Module 15:  Fresnel Lenses and Prisms

In an earlier module I briefly discussed a young man who needed plano power and 15 diopters of prism in one lens.  Look at the cross-section of that lens:

   Now, consider the difference in the base edge thickness if the lens is large or small -- the size of the lens does not change the amount of the prism, just the edge thickness.

Look at FIGURE 22-24 on page 517 of Systems for Ophthalmic Dispensing.  This is a cross-section of a strong prism -- made up of a series of very short prisms.  Look at all of the thickness that we can eliminate, just by changing that one thick prism into a series of short prisms, all of the same power, stacked one on top of the next.

Read pages 516 - 519 (first column) of Systems for Ophthalmic Dispensing.   Although the Fresnel optics are more likely to be found in an Ophthalmology practice than in a one hour chain store, that is only because many Opticians are not aware of the availability of this product.  It is an inexpensive way to provide the occasional problem solver, and should be available to every Optician.

From the front the press-on prism (like my friend's 15 prism) looks like straight lines, with the base direction perpendicular to the lines.  From the front, a press-on lens will look like concentric circles.

Remember that a plus lens is increasing amounts of prism, bases together, as you progress out from the center of the lens?  And a minus lens is increasing amounts of prism, apex together, as you progress out from the center of the lens?  Well, that is all that is happening here.

After doing the reading assignment above, see if you have any Fresnel optics in your place of business.  If so, ask to practice with one.  If not, ask if you can order one to practice with.  Try making temporary FT40's out of a plus Fresnel lens -- cut it round and 40 mm wide with the OC in the center, then cut it in half and apply it to the back of a pair of old Rx glasses.  Instant temporary bifocals.