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Module 17 Section 2 |
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Module 17: |
Clinical Optics Part 1 |
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Section 2: |
Base Curve | |||
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The base curve of a spectacle lens and its affect on the wearing comfort of the patient is one of the most ignored and least understood principles of refractive optics.
In order to understand the concept of the base curve, it helpful to understand how a glasses lens is made. Although some labs make lenses in a mold, most lenses are ground from a lens blank. The blank has a front surface curve that is not changed in the fabrication process. The prescription is made by cutting away material from the back of the blank to form one (sphere) or two (cylinder) "ocular" curves.
This is called "minus cylinder form". The back surface minus curvature allows the lens to fit closer to the eye and maintain a more constant vertex distance as the eye rotates. The slightly plus or flat front curve creates a cosmetically pleasing profile and forms a edge that fits well into the frame. Lens blanks come with a range of front surface curves. Lens manufacturers apply mathematical formulas according to a "Corrected Curve Theory" that computes the optimum front curves and back curves for a given prescription. The idea is that there is an optimum combination that will minimize lens aberrations.
There is more than one definition of the base curve:
Lens blanks are generally made with base curves in two diopter increments from plano through 10 D (plano, 2, 4, 6, 8, 10). A given prescription can be ground in several combinations of front curves (base curves) and back curves (ocular curves).
Take for example a +4.00 D sphere:
Each of the following base curve/ocular curve combinations would yield a +4.00 D power lens. Algebraically add the front curve value and the back curve value (+8-4 = +4).
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So how do you decide which combination is best? The Corrected Curve Theory tells you which combination is optimal to reduce aberrations and give the patient comfortable viewing. Generally, minus lenses will have base curves from plano (high minus) to 4 D (low minus). Plus lenses will have base curves from 6 D (low plus) to10 D (high plus).
If the Corrected Curve Theory gives the optimum curve combination for a given prescription, why might a patient have an adjustment problem when the base curve is changed? Every glasses lens distorts the image as compared to the view of the emmetropic eye. It is the back curve (ocular curve), not the base curve, that affects the patient's view of the world. The patient gets used to this distortion and it becomes the patient's "normal" view of the world. If the back curve of a new prescription is changed significantly from the back curve of the old prescription, the patient complains of distortion. The patient might not use this terminology. The patient might simply say that the new glasses "just aren't right." To eliminate this problem, try to keep the new back curve within one diopter of the old back curve.
Realize that, depending upon the Rx change, the back curve may change even if the base curve is kept the same, and it may be necessary to change the base curve in order to keep the back curve the same. When evaluating glasses problems, remember that it is the ocular curve (back curve) that matters most. You will need to be able to read the base curve and calculate the ocular curve.
The ocular curve is found with the following formula:
Ocular Curve (OC) = Lens Power (P) - Base Curve (BC) Values in diopters
Example: Lens power = +2 D, Base Curve = 8 D
Ocular curve = (+2) - (+8) = -6
Example: Lens power = -4.00 D, Base Curve = 4 D
Ocular curve = (-4) - (+4) = -8
Example 1:
Mrs. Kadidlehopper comes into the office wearing a +4.00 diopter lens. Her base curve measures 8 D. She has an early cataract and her prescription is changed to +2.00. Next week she comes back complaining of slanting lines when reading.
Her glasses are checked and the new prescription measures +2.00 D with an 8 D base curve. We can calculate the ocular curve of the old prescription to be -4 D (4 - 8 = -4). We measure the base curve of her new glasses lens and it is also 8 D, just like her old glasses. We calculate the ocular curve on the new lens to be -6 D (2 - 8 = -6).
So, Mrs. Kadidlehopper's base curve was kept the same, at 8 D, but we have learned that it is the ocular curve that really matters with Mrs. K's perception of the world, and that changed from -4 to -6. Could this two diopter change be causing her problem? Probably. It would be worth remaking the lens to find out.
How would we keep the ocular curve the same, at -4 D? Of course there is a formula for that too, and it is similar to the ocular curve formula:
Base Curve (BC) = Lens Power (P) - Ocular Curve (OC)
In our example, we want to keep the ocular curve at -4, but we are changing the lens power to +2. What base curve will we need to change to?
BC = (+2) - (-4) = +6 So, we will be changing the base curve from 8 to 6 in order to keep the ocular curve the same. Since the lens manufacturer may use an 8 base curve for a +2 D lens power, you would need to make a note on the Rx that you are requesting a 6 D base curve.
Example 2:
Mr. Guy Wire has had cataract surgery on his right eye. Before surgery he was a +1.00 D hyperope in each eye. After surgery he became a -1.00 D myope in his right eye. After receiving a new right lens in his glasses, he comes back into the office complaining that the glasses "don't seem right".
A re-refraction confirms his prescription, the optical centers of the lenses match his PD, and the lens was ground to the correction. A check of the old and new base curves reveals the following:
Old right lens: Rx = +1.00 BC = 6 D OC = -5, change to: New right lens: Rx = -1.00 BC= 4 D OC = -5
At first glance it may seem that a change in the base curve may be causing his problem. Remembering that it is the ocular curve that matters most, we calculate the ocular curve for each lens:
Ocular curve (OC) = Lens Power (P) - Base Curve (BC)
Old right lens: P = +1.00, BC = +6.00 OC = +1.00 - (+6.00) = -5.00 D
New right lens: P = -1.00, BC = +4.00 OC = -1.00 - (+4.00) = -5.00 D
So, the ocular curve in each lens is the same. It appears that someone has changed the base curve to keep the ocular curve the same, a procedure that should help the gentleman adjust to his new glasses. In this case the base curve change is probably not contributing to the patient's problem.
A more likely cause for his difficulty would be the mismatch of having a plus lens in front of one eye and a minus lens in front of the other eye. The plus lens causes a magnification of the image on the retina of the left eye, and the minus lens causes a minification of the image on the retina of the right eye, making it difficult for the brain to fuse the images.
What can be done for this patient? In terms of a glasses prescription, not much more, except perhaps slab-off grinding if the patient wears a bifocal and is complaining of reading difficulty. If the patient needs cataract surgery in the left eye, then the outcome could be adjusted so that both eyes are slightly nearsighted after surgery. If that is not a option, contact lenses could be used to minimize the retinal size difference. With such a minimal correction in each eye, this patient may do best by only wearing glasses to read.
How To Measure the Base Curve
The base curve of a glasses lens is measured with a Geneva Lens Measure, sometimes called a "lens clock".
The lens clock has an indicator that points to plus (black) or minus (red) diopter powers. It has three prongs that are placed on the lens surface. The center prong moves the indicator hand and is placed on the optical center of the lens. The diopter power of the base curve is read from the dial. Since the curve on the front surface will always be either positive or flat, you will read the black numbers on the dial. The prongs are placed in a perpendicular fashion on the front surface of the lens to measure the base curve. When reading a multifocal lens, the prongs of the lens clock are placed horizontally with the center prong placed over the optical center (just above the line on a flat-top bifocal).
The clock can be checked for calibration by placing the prongs on a flat surface. The clock should read zero in this position. Computing ocular curves is made easier by using the calculator provided with the Clinical Optics Calculator.
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