Fresnel Prisms

 

Avoiding Image Jump

 

 

This 40D prism bends the light ray 20° (degrees). 

This principle is helpful for visual field loss using prisms

to shift images from the blind field into the seeing field.

Figure A

 

 

 

Blind spots are always created at the apex of the prism.  The stronger the prism, the larger the blind spot that is created.

This blind spot creates “image jump”.

Figure B

Fresnel prisms are made up of tiny prisms lined up base to apex on a thin platform of plastic.  Fresnel prisms are thinner and lighter than conventional prisms and bend light in the same manner.  Their optical quality is not quite as good as conventional prisms. 

 

Fresnel prisms are made in two materials. One is a flexible plastic which can

be applied to an eyeglass lens and comes in a variety of powers from 1∆ to 40∆. These are called “Press-On” or temporary prisms. The 40∆ is used to create the Peli Lens configuration.  The other is a rigid plastic material or permanent prisms currently available in powers from  1∆ to 57∆.  The 40∆ and the 57∆ are used in the permanent Peli Lens™ .

 

Example of Image Jump (Blind Spots)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 1

  2

 3

Placing prisms over (and/or under) the line of sight allows simultaneous view of the child and the stop sign. Image jump takes place in a non-critical visual area circled in red.

 

Placing the prisms over (and/or under) central vision allows the use of larger amounts of prism to create more visual field expansion and eliminates image jump.  This is the concept used in the Peli Lens™. 

Picture 1 illustrates a full scene.   

Picture 3 shows how the prism shifts the image of the stop sign into view, a 9° shift, but the blind spot caused by the apex of

the 18∆ prism (the blurry arc in the center

 of the picture) eliminates the child from view.  This is the result of “image jump”.

Text Box:

Picture 2 illustrates a right side visual

field loss that allows the child to be seen,

but cuts the view of the stop sign. 

Text Box:

 Another example is of a 5° image jump

on an InWave™ type lens with 10D prism.  

At the apex of the prism, half of the

image of the little wooden dowel disappears behind the lens on the left.

 

The Peli Lens™ to the right has no image jump or blind spots in the visual field.  This lens design uses a thin profile 40∆ Fresnel prism and shifts images to create visual field expansion of 20º in peripheral, not central view.

Apex

Base

Prism amounts are measured in diopters signified by the symbol D

 

The higher the D amount, the more they bend light. 

 

Thick prisms bend  light further than thinner prisms.

 

Prisms bend light at about 1/2 their D value. (Figure A)

 

Blind spots are created at the prism apex. (Figure B)

This is a simple lesson on optical prisms and how they work.  This knowledge will be useful in understanding the different types of prism glasses for Hemianopia.

How Prisms Work

How Prisms Work

Prism Glasses

History

How Hemianopes See

New Developments

Hemianopic Product Guide

Hemianopia Home

Learning Center

Getting Help

Find A Doctor

How Prisms Work

Prism Glasses

History

How Hemianopes See

New Developments

Hemianopic Product Guide

Estimated Costs

Helpful Hints

Testimonials

References and Links

About Us

Contact Us