Module 18 

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Module 18:

Clinical Optics:

Part 2

Lens Design and Slab-off

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:  Modules 16 and 17 or an intermediate level knowledge of optical principles.

Date of original release:  March 7, 2003

Section 1:

Description of content:  This section discusses how elements of lens design affect the final glasses product and the visual performance for the patient. This information is helpful when evaluating and preventing glasses complaints.  Concepts discussed include:

• Lens thickness

• Lens size

• Lens decentration

• Lens shape

• Lens style

• Lens weight

• Lens material

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

• Name the factors that increase lens thickness

• Describe how lens decentration affects lens thickness

• Explain how to calculate the frame PD

• Calculate the amount of lens decentration needed for a given frame and a given patient

• Define effective diameter

• Explain how lens shape affects lens thickness

• Define saggital value

• Explain how lens style affects sag value and lens thickness

• Explain the effect of the index of refraction on lens weight

Section 2:

Description of content:  This section discusses how slab-off grinding is used to correct unwanted prism caused by anisometropic lens corrections in the reading area of glasses lenses .  This information is helpful when evaluating and preventing glasses complaints.  Concepts discussed include:

• anisometropia

• induced prism

• the power of the lens in the 90 degree meridian

• how a slab-off works

• alternatives to a slab-off

• calculating the need for a slab-off

• a quick estimation technique

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

• describe what a slab-off does

• name the formula used to calculate induced prism

• explain how the formula is used

• construct a power cross from a glasses prescription

• estimate the power of a lens in the 90 degree meridian

• list alternatives to the use of a slab-off

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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