Description
Geometric equal-angular-size chart scaling, without asserting clinical equivalence between chart designs.
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.
Geometric equal-angular-size chart scaling, without asserting clinical equivalence between chart designs.
When to use this tool
Use Ophthalmic Chart Distance Letter Height Factor 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 referenceDistance, referenceLetterHeight, targetDistance. Use the units and conventions below.
- Run the tool. Correct any format, range, or cross-field input error before interpreting the output.
- Read scale, targetLetterHeight. Use the output convention below.
For positive referenceDistance and targetDistance and nonnegative referenceLetterHeight return scale=target/reference and targetLetterHeight=referenceHeight*scale.1
| Input field | Meaning and units | Accepted input |
|---|---|---|
| referenceDistance | Reference Distance | Required. Finite number; strictly above 0. |
| referenceLetterHeight | Reference Letter Height | Required. Finite number; minimum 0. |
| targetDistance | Target Distance | Required. Finite number; strictly above 0. |
| Output field | Interpretation | Returned shape |
|---|---|---|
| scale | Supplied nonzero or positive multiplier under the stated transform convention. | Required. Finite number. |
| targetLetterHeight | Target Letter Height | Required. Finite number. |
{
"input": {
"referenceDistance": 6,
"referenceLetterHeight": 10,
"targetDistance": 3
},
"output": {
"scale": 0.5,
"targetLetterHeight": 5
}
}Limitations and assumptions
- Distances and heights need compatible length units within their ratios. This preserves an ideal geometric angular-size relation but does not validate optotype design, chart calibration, viewing conditions, or a clinical acuity test.
- 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
Similar or alternative tools
- Chart Distance Unit Basis Flag
Compare a supplied chart distance to an explicit reference after unit conversion; no acuity score is inferred.
- Ophthalmology Interpupillary Half Distance Offset
Compare explicit total and monocular distance records, without selecting a spectacle fit or determining patient measurements.