Duochrome test principle

Duochrome test is a manual procedure that is performed as part of the subjective refraction process. Duochrome test is performed using a distance visual acuity chart with black letters, and the chart is split equally into two identical halves. Left side of chart have letter on a red background and right side of chart have letter on green background, or vice versa.

Duochrome-test

Principle of Duochrome test

Duochrome test is based on longitudinal chromatic aberration. Short wavelength (Green 535nm) tends to bend more compared to longer wavelength (Red 620nm). In emmetropic eye or corrected eye, Green light focus 0.20 D in front of the retina and red light focus 0.24 D behind the retina.

When to perform Duochrome test

• Duochrome test can be performed before JCC to ensure that circle of least confusion is on the retina.
• And after JCC before you finalize final refractive correction.

Procedure of Duochrome Test

• Monocular
• Semi dark room: Helps dilating pupil and slightly increase the chromatic aberration.
• Ask the subject: whether the letters/rings/dots are equally visible or prominent on the both backgrounds or more visible or prominent against one of the background.

If patient says: Letters on the red background are more clearer or sharper, when compare to green background.

It means:

Patient overcorrected hyperope

Or under corrected myope

Add  -0.25DS until patient reports both same.

If patient says: Letters on the green background are more clearer or sharper, when compare to red background.

It means:

Patient overcorrected myope

Or under corrected hyperope

Add plus +0.25DS until patient reports both same.

Note: If more than +/- 0.50 DS is required to balance, this usually indicated the duochrome test is not reliable for this patient and should be ignored.

For final power

If clarity or sharpness of letters or rings changes from green to red with +0.25 or red to green with -0.25. Keep young patient on red side.

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