Duochrome (bichrome) test in refraction uses red and green filters. In a myopic patient whose red letters are clearer than green, the clinical interpretation and appropriate action is:
- A Patient is overcorrected (too much minus); reduce minus until letters appear equally clear ✓
- B Patient is myopic; add more minus (or less plus) to shift focus to green clarity
- C Patient is hypermetropic; add more plus
- D Duochrome test is unreliable in myopes
Explanation
The duochrome test exploits chromatic aberration: red light focuses ~0.5 D behind green on the retina. When red letters are clearer, the eye's circle of least confusion falls closer to the red end — meaning the focus point is behind the retina (or the green-red boundary is at the retina), suggesting the prescription has too much minus (over-minused). The endpoint is equal clarity of red and green letters. In emmetropia/optimal correction, both are equally clear. If red is clearer, reduce minus (add plus); if green is clearer, add minus. The test works most reliably at 6/9 to 6/12 visual acuity.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.