Check Tests Flashcards
Bichromatic Test
The eyes natural Chromatic Aberration can be exploited for refraction purposes.
Duochrome
Red and Green glass filters
Why Duochrome?
This Bichromatic Test is used as a Check Test on the Best Vision Sphere during Monocular Refraction.
After the initial determination of the BVS and prior to the use of the (JCC) - to ensure the Circle of Least Confusion is on the Retina prior to the use of the JCC.
A standard test to help find the spherical refractive error of an eye.
What is Duchrome based on?
It is based on the principle of Axial Chromatic Aberration where light of shorter wavelength (green light) is refracted more by the eye’s optics than light of longer wavelengths i.e. Red light.
It examines the position of the focus of the green and red wavelengths with respect to the retina and theoretically enables precise determination of the spherical correction or of the Circle of Least Confusion (COLC).
Duochrome Test is standardised by the British Standards which:
- Define the Characteristics of the filters used.
- Define wavelengths of light used
- Amount of Longitudinal Chromatic Aberration that is produced.
What is Green filters peak luminosity?
535nm
What is Red filters peak luminosity?
620nm
What is Yellows peak luminosity?
570nm
How to use Duochrome?
If the patient responds that the rings are clearer on the red background, then a -0.25DS lens is added
If the patient responds that the rings are clearer on the green background, then a +0.25DS is added
The objective of Duochrome is to _______
Achieve a balance where both red and green based targets are equally clear.
Myopes will see the circles on the _______ background clearer.
Red
Hypermetropes will see the circles on the _______ background clearer.
Green
Why do Myopes see circles on the red background clearer?
The focus for a low Myope will fall a short distance in front of the retina, hence why the rings on the red are seen more clearly.
Why do Hypermetrope see circles on the green background clearer?
The focus for a Hyperope will fall behind the retina, hence why the rings on the green are seen more clearly.
When conducting the Duochrome test, what should you stress as a Practitioner?
They should not pay attention to the apparent brightness difference.
Procedure for Duochrome:
- Occlude one eye
- Turn room lights off to dilate the pupil, which increases the chromatic aberration of the eye.
- Ask the patient:
“Are the rings clearer and blacker on the red or on the green or are they the same?” - If the rings on the green are clearer, add plus +0.25DS until you obtain balance.
- If the rings on the red are clearer, add minus -0.25DS until you obtain balance.
If patient responds with:
“It Is the same.”
What does this suggest?
This suggests the Best Vision Sphere has been obtained and the Circle of Least Confusion is on the retina.
What can you to ensure you have met your endpoint?
You can check by adding:
+0.25DS – the rings on red should look clearer
-0.25DS for the rings on green to look clearer
If more than ±0.50DS is required to balance, this usually indicates……
That the Duochrome test is unreliable for this patient and should be ignored.
Prior to the use of the JCC, if the clarity of the rings changes from:
Green to red with +0.25DS
or
Red to green with -0.25DS then…..
You should leave the young patient on the green as they will be able to accommodate to bring the Circle of Least Confusion onto the retina.
After the use of the JCC and before finalising the refraction, if the clarity of the rings changes from green to red with +0.25DS or red to green with -0.25DS, note the additional spherical power required to leave a young patient ‘on the red.’
Use the additional lens power suggested by the duochrome test and double-check whether this additional power is preferred by the patient using the plus-minus technique of best vision sphere assessment.
Why smaller and blacker?
If a Myopic eye is slightly over-corrected i.e. too much minus power or a Hypermetropic eye is slightly under-corrected; the test letters generally appear smaller and blacker.
The results from the Duochrome may not always be reliable, because:
Peak wavelengths may vary from test to test.
Relative brightness of red and green may affect reliability.
Chromatic Aberration of the eye decreases with age.
Crystalline lens becomes yellow – older px like red more ~ RED BIAS.
Blue-green light is partially absorbed and scattered, red bias to test.
Colour defectiveness; can still do the test as sharpness of focus not affected only the appearance of the colour.
Older px, due to small pupils and the increased absorption of low wavelength light by the lens particularly nuclear cataract tend to give unreliable duochrome results.
What should be the end-point in Duochrome?
Give maximum plus that provides optimum VA e.g. night drivers.
What is the Cobalt Disc Test?
Simple form of Subjective Test
What does the Cobalt Disc Test do?
It absorbs the middle region of the Visible Spectrum whilst transmitting a proportion of red and blue .
The retinal image of a small white source will be formed by overlapping red and blue blur circles:
If myopic, the red circle is smaller than the blue as its less blurred.
If hyperopic, the blue circle is smaller than the red as its less blurred.
What is the aim of the Cobalt Disc test?
Both blur circles to be equal size.
+1.00 Blur Test
This test is used to ascertain if the patient’s accommodation has been sufficiently controlled during the subjective refraction procedure.
Adding +1.00DS should blur a patient’s vision by 4 lines with average sized pupil.
Some patients find it difficult to relax Accommodation once it has been exerted i.e.
- Psuedomyopes
- Low Hypermetropes
Adding +1.00 DS should Blur a Pre-presbyopic patient’s vision by _______
4 lines with average sized pupil from their optimum VA, if accommodation has been sufficiently controlled.
Adding +1.00 DS should Blur a Presbyopic patient’s vision by _______
3 lines with small pupils, if accommodation has been sufficiently controlled.
Procedure for +1.00DS Blur Test:
- Add +1.00 DS as a separate trial lens on top of the prescription lenses.
- Ask patient:
“What is the lowest line you can read on the chart?” - If the patient does not sufficiently blur back, add more plus to the sphere of the prescription.
What is the final goal with +1.00DS?
A prescription that gives clear vision but blurs back when a +1.00DS is added.
If the patient’s acuity is 6/6 or better, then addition of +0.25DS should blur the line fractionally.
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