|READING RANGE This lens clock is representative of the one you probably have in your office. While most illustrate plus power using black numbers and red to illustrate minus power, this one uses a + and – sign on the respective side of the clock’s dioptric scale. This clock has a reading range from 0.00D to +/-20.00D. Notice that at rest, it does not indicate plano. It will only do that when its three pins are touching a plano surface.|
EACH ISSUE, TECH TIPS WILL EXPLORE SOME INTERESTING ASPECT OF OPTICAL TECHNOLOGY. THIS MONTH WE LOOK AT NOMINAL BASE CURVE.
UNDERSTANDING WHAT THE LENS CLOCK IS TELLING YOU.
The lens clock hasn’t changed in the last 50 years. When you need to know the base curve of a lens, you simply pick it up, touch it to the lens, and read the dioptric value the needle indicates. While this sounds simple, there’s a lot more going on here than some eyecare professionals (ECPs) realize.
It’s important to understand what the lens clock is telling you. The index of refraction the lens clock is designed upon is 1.530. If you were to read a lens made with Trivex® material (which has an index of 1.530), you would get an accurate reading of the refractive power of that surface from the lens clock. That’s because the index of the lens and the index of the lens clock are the same. However, if you read a 1.67 index lens, the surface power reading on the lens clock would be inaccurate because the two indices do not match.
Think about how confusing this could be in today’s optical world that has a slew of lens materials commonly used and a few that are occasionally used. If you want to know the accurate surface power of a lens, the lens clock you use should have the same index of refraction as the lens material you are measuring. When they don’t match, you have to do a little math (or consult a conversion table) to obtain the accurate surface power of the lens from the lens clock’s reading(s). That’s impractical today because we use so many lens materials on a daily basis. Even decades ago when there were only two or three glass lens materials commonly utilized by ECPs, this problem existed.
Notice in Fig.1 below how all the lens materials with indices higher than 1.53 have a higher surface power. This makes a lot of sense since we know that a higher-index lens material creates more power from a surface curvature than a lower- index material does. That’s why higher-index lenses use less curvature and thickness than lower-index lenses do. Notice too that the 1.50 material (CR-39®) has a lower surface power.
LENS CLOCK LENS CLOCK
|Here are samples of the error the lens clock produces when the clock’s index of refraction doesn’t match the lens’ index of refraction.|
To overcome this problem, the optical surfacing world decided to use the concept of “nominal base curve.” Here’s how it works. No matter what the index of refraction of a lens you wish to measure is, you take a lens clock reading of a lens’ front spherical curve—its base curve. Whatever that reading is, that’s the nominal base curve of that lens. In other words, when you take a base curve reading of a lens, you just ignore the fact that its index of refraction is different from the lens clock’s index. Simple, right? Sure it is…for those who work in a dispensing environment. That’s because all you have to do is write down the reading you obtain from the lens clock. Understand that this nominal base curve has to be converted by a surfacing lab in order to surface the lens because the lens’ accurate surface power for its base curve (and the other curves on the lens) is not the same as its nominal base curve power when the lens’ index is different from the clock’s (as illustrated in Fig. 1). In today’s surfacing world where computers calculate and control everything, this is no problem at all.
Nominal base curve is a simple solution to using a single lens clock for measuring the base curve of all lenses, regardless of their index of refraction.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.