Visual Acuity logMAR
Score acuity using a caller-specified starting score and per-letter increment, then convert to decimal acuity; the chart scoring convention must be supplied.
Description
Letter-scored logMAR acuity subtracts a fixed increment for each correctly identified letter.
Letter-scored logMAR acuity subtracts a fixed increment for each correctly identified letter. Decimal acuity is an exponential transformation of the resulting log score.
The tool returns letterScoredLogmar (letter scored logmar); decimalAcuity (decimal acuity). Input measurements and model parameters are supplied by the caller; the tool computes the stated summary rather than acquiring those measurements.
When to use Visual Acuity logMAR
Use Visual Acuity logMAR when you already have startingLogmar, lettersCorrect, lettersPresented, logmarPerLetter and want letterScoredLogmar, decimalAcuity. This is useful for checking a supplied calculation, documenting a measurement convention, or comparing observations prepared on the same basis.
How to use Visual Acuity logMAR and understand its results
- Enter startingLogmar, lettersCorrect, lettersPresented, logmarPerLetter using the field table below. Match the units, ordering and data boundaries described in the calculation.
- Run the tool after checking the stated bounds and matching paired observations or categories. A malformed request gives a tool error rather than a usable measurement.
- Read letterScoredLogmar, decimalAcuity. Use the calculation and interpretation below to distinguish amounts, ratios, signs and decision flags where applicable.
- Review the supplied parameters and assumptions before using the result in a report or comparing it with another observation.
| Input field | Meaning | Value type and bounds | Requirement |
|---|---|---|---|
| startingLogmar | Starting logmar | number | Required |
| lettersCorrect | Letters correct | integer | Required |
| lettersPresented | Letters presented | integer | Required |
| logmarPerLetter | Logmar per letter | number | Required |
Validate integer correct counts against positive presented counts and a positive score increment.
logMAR = startingLogMAR−lettersCorrect×logMARperLetter; decimal = 10^(−logMAR) The symbols refer to the supplied quantities or generated values described above; unit conversions and comparison boundaries follow the stated implementation.
{
"input": {
"startingLogmar": 1,
"lettersCorrect": 25,
"lettersPresented": 50,
"logmarPerLetter": 0.02
},
"output": {
"letterScoredLogmar": 0.5,
"decimalAcuity": 0.31622776601683794
}
}Limitations and assumptions
The starting score and increment must match the chart protocol. No chart identification, refraction or impairment classification is performed.
Numerical inputs must be finite. The tool checks its declared input shape and calculation constraints; those checks do not verify the quality, calibration or representativeness of the supplied observations.
Alternative or Complementary methods
Spherical Equivalent Refraction: Convert a signed sphere/cylinder prescription into spherical equivalent and J0/J45 power vectors; plus- and minus-cylinder prescriptions are both accepted. Contrast Sensitivity: Build a contrast-sensitivity function and trapezoidal area in log10 frequency/log10 sensitivity space from measured Michelson thresholds.
Methodological background: Power vectors: Fourier analysis of refractive error.1
References
-
Power vectors: Fourier analysis of refractive error — Thibos, Wheeler and Horner; Optometry and Vision Science
Similar or alternative tools
- Spherical Equivalent Refraction
Convert a signed sphere/cylinder prescription into spherical equivalent and J0/J45 power vectors; plus- and minus-cylinder prescriptions are both accepted.
- Contrast Sensitivity
Build a contrast-sensitivity function and trapezoidal area in log10 frequency/log10 sensitivity space from measured Michelson thresholds.