Factors of Lens Design
You will probably get at least ONE question on your ABO on lens curvature and design and the factors that affect it. You might even get a LOT of questions on lens design depending upon which ABO test you are taking. The National exams are different each testing period. Understanding of the product we sell and dispense is essential.
So… what ARE the factors that determine the design of a lens beyond the patient’s actual prescription?
- front curve and index
- front and back curves, and index
- front and back curves only
- front and back curves, thickness, and index
- front curve, thickness, and index
The answer is d: front curve, back curve, thickness, and index
Front and Back Curves
The power of a lens is derived from the combination of its front and back curves. That is Nominal Power as is measured with a lens clock. The front curve and the back curve are added together. Since most lenses today are meniscus lenses, the front curve is almost always a positive value and the back curve a negative value.
A lens with a front curve value of +6.00 and a back curve of -8.00 would give us a -2.00D lens.
It is VERY important for you to know Corrected Curve Theory and be able to determine what the front and back curves should be for any given Rx. There will likely to be questions concerning this. Memorize the tables!
Now, if the Rx contains cylinder, the two principle meridians will comprise two curves, usually on the back surface of the lens. Why is this? Because most lenses today are in MINUS cylinder. You should make sure you know how to interpret a toric cross diagram and know the terms involved.
You also should be aware of the concept of aspheric or atoric lens designs, the advantages and disadvantages of them, as well as how to order and fit these lens designs. It is likely there will be a question on it.
Thickness as a Design Factor
Usually when we think of lens thickness we consider it as a product of the lens design, not a factor in it. I mean… How can the thickness be a factor of the thickness?
However, opthalmic lens designs start out as theoretical concepts, pure optical physics. A lens design can be initially described as a thin lens in air. But that’s not a real lens! Real life is three dimensional. Real lenses have to have a certain amount of thickness between the front and back curves to make the design able to be manufactured and fit to a frame. There are also other factors that determine how much that thickness should be.
Lenses sold in America have to be thick enough to pass the standard Drop Ball Test. That thickness can vary. Many lens materials can pass that test with lower thicknesses than basic CR39 plastic. Lenses inserted into Safety frames often must be made to specific thicknesses. Be sure to read up on your ANSI Z87.5 standards!
Index of Refraction
Index of refraction profoundly affects the shape of a lens. Lenses of higher index material bend light more efficiently. This can cause the lens to be thinner as the curvatures needed to do the desired job can be flatter.
It is important to remember that the index of our lens clock is 1.53 which is not actually in use today. When we measure with a lens clock, we are getting an approximate reading, Nominal Power. We cannot actually neutralize a lens with a lens clock unless we use a conversion formula that takes into account the index of refraction. (Using a lensometer to find the back vertex power is much easier!)
Questions on Lens Design
At www.passyouropticalboards.com there are hundreds of questions on lens design in our database. Some of those questions are basic, others more challenging. By purchasing our special 10 test package you will have the opportunity to practice again and again: finding Nominal Power, calculating base and ocular curves, using the toric cross, applying corrected curve theory, and identifying index.
The best way to study for a multiple choice test is to PRACTICE! The experience is even better when you can learn from every wrong answer!