Anisometropia
Interocular image-size comparison and spectacle-magnification estimate.
For educational purposes only. A teaching and screening aid, not a medical device; it does not replace a standard clinical examination.
How to use
- Size comparison: the patient wears red/green glasses over their full distance correction — set which eye is behind the red lens. Each eye then sees only one half of the figure.
- Ask the patient to look at the central dot and say whether the red half and the green half look the same size. Enlarge or shrink the green half until they report the two as equal, then read off the difference.
- Repeat with the halves stacked vertically to measure the vertical meridian, which can differ from the horizontal one.
- Spectacle estimate: enter each eye’s spectacle power in the meridian of interest (or the spherical equivalent) and the vertex distance, to see how much image-size difference the spectacles themselves introduce.
How it is scored
- Size comparison: if the green half had to be made p % larger to look equal, the eye behind the green filter perceives images p % smaller than the other eye (and the reverse for a reduction).
- Spectacle estimate: power factor = 1 / (1 − h × F), with h the vertex distance in metres and F the lens power in dioptres. The difference between the eyes is (PF right ÷ PF left − 1) × 100 %.
- Reviews describe small image-size differences as commonly tolerated and larger ones as increasingly likely to cause symptoms such as asthenopia, headache and reduced stereopsis; tolerance varies between patients, so no single cut-off is applied here.
Limitations
- This is a demonstration and screening tool, not a validated aniseikonia measurement. Direct-comparison tests of this design are known to underestimate aniseikonia.
- Red/green separation depends on the glasses and on the screen: cross-talk (seeing a faint ghost of the other half) makes the comparison less reliable.
- The spectacle estimate uses the power factor only. The shape factor (lens thickness and front curve) and the eye’s own contribution (axial versus refractive anisometropia) are not modelled.
References
- Awaya S, Sugawara M, Horibe F, Torii F. The "new aniseikonia test" and its clinical application. Nippon Ganka Gakkai Zasshi 1982;86:217-222.
- McCormack G, Peli E, Stone P. Differences in tests of aniseikonia. Invest Ophthalmol Vis Sci 1992;33:2063-2067.
- Achiron LR, Witkin N, Primo S, Broocker G. Contemporary management of aniseikonia. Surv Ophthalmol 1997;42:321-330.
- Bennett AG, Rabbetts RB. Clinical Visual Optics — spectacle magnification, power and shape factors.
For educational purposes only. The Clinical Exam tools are part of an educational platform. They are screening and teaching aids, not certified medical devices: a result depends on the screen (letter sizes are estimated for the type of device), the lighting and the testing distance. They are not a diagnosis, and any abnormal or doubtful result should be confirmed with a standard clinical test.