Absorption

Start by reading Chapter 12 in Systems for Ophthalmic Dispensing. Read through for basic understanding.  We are going to go through some of the sections more carefully, and some we will leave with just basic knowledge.

Early in the first Optical Theory course you learned that one of three things will happen to a ray of light when it travels through a material and enters another material.  Either it will be reflected, it will be refracted, or it will be absorbed.  You have already learned a great deal about refraction, and in the lesson on lens surface reflections you learned (or will learn) what happens to those rays.  We are left with absorption.

When a photon of light is absorbed one of three things happens.  It may be converted into thermal energy, chemical energy, or electrical energy.  Mostly we do not care which one of these three happens.  None of our ophthalmic materials purposely changes light into electrical energy.  The only time we are purposely dealing with chemical changes that are the result of absorption of light is when we are dealing with photochromatic lenses.  In photochromatic materials it is chemical changes in the lens material resulting from the absorption of light (mostly the UV range of the electrical spectrum) that increases the absorption of rays in the visible range.

Systems for Ophthalmic Dispensing pages 329 through page 341 (up to but not including "Antireflection Coatings")  contains basic information on colors and densities (transmission) that is true regardless of the type of material and method used to supply absorption.  You will be responsible for all of this material.

Items to note:

1. The popularity of various colors and densities of color changes with fashion.  There is nothing wrong with this, as long as you, the dispenser or eyecare professional, are aware of the advantages and disadvantages of each.
2. Light tints, which many people consider just cosmetic or fashion tints, can be a hindrance in low-contrast and low-intensity situations.  Many people believe that a light tint helps them with night driving because in fact these light tints decrease the internal reflections from headlights that increase the glare from the headlights.  These light tints decrease the visual acuity and contrast sensitivity for the driver.  Anti reflective coatings increase the low-illumination sensitivity and also eliminate the same internal reflections from headlights, so they have the same positive effect without the negative effect.
3. We see the color of the wave that enters the eye.  So, if we are looking at an opaque surface that is red, that means that red waves are reflected from the surface and most of the rest of the spectrum is absorbed by the surface.  On the other hand, if we are looking at a piece of glass where the light emerging is red, then the red waves were transmitted through the glass and the rest of the spectrum was absorbed by the glass.
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Systems for Ophthalmic Dispensing pages 346 - 357 contains information about photochromatic glass and about polarizers.  You will notice that I skipped 342-345.  That is information about antireflective coatings, which we did (or will do) in another lesson; and scratch resistant coatings, which you may want to read for your own information but which we will not cover in this course.

Polarized lenses have been around for a long time, and many in the optical professions (including me) consider them -- with the appropriate coatings -- to be the best that we have to offer for a sunglass lens.  If you offer polarized lenses and do a decent job of describing and demonstrating them you will sell them.  So pay attention to the concepts that are offered here.

Keep in mind that the plastic photochromatics have been going through serious development since the material on pages 350-351 was written.  Your instructor may ask you to go to a search engine and see what you can find about about the current generation of plastic photochromatic materials and lenses.  If so, or if you want to find out more on your own, a good place to start your search may be the National Federation of Opticianry Schools (NFOS) web link page:  http://www.nfos.org/links.htm

Systems for Ophthalmic Dispensing pages 358-360 involves using the formulas for reflection and absorption.  We will talk about the formulas on page 358 in the next section, Absorption - Formulas -- part b.  You may not be requires to do this part;  find out from your instructor if you will be responsible for the formulas.
 

REFERENCES:
Brooks & Borish, Systems for Ophthalmic Dispensing, 2nd edition, Butterworth-Heinemann 1996, pages 329 - 364.
Fannin & Grosvenor, Clinical Optics, 2nd edition, Butterworth-Heinemann 1996, pages 167-195.
Stoner, & Perkins, Optical Formulas Tutorial, Butterworth-Heinemann 1997 page 142-149.