Description
Numerical Snellen fraction, decimal and logMAR crosswalk, excluding counting-fingers or other non-Snellen categories and clinical interpretation.
Vision research uses stimulus angles, chart notations, image scales, viewing distances, coordinate origins, and observation windows. These quantities have different denominators and measurement conventions. A conversion or support share does not independently establish a visual measurement or its clinical interpretation.
Numerical Snellen fraction, decimal and logMAR crosswalk, excluding counting-fingers or other non-Snellen categories and clinical interpretation.
When to use this tool
Use Snellen Decimal Logmar Source Crosswalk when you need this image, stimulus, notation, coordinate, or observation component for supplied research data.
How to use the tool and how it works
- Enter numerator, denominator. Use the units and conventions below.
- Run the tool. Correct any format, range, or cross-field input error before interpreting the output.
- Read decimalAcuity, logMar. Use the output convention below.
For positive numerator and denominator return decimalAcuity=numerator/denominator and logMar=log10(denominator/numerator).1
LogMAR uses the negative base-ten logarithm of decimal acuity. This changes the notation of a supplied positive score; it does not make different chart procedures clinically equivalent.2
| Input field | Meaning and units | Accepted input |
|---|---|---|
| numerator | Numerator in the numeric or exact integer-text representation declared below. | Required. Finite number; strictly above 0. |
| denominator | Denominator or exact integer denominator text under the stated sign restriction. | Required. Finite number; strictly above 0. |
| Output field | Interpretation | Returned shape |
|---|---|---|
| decimalAcuity | Decimal Acuity | Required. Finite number. |
| logMar | Base-ten log of denominator/numerator; a notation value, not a chart procedure. | Required. Finite number. |
{
"input": {
"numerator": 20,
"denominator": 40
},
"output": {
"decimalAcuity": 0.5,
"logMar": 0.3010299956639812
}
}Limitations and assumptions
- Numerator and denominator must use the same distance unit and acuity convention. This converts finite positive notation only; counting fingers, hand motion, light perception, chart scoring, and clinical interpretation are not supplied.
- Inputs must satisfy both the field types and the cross-field conditions above. Invalid input causes an error; a successful false flag or documented placeholder is a different result.
- Numeric calculations use binary floating point unless exact decimal-integer arithmetic is explicitly stated. No hidden measurement tolerance or decimal rounding is added; nonfinite or out-of-contract results are rejected. Numeric signed-zero results are exported as zero.
Alternative or Complementary methods
Use calibrated acquisition and a documented visual or imaging protocol for complete interpretation. Confirm the reporting notation, viewing geometry, image scale, and support of the underlying observations.
References
-
Calculation and input contract: ophthalmology/snellen-decimal-logmar-source-crosswalk — tool.io
-
DICOM PS3.17 2018a: Visual Acuity Notations — National Electrical Manufacturers Association (NEMA)
Similar or alternative tools
- Optometric Chart Distance Acuity Factor
Scale an explicitly calibrated Snellen chart fraction for a supplied test distance.
- Optometric Acuity Log Scale Step Factor
Apply an explicit logMAR step without selecting a chart scoring convention.