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When Physics Meets Fashion

Challenged with wading through a multiplicity of available lens options,
the eyecare professional must be able to distinguish some of the more
innocuous characteristics of lenses.

Sometimes some of the more intangible variances between lens materials can make all the difference in the world. Being able to understand and utilize a working knowledge of refractive index, specific gravity and abbe value can help the optician make better choices for their patients. These and other distinguishing lens traits are the foundation upon which we create our spectacle masterpieces. Sometimes it is important to understand the why? in order to best serve our patients in the dispensary.

Index of refraction, or refractive index, can be defined as the ratio of the speed of light in a vacuum to the speed of light in a medium (lens). The vacuum represents a variable controlled environment in which light has no obstructions, obstacles or meandering to compete with. Under perfect, un-deviated circumstances light travels at an alarming one hundred eighty-six thousand miles per second! In ratio form we see that the speed of light in a vacuum divided by the speed of light in a vacuum (186,000 mps / 186,000 mps = 1 or n=1).

Thus, the number 1 is the basis for all other indices of refraction. When light passes through a medium other than air it is necessarily slowed down and refracted, or bent. The higher the refractive index of a material the easier it can bend light. For example the refractive index of columbia resin-39 is 1.498. Effectively this is stating that light travels 1.498 times faster in air than it does through the CR-39 lens. Likewise, a polycarbonate lens with a refractive index of 1.586 slows light down 1.586 times more traveling through the lens than through the air. This is why higher index lenses of the same dioptric strength as their lower index counterparts are manufactured thinner and more cosmetically appealing; the lens is able to slow the light down and refract, or bend, it faster. Basically - it takes less lens material thickness to get the same desired Rx because the higher indices medium is able to produce a harsher refraction.

Knowing the refractive indices of the various ophthalmic materials is of paramount importance in the dispensary so the eyecare professional is able to determine what the finest lens will be for the patient. With plastic lenses achieving refractive indices of up to 1.74 the industry is able to create thinner lenses than were once ever imagined. The higher the index of refraction, the thinner the lenses will be for your patient; and with lens thickness being mentioned as the number one lens concern of eyewear customers it is extremely important that the optician understands this concept. Then shouldn’t everyone receive high index lenses? Not necessarily. There are other variables and lens characteristics to consider while selecting the proper material for you patient.

Specific gravity is the ratio of the density of any substance to the density of water (water is the standard variable, much like air in a vacuum is the standard variable for index of refraction). Density is a measure of how compact or heavy an object is in a given volume. The higher the specific gravity is of a given lens material generally the heavier the finished spectacles will be. Some common specific gravities are CR-39 at 1.32, polycarbonate at 1.20, Trivex at 1.10 and crown glass at 2.54. We all know that glass lenses are generally heavier than their plastic rivals, and that polycarbonate is lighter than CR-39. This is clearly reflected in the specific gravity values. It is interesting to consider; however, that though Trivex is the less dense (lightest) material per volume (1.10 sg) versus polycarbonate (1.20 sg), a pair of glasses with the same prescription on the two materials will be thicker in the Trivex due to its lower index of refraction (n = 1.53). Having the ability to sort out these lens inequalities and do what is best for your patient is incredibly valuable as a practitioner.

Abbe value, notoriously and affectionately known in the optical field as “I can’t stand these polycarbonate lenses!” is also of principal consideration. The abbe number, which is also known as the (V) number is a measure of a material's dispersion in relation to the refractive index. Higher abbe values indicate lower dispersion and less chromatic aberration. Lenses that have inherently lower abbe numbers have less refractive index consistency throughout the material which translates to a poor visual experience for the patient characterized by color fringes and inadequate visual acuity.

Glass lenses which are optically superior to most plastics have abbe values in the upper fifties, while polycarbonate has an abbe value of only thirty. This is why many of the high end “over the counter” sunglasses are equipped with glass lenses. The optics are generally noticeably sharper. This is not to say that polycarbonate is optically unacceptable, as many patients are imperceptive of the chromatic aberrations. In fact, it is very rare that one will actually cite a material as the reason for poor visual acuity. This is where the eyecare professional must pull from their knowledge base and guide the patient towards the best lens option. Even with polycarbonate’s lack-luster abbe number it still equates for the brunt of ophthalmic lens sales, but maybe that has more to do with its higher index of refraction, lower specific gravity and impact resistance.

One caveat – do not use the words index of refraction, specific gravity or abbe value while selling patients spectacles. Not only will your patients not understand you, but you also run the risk of coming off as arrogant or condescending. Use language that is easily digestable. “Ma’am, have you considered the implications of a lower specific gravity lens?” or, “Your current lenses have an egregious abbe value resulting in increased refractive inconsistencies and color dispersion” just don’t resonate with patients quite as well as “Would you like lightweight lenses with superior optics?” It is OK to secretly be an ophthalmic-science-physics-geek-superhero, just don’t reveal this to your patients!

Sam Winnegrad, MBA, LDO

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