Hi, does anyone know of a formula to calculate oblique cross prisms as you you would calculate oblique cross cyls which normally occur with a over refraction?
Hi, does anyone know of a formula to calculate oblique cross prisms as you you would calculate oblique cross cyls which normally occur with a over refraction?
Split them into their horizontal and vertical components and they are additive.
If I understand the question, and Harry's answer, you would consider each of two prisms as separate, resolve each into horizontal and vertical components, then add the horizontal to horizontal, and vertical to vertical. You could then compound the resulting prisms into a single base orientation.
That is correct, if one eye has BASE IN and the other eye BASE OUT that would cancel each other out if same power. If different power then you subtract each other.
In terms of Oblique Prism, that would be measuring Horizontal Prism and Vertical Prism.
In phoropters we are able to measure horizontal and vertical prism separately.
When I prescribe prism, I neutralize the vertical prism first, using the lowest as possible prism first, and then I try to find the lowest amount horizontal prism.
Then I split the prism in both eyes.
For example, during the trial frame testing I find the patient needs 4 UP OS and 6 BO OS.
I would split this way
OD: 3 BO and 2 DOWN
OS: 3 BO and 2 UP
Of course if you find vertical prism split 2 UP OD and 2 UP OS, that would yield relative no prism.
Thanks that as I thought
Rob
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