During the duochrome test, the patient states that the letters in green are more clear. What should be done in the phoropter?
Refractometry
No
U
D
The duochrome test uses chromatic aberration to help refine the spherical component of a person's refractive error. Therefore, it does not matter if the patient is color blind since this patient can still discern differences in wavelengths of light.
During refraction, if the patient sees the red side better, then the current refraction is too hyperopic and more minus power must be added. Conversely, if the patient sees the green side better, then the refraction is too myopic and thus more plus power must be added. This can be remembered with the "RAM-GAP" mnemonic.
Again, this is due to the fact that light with shorter wavelengths (e.g. green) are normally focused more anteriorly while longer wavelengths (e.g. red) are focused more posteriorly. Your goal in refraction is to have yellow light fall on the retina (i.e. how far the green light is in front of the retina should equal how far the red light is behind the retina).
The duochrome test should not be used in patients whose best corrected visual acuity is worse than 20/30 since the small distance (0.50 diopters) between the red and green sides is too small to distinguish.
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