Lens Basics

Transposition

To transpose a lens is to rewrite the expression of its powers without actually changing them.

To transpose from one sphere-cylinder form to another:

  1. Add the sphere and cylinder powers together. (If the signs are the same add; if they are not the same subtract.) The result is the sphere power.
  2. Retain the power of the cylinder, reverse its sign and change the axis by 90 degrees.
Sphere
Cylinder
Axis

Basic lens Materials

Hi Index Materials are selected when higher powers are prescribed to reduce lens thickness, reduce overall weight and to enhance the physical appearance of the completed eyewear. As the index of refraction increases, several changes take place. 1) The material becomes softer and a protective scratch resistant coating is necessary. 2) Color dispersion (Abbe Value) is more noticeable so a Anti-Reflective Coating is recommended. 3) The patients vision is greatly enhanced with Anti-Reflective Coating.

Crown Glass - The standard glass material with a refractive index of 1.53.

CR-39 - The standard plastic material. Slightly thicker than standard glass but much lighter with a refractive index of 1.498.

Hi Index Glass - Index of Refraction ranges from 1.60 to 1.90.

Spectralite - Aspheric in design with a refractive Index of 1.54.

Mid-Index - Lighter and thinner than regular plastic with a refractive index of 1.55 to1.57. Available in standard and aspheric lens designs.

Polycarbonate - Index of refraction is 1.586 and is slightly thinner and lighter than CR-39. It is softer than regular plastic and must have a scratch resistant coating on front and back surfaces. Poly will withstand high impacts, has a built in UV inhibitor (380nm) and is a economical high index lens.

Hi Index - 1.60 index of refraction. Available in either standard or aspheric designs. Must have a front and back scratch resistant coating. Recommend Anti-Reflective Coating.

Hi Index - 1.66/1.67 index of refraction. Available in aspheric design. Must have front and back scratch resistant coating. Strongly suggest Anti-Reflective Coating.

 

Lens Measure Gauge

The (+) plus side of a lens is the front curve or base curve. The (-) minus side is the inside or ocular side of the lens.

The base curve of a lens is measured with a lens measure, also referred to as a lens clock.

The base curve reading is taken using the inside numbers or black numbers on the face of of the lens measure. The lens measure is depressed over the front surface of the lens until all three points of the lens measure are touching the lens.

The ocular or inside surface of the lens is also measured with the lens measure. However, the reading is taken from the outside or red numbers on the face of the lens measure.

The cylinder can also be measured by holding the lens measure firmly but carefully against the inside of the lens and rotating the lens or lens measure. The difference between the high and low reading is the total amount of cylinder.

The amount of prism ground into a lens to create a slab off can also be measured with a lens measure. Depress the lens measure carefully against the front surface of the lens, parallel to the slab off line. Make note of the base curve reading. Then place the lens measure perpendicular to the slab off line with the center point of the lens measure right on top of the slab off line. Once again depress the lens measure. The difference will be the amount of slab off prism.

Lens measure gauges should be checked for accuracy periodically. Find a perfectly flat surface, the frame table on your lensometer is usually a good surface to use. Place the three points of the guage on the flat surface, depress the gauge until all three points touch the surface. The dial indicator should read (0) Plano. If it doesn't send it to your lab and they can calibrate it.

 

Lens Thickness Caliper

There are several different kinds of thickness calipers. Basically they pretty much the same. The only difference may be in the calibration of them. Before measuring a lens make sure the indicator is at (0) zero. If it is not at (0) zero, make the necessary adjustment. Center thickness on minus lenses and edge thickness on minus and plus lenses are easily measured. Plus lens center thickness may be easier to read if the optical center is spotted in the lensometer first. In all cases be careful not to slide the gauge over the lens. Depress the gauge, lift it onto the lens and repeat until you reach the point of measurement.