L10 Flashcards
(29 cards)
What is the goal of subjective spherical refinement?
To determine the most positive lens that provides maximum vision, minimizing accommodation to increase comfort.
What are the main techniques for spherical refraction?
BVS Plus/minus test, Duochrome test, and +1.00DS test.
What is the first step after retinoscopy when starting monocular subjective refraction?
Turn the room lighting back on, make adjustment for working distance, leave cylinder untouched, and occlude the eye not being tested.
What is the basic procedure for the plus/minus technique?
Add +0.25DS and ask if the letter is clearer with or without the lens or just the same; continue until vision begins to blur.
What should you do when vision blurs with plus lenses in the plus/minus technique?
Check with a -0.25DS lens and ask if the letter is clearer with or without the lens or smaller and blacker.
When should you incorporate minus lenses in the plus/minus technique?
Only when there is an improvement in vision (more letters read), not just when letters look smaller and blacker.
What is the endpoint of the plus/minus technique?
The lens in between where adding more plus would cause blur and where adding minus does not improve vision.
What is the basis for bichromatic tests?
They use the eye’s longitudinal chromatic aberration to determine the spherical component of refractive error.
What is chromatic aberration?
The dispersion of light into different wavelengths in the ocular media, causing rays of different wavelengths to focus at different points.
How does the refractive index of the eye vary with wavelength?
The refractive index is greater for shorter (blue) wavelengths, so blue light is refracted more than green, which is refracted more than red.
How do different wavelengths focus in an emmetropic eye?
Green light focuses in front of the retina, red light focuses behind, and both are approximately equal distances from the retina.
How do different wavelengths focus in a myopic eye?
Red light focuses closer to the retina, and green light focuses further away, making targets on the red background appear clearer.
How do different wavelengths focus in a hyperopic eye?
Green light focuses closer to the retina, and red light focuses further away, making targets on the green background appear clearer.
What wavelengths are used in the standard duochrome test?
Red filter with peak wavelength 620nm and green filter with peak wavelength 535nm, relative to a best focus in yellow at 570nm.
How much longitudinal chromatic aberration is produced by standard duochrome filters?
Green focus is 0.25D forward and red focus is 0.25D behind the yellow focus.
What do different duochrome responses indicate?
Emmetrope = equal clarity; Myope = red clearer; Hyperope = green clearer.
How should you adjust lens power based on duochrome responses?
Add minus if red is clearest, add plus if green is clearest, repeat until no difference is seen.
What should you be wary of when performing the duochrome test?
Accommodation and red-preference; stress to the patient not to pay attention to brightness differences.
When should you consider the duochrome test unreliable?
If more than +/- 0.50DS is needed to balance clarity (assuming you did a good job finding the BVS).
What should you do if the patient says ‘they look the same’ at first presentation of the duochrome test?
Confirm by using +0.25DS (red should now be clearest); if no change in response with lens change, move to another test.
What should you do if the clarity changes from green to red with +0.25DS or +0.50DS?
Do not add plus lenses and leave the prescription on green.
What should you do if the clarity changes from red to green with -0.25DS or -0.50DS?
Add the minus lens and leave the prescription on green.
What is the main purpose of the duochrome test?
Used mainly as a check test for spherical refraction, refining focal power, and binocular balancing.
Why is the duochrome test unaffected by color perception anomalies?
Because the patient is instructed to judge the sharpness of focus, not the colors.
