The College of Optometrists

Hi when writing the final RGP specification in the CL fit exam is it necessary to include the peripheral curves?if so how is this calculated?

Also at a mock several friends performed binocular balance with the Humphriss Immediate Contrast method (i.e fogging lens in one eye with immediate contrast of plus lens and then minus lens infront of the other) and were marked down for it. Why is this so? what would be a more appropriate method if binocular balancing is necesary?

This has been delt with before lower down in the posts, I think.

Basically, the spec can be from the fitting lens -if you fit a maxim - then thats what you order.

You judge the overall size by the ride and how it centres over the pupil -so as to minimise pupil flare with the lens slipping off the pupil - so the eye is looking through the periphery.

You can alter the central optic radius in relation to the tear film and lens power. An aligned central curve will have a power similar to the corneal refraction and any deviations from this will usually need changes to the central optic radius, as you already know.

The perip spec is typically 0.8 flatter than the central optic radius with first diameter 0.9 to 1mm wider.

So to design a lens from scratch -

Say the COR is 7.85 and we decide the overall lens diameter to be 9.4mm and we want 2 peripheral curves.

so spec is 7.85, / , / , / 12.25, 9.4 -the last curve is typically 12.25 - which is the radius of the sclera of the average eye (you don't want the lens to dig in there)

The central optic diameter is usually half the ride plus the pupil size - so ride looks like 2mm vertical, pupil is 6mm = 7.00mm

The peripheral diameters are simply overall size - central optic diameter devided by the number of curves you want ie: 9.4 - 7.00 divide by 3 = 0.8mm

so puttin g that all in - we have

7.85,7.00/ ,7.80/ ,8.60/12.25,9.4

The remaining curves are typically 0.8 flatter than each other so: we finish up with 7.85,7.00/8.65,.780/9.45,8.6/12.25,9.4

I really don't know why they were marked down for Humphriss - it is a fine method - maybe Tim can answer that one! =steve

I'd agree whilst it is nice to write out all the curves and understand how to adjust them for better fitting it is not required for the basic level of ability we are testing at PRP. However if you do not write down the manufacturer and lens design then you must write out the back and front curve specifications in full and don't forget about specifying the material.

I can only assume they did the modified Humphries incorrectly.