You should add more plus power. On the duochrome test, a patient who prefers the green side is focusing the image behind the retina. That is the same picture as hyperopia, or farsightedness. Plus power pulls the focus forward onto the retina, so you turn the sphere toward plus until the two sides look equal.
The test uses the sphere, not the cylinder. Myopia means the eye has too much plus power for its length, so distant images land in front of the retina. Hyperopia, or farsightedness, is the opposite: too little plus power, so the image lands behind the retina. Green light focuses in front of red light inside the eye. A patient left with too much minus in the phoropter is pushed toward the hyperopic side, so green lands on the retina and looks clearer. A patient left with too little minus is still myopic, so red looks clearer. Many techs learn this as RAM-GAP: red add minus, green add plus. Change the sphere in small steps and stop when the red and green sides look the same. Work on one eye at a time, with the other eye covered or fogged. Then check the acuity again to be sure the change did not blur the letters. Young patients can accommodate through extra minus power, so they may accept lenses that are too strong. A patient who prefers green while reading 20/20 is already close to the end point.
Green clearer means add plus. Red clearer means add minus. Refining the sphere and reading the patient's answers is a core part of the COA refraction content area.
Incorrect Answers
- A. Cylinder power corrects astigmatism, which is uneven corneal curvature. The red and green sides test the sphere, not the cylinder.
- B. Taking cylinder away also changes the astigmatic correction and would blur both colored sides. It does not answer a color preference.
- C. More minus pushes the focus even farther back. That makes the green side look better still and drives the patient away from the right answer.
Testing Pearls
- RAM-GAP: red add minus, green add plus.
- The duochrome test refines the sphere only; the cross cylinder refines axis and cylinder power.
- Myopia = too much plus for the eye's length. Hyperopia = too little plus.
- Color blindness does not block the test, because the patient compares clarity, not color.
- Stop when the two sides look equal, and never over-minus a young patient. Test one eye at a time, with the other eye covered or fogged, and recheck the acuity after any sphere change.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 5, Optics and Refractive States of the Eye.
