighting conditions for optotypes, regardless of changes in ambient room lighting. LED technology also delivers long-term stability of luminous parameters and removes the fading problem inherent to paper charts.

Full-surface illumination technology

Backlit charts are an evolutionary step between traditional and digital solutions. They feature full-surface illumination — the entire chart face is lit uniformly. The front plate is usually acrylic glass, and the lighting system is an LED array.

Limitations of backlit charts

A key limitation of backlit charts is the lack of selective illumination — all optotypes are lit simultaneously. There is no ability to illuminate a specific row or a single character. This limitation affects test precision because the patient sees all elements at once, which can distract attention when testing a particular line or optotype.

OKO digital charts — the only point-illumination system

The OKO vision chart from Meden-Inmed is the only eye chart on the market with point-illumination LED technology. Unlike other backlit charts that illuminate the entire surface, OKO enables precise illumination of selected elements — entire rows or individual characters.

Unique illumination control

The control software for the OKO chart allows illumination of all characters in one row or only a single, chosen character. This functionality is a key difference compared with all other eye charts available on the market, which do not offer such precision of illumination.

Advanced OKO remote control

The infrared remote control, equipped with six (or multiple) buttons, enables:

• activation of an optotype;
• illumination of a whole row of characters;
• illumination of a single selected character;
• selection of illumination type;
• changing the operating mode (selecting row or single character);
• dimming/extinguishing optotypes;
• selecting the cursor movement direction;
• automatic shutdown after 5 minutes of inactivity.

Built-in quick-change optotype mechanism

The OKO chart offers a built-in quick-change mechanism for swapping optotype sets without interrupting the examination. Available optotypes include:

• CN — number optotype
• LN — letter optotype
• DN — hand optotype
• ON — picture optotypes for children
• HN — Snellen hooks / Tumbling E

Comparison of illumination technologies in eye charts

The fundamental difference between chart types lies in the method of illuminating tested elements:

• Traditional charts — no built-in illumination; susceptible to glare and reflections from paper surfaces.
• Backlit charts — uniform full-surface illumination, but no selective control.
• OKO digital chart — the only chart with precise point illumination of selected elements and full control.

How illumination technology affects test precision

Point illumination on the OKO chart eliminates patient distraction by directing gaze to the specific tested element. Unlike other backlit charts where all optotypes are lit simultaneously, only the OKO chart allows isolation of the tested character or row. This approach increases test accuracy and patient comfort during vision assessment.

Light uniformity in eye charts

The OKO Snellen chart provides optimal illumination of tested elements with proper background contrast by using LED technology. Research indicates that as the contrast between the target and background increases, the accuracy of visual detection improves. OKO’s LED system ensures optimal lighting conditions regardless of clinic ambient light.

Eye charts in specialist clinics

In professional ophthalmology clinics, the OKO digital chart is becoming standard equipment due to its diagnostic precision and capacity for advanced vision tests. The selective illumination function enables accurate testing of individual acuity levels, which is something other charts cannot achieve.

Eye charts in occupational medicine

In occupational health settings, where hundreds of screening tests may be performed daily, diagnostic speed and precision are essential. Traditional backlit charts, while offering better optotype visibility than cardboard charts, still have limitations — they illuminate the whole surface at once, which can distract examinees and lengthen testing time.

The OKO chart revolutionizes screening work in occupational medicine thanks to its unique point-illumination LED technology. Unlike standard backlit charts, OKO can precisely illuminate only the tested row or a single optotype, which significantly:

• shortens test time — patients focus immediately on the intended element;
• improves result accuracy — it eliminates distraction by other optotypes;
• enhances patient comfort — reduces visual fatigue;
• streamlines clinician workflow — precise remote control operation.

For occupational health units where efficiency and accuracy are priorities, the OKO chart is the only available market solution with point-illumination technology, offering a clear advantage over traditional backlit charts in speed and diagnostic precision.

Eye charts in pediatric testing

In pediatric examinations it is particularly important to be able to switch optotypes quickly from letters to picture symbols suitable for a child’s age. The OKO chart allows instant transitions between different test types during the examination of a young patient — an important and distinctive advantage.

Warranty and technical support for OKO

The OKO eye chart is covered by a 36-month manufacturer’s warranty, which confirms the device’s high build quality and reliability. The extended warranty covers all electronic components, the LED illumination system and the remote control, ensuring worry-free operation throughout the service life.

Diagnostic needs analysis

Choosing the right type of eye chart should consider the specifics of the medical practice and diagnostic requirements:

• For specialist clinics — the OKO digital chart offers the highest precision and functionality thanks to its unique point illumination.
• For occupational medicine — backlit LED charts provide adequate functionality for screening tests.
• For educational institutions — traditional charts may be sufficient for basic screenings.

A review of solutions available on the Polish market shows that the Meden-Inmed OKO chart is currently the only chart with point-illumination technology. All other backlit charts provide only full-surface illumination, which is a significant limitation for selective testing.

Eye Charts - Summary

The OKO chart from Meden-Inmed is a breakthrough in vision diagnostics: it is the only eye chart on the market with point-illumination LED technology. Unlike all other backlit charts that light the entire surface simultaneously, the OKO chart allows precise illumination of selected elements — entire rows or single optotypes.

Traditional charts remain an option for basic screening, where simplicity is more important than advanced features. Backlit LED charts improve visibility compared with cardboard charts but do not offer fine control over tested elements.

The key difference between lighting systems — from no control (traditional) to full-surface lighting (backlit), to revolutionary point illumination (OKO) — directly affects the quality and accuracy of vision testing. Point illumination represents the future of vision diagnostics, giving clinicians tools to perform more precise and patient-friendly examinations. The OKO chart’s unique point-illumination makes it an indispensable instrument in modern ophthalmology clinics where diagnostic precision is vital for patients’ eye health.