Question
While performing this test, if the patient reports that he sees red letters more clear than green, it indicates that he is slightly:
A. Myopic.
B. Hypermetropic.
C. Presbyopic.
D. None of the above.
Show Answer
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Correct Answer » A Explanation |
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Ans:A. Myopic.
This image shown is of a Duochrome test
Subjective Refraction
- It is an attempt to determine, by trial and error using the patient’s cooperation, the combination of lenses that will provide the best corrected visual acuity
- The following equipment is used to complete a Subjective Refraction:
- Trial Frames
- Trial lens box & confirmation set: Including the pinhole and occluder
- Jackson Cross Cylinder: This is used to search for astigmatism.
- Snellen chart
- Duochrome test
Duochrome test
- It is a test commonly used to refine the final sphere in refraction, which makes use of the chromatic aberration of the eye. Because of the chromatic aberration of the eye, the shorter wavelengths (green) are focused in front of the longer red wavelengths.
Testing :
- The patient is asked to compare the clarity of the letters on the green and the red side.
- If the focus is anterior to the retina, as in the case of myopia, the letters will be blurred more for the green side.
- If the focus is posterior to the retina, as in the case of hyperopia, the letters will be blurred more on the red side
- With optimal spherical correction, the letters on the red and green halves of the chart appear equally clear.
- Because this test is based on chromatic aberration and not on color discrimination, it is used even with people having color vision deficiency.
- The duochrome test is not used with patients whose visual acuity is worse than 20/30 (6/9), because the 0.50 D difference between the 2 sides is too small to distinguish.