Module 17 

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Module 17:

Clinical Optics:

Part 1

Lens Aberrations, Base Curve, and Optical Center

OVERVIEW

Estimated study time:  One hour.

Program Level:  Intermediate and Advanced 

Target Audience:  OMPs wanting a more complete understanding of the optics of glasses lenses.  OMPs studying for the COT and COMT exams.

Pre-requisites:  Module 16 or a knowledge of basic optical principles.

Date of original release:  March 7, 2003

Section 1:

Description of content:  This section discusses the aberrations that are inherent with a glasses lens and how to minimize aberrations, including: 

• Spherical Aberration

• Distortion

• Curvature of Field

• Marginal Astigmatism

• Coma

• Chromatic Aberration and Abbe value

• Magnification and Minification

• Corrective Curve Theory

Instructional  objectives:  Upon completion of  this section,  the student should be able to:

• Name common lens aberrations

• Describe the effects of common lens aberrations

• Describe methods of minimizing particular aberrations

Section 2:

Description of content:  This section defines base curve and discusses how it relates to the ocular curve and the viewing comfort of the patient.  Measuring the base curve and calculating the base curve and ocular curve are discussed. This information is helpful when evaluating and preventing glasses complaints.

Instructional objectives:  Upon completion of this section,  the student should be able to:

• define base curve and ocular curve

• calculate the base curve given the ocular curve and the lens power

• calculate the ocular curve given the base curve and the lens power

• describe how the base curve is measured

Section 3:

Description of content:  This section defines the optical center and discusses how induced prism is created when the optical center is decentered relative to the PD.  Formulas are presented and calculations are discussed for spherical and astigmatic prescriptions.  This information is helpful when evaluating and preventing glasses complaints.

Instructional objectives:  Upon completion of this section,  the student should be able to:

• Define optical center

• Describe how to mark the optical center of a lens

• Calculate the decentration given the PD and the OCD

• Identify the direction of prismatic effect given the lens power, the OCD, and the PD.

• Calculate the amount of induced prism given the lens power and the amount of decentration.

About the Author: Gary Gallimore is a certified ophthalmic technologist who assists with research at the Eye Foundation of Kansas City and works for a large multi-specialty ophthalmology practice in the metro area of Kansas City.   He has taught CE courses for many years on all subjects of ophthalmic technology. 

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