How accurate are our near reading charts? An assessment of 19 charts against ISO standards.

This study evaluated 19 commercially available near reading charts against ISO standards and found that none fully met all requirements due to significant variability in text size and surface finish, prompting the development of a new compliant chart to ensure accurate near vision assessment.

The Gist

Imagine you are trying to measure the height of a building. You have a ruler, but the problem is that every time you switch to a different ruler, the "inch" mark changes size. One ruler says a foot is 12 inches, another says it's 14, and a third says it's 10. If you were building a house, this would be a disaster. You might think a wall is safe, but it could actually be too short or too tall because your measuring tool was lying to you.

This is exactly what researchers Timothy Murphy and his team discovered about near reading charts—the little cards eye doctors use to test how well you can read small text up close.

Here is the story of their investigation, broken down simply:

The Problem: The "Ruler" Was Broken

Eye doctors use these charts to track vision changes over time, especially for people with conditions like macular degeneration or cataracts. If a patient's vision gets slightly worse, the doctor needs to know exactly how much worse.

The researchers decided to check if the charts currently used in clinics were accurate. They treated the charts like a set of rulers and compared them against a new, strict international rulebook called ISO 7921:2024. This rulebook is like the "Gold Standard" for how a reading chart should be made.

The Investigation: Scanning the Charts

The team gathered 19 different reading charts from various manufacturers and countries. They didn't just look at them with their eyes; they used a high-tech scanner (like a super-powered photocopier) to turn the charts into digital images.

Think of this like taking a photo of a map and then using a computer to measure every single street on it with perfect precision. They measured the height of the letters (specifically the lowercase "x") to see if they matched the "Gold Standard."

The Shocking Results

The findings were a bit like finding out that 18 out of 19 rulers in a classroom were the wrong size.

1. The "Perfect" Chart Didn't Exist: Out of 19 charts, zero of them met every single requirement of the new international standard. Only one chart got the text sizes right, but even that one had confusing labels.
2. The "Font" Trap: The researchers found that the type of font mattered a lot.
   • Serif fonts (letters with little feet, like Times New Roman) were generally printed too small. It's like the manufacturer used a template for a big font but shrank it down without adjusting the numbers.
   • Sans-serif fonts (letters without feet, like Arial) were generally printed too big.
3. The "Labeling" Confusion: Many charts had labels that didn't match the actual text. One chart might say "N8" (a standard size), but the letters were actually the size of "N10." It's like a clothing store selling a "Medium" shirt that actually fits a "Large."
4. The Shiny Surface Issue: The standard says charts should be matte (non-shiny) so light doesn't bounce off them and make the text harder to read. Several charts were printed on shiny plastic or laminated paper, which is like trying to read a menu under a bright spotlight—it creates glare.

Why Does This Matter?

Imagine you are tracking your weight. If you step on a scale that says you weigh 150 lbs today, and a different scale says 155 lbs tomorrow, you might panic and think you gained 5 pounds. But if the scales are just calibrated differently, you haven't gained anything.

In eye care, this is dangerous.

• If a doctor uses Chart A today and Chart B next month, they might think a patient's vision has gotten worse (or better) when it hasn't.
• This could lead to wrong prescriptions for glasses or magnifiers.
• It makes it hard for scientists to compare research results from different clinics.

The Solution: A New "Master Ruler"

Because the existing charts were so unreliable, the researchers didn't just complain; they built a new one.

They created the UC/UWA Reading Chart.

• It is free for anyone to download.
• It is designed to be perfectly accurate according to the new international rules.
• It comes in two versions (with and without "feet" on the letters) so doctors can choose what they prefer, but both are mathematically perfect.

The Takeaway

The study is a wake-up call for the eye care world. Just because a chart looks like a standard reading card doesn't mean it's accurate.

The main lesson: If you are an eye doctor, don't switch between different brands of reading charts if you are monitoring a patient's vision over time. Stick to one chart, or better yet, use the new free, standardized one the researchers built. If you are a patient, it's a reminder that the tools we use to measure our health need to be as precise as the health itself.

In short: We found that our measuring tapes were broken, so we made a new one.

Technical Summary

1. Problem Statement

Near reading acuity charts are fundamental tools in community optometry for assessing functional vision, particularly for managing conditions like age-related macular degeneration, cataracts, and presbyopia. However, there is a lack of evidence regarding the accuracy of commercially available charts and those provided by lens manufacturers.

• Standardization Gap: While the new international standard ISO 7921:2024 defines strict parameters for font size (based on x-height), line spacing, contrast, and surface material, many community clinics use non-specialized charts.
• Clinical Risk: Inconsistent chart accuracy can lead to misinterpretation of vision changes, especially in shared-care models or when monitoring progressive diseases. Clinicians often rely on labels (e.g., N-size, logMAR) that may not correspond to the actual physical dimensions of the text.
• Unit Confusion: There is confusion between logMAR (based on letter width for distance) and logRAD (based on x-height for near vision), leading to potential mislabeling of text sizes.

2. Methodology

The study employed a rigorous quantitative approach to assess 19 distinct near reading charts against ISO 7921:2024.

• Sample: 19 charts were collected via convenience sampling from optometry clinics and universities in Australia, New Zealand, and Hong Kong. These included commercially available charts and those provided by device manufacturers.
• Data Acquisition:
  • Charts were scanned at 600 dots per inch (dpi) using a Konica Minolta C450i.
  • Images were processed using Gaussian adaptive thresholding to convert them to binary black-and-white, eliminating grey pixels to ensure precise edge detection.
  • Validation: The scanning method was verified against a 20cm ruler, yielding a measurement error of only 0.12%.
• Measurement Protocol:
  • X-Height Measurement: Three researchers independently measured the height of lowercase letters without curves (x, v, w, z) or derived x-heights from capital letters where necessary.
  • Line Spacing: Estimated based on the ratio of baseline-to-baseline distance to x-height, assuming fonts similar to Helvetica (sans-serif) or Times New Roman (serif).
  • Contrast: Calculated using the Weber formula ($C = \frac{L_S - L_T}{L_S} \times 100\%$) on gamma-corrected grayscale images.
• Statistical Analysis:
  • Deviations from ISO expected values were calculated as percentages.
  • Inter-rater reliability was assessed using a two-way random, single-score Intraclass Correlation Coefficient (ICC).
  • Comparisons were made between serif and sans-serif fonts.

3. Key Contributions

• Empirical Assessment: This is the first study to systematically measure a wide variety of community-available near reading charts against the newly published ISO 7921:2024 standard.
• Methodological Innovation: The authors developed and validated a highly repeatable digital measurement technique using adaptive thresholding and high-resolution scanning, achieving near-perfect inter-rater agreement.
• Open-Source Solution: In response to the findings, the authors developed and released the UC/UWA Reading Chart, a free, ISO-compliant chart available in both serif and sans-serif versions under a Creative Commons license.
• Clarification of Font Ratios: The study highlighted the critical difference in x-height ratios between serif (Times New Roman: 0.450) and sans-serif (Helvetica: 0.523) fonts, explaining why charts labeled with the same "point size" or "N-size" have vastly different actual acuity values.

4. Key Results

• Overall Compliance: None of the 19 charts met all requirements of the ISO 7921:2024 standard. Only one chart (5.26%) had all text sizes within the permitted tolerance.
• Text Size Variability:
  • Serif Fonts: 12 charts used serif fonts. They tended to be significantly smaller than required (mean deviation $\mu = -9.63\%$), leading to an underestimation of near acuity. All serif charts were outside tolerance for at least one size.
  • Sans-Serif Fonts: 7 charts used sans-serif fonts. They tended to be slightly larger than required (mean deviation $\mu = +4.96\%$). Only one sans-serif chart (Chart 17) met all size tolerances.
  • Labeling Errors: Many charts contained incorrect conversions between N-size, logMAR, and Snellen fractions. Some charts lacked the required 0.00 logRAD size, limiting the ability to detect high-acuity vision.
• Line Spacing: All charts met the maximum line spacing requirement (ratio $\le 1.5$). However, spacing was inconsistent across different text sizes on the same chart, which may affect visual crowding.
• Contrast and Material:
  • All charts met the minimum contrast requirement of 85% (lowest was 88.9%).
  • Surface Material: 8 charts (42%) were printed on laminated, satin plastic, or satin card. The ISO standard requires a matte surface to minimize specular reflections; thus, these charts were non-compliant.
• Reliability: The measurement technique demonstrated exceptional repeatability with an ICC(2,1) of 1.00.

5. Significance and Clinical Implications

• Clinical Caution: The study concludes that clinicians must exercise extreme caution when comparing near acuity results obtained from different charts. Switching charts can introduce errors greater than the natural progression of many eye diseases, potentially masking deterioration or falsely indicating improvement.
• Standardization Urgency: The high variability in text size (ranging from -29.8% to +23.95% deviation) underscores the urgent need for universal adoption of ISO-compliant charts, particularly in research and shared-care models.
• Recommendation: Clinicians should record the specific chart name and test distance with every acuity measurement. Where ISO-compliant charts are unavailable, the same chart must be used consistently for a single patient over time.
• Future Work: The authors call for further research to determine if these physical discrepancies translate to clinically significant differences in patient outcomes and encourage the use of their newly developed UC/UWA Reading Chart to bridge the gap between research standards and clinical practice.