Australia's Leading Ophthalmic Magazine Since 1975

     Free Sign Up     

Australia's Leading Ophthalmic Magazine Since 1975

     Free Sign Up     
Lenses

Does CR-39 have two refractive indices

31/03/2019
By Steven Daras
Share:
Students frequently believe that CR-39 lenses, despite all being made of the same material, can come in a variety of different indices. STEVE DARAS examines where this confusion stems from.

I often hear students talking about using hi-index CR-39 lenses in their workplace. They say that this is what they have been told by their supervisors, and believe that all resin lenses are some form of CR-39. So, does this material have different indices?

To answer this, we need to look at what constitutes white light and how this affects the refractive index of a lens material. White light is made up of all the visible colours of the spectrum; it does not have a single wavelength. This means that we need to have a reference wavelength to represent the visible spectrum. One wavelength, somewhere in the green-yellow band, needs to be selected to ‘represent’ white light.

The world of ophthalmic optics uses two such reference wavelengths. Some countries, such as the United States, the United Kingdom and Australia, use the helium d line, helium being the source of the light generating the line. The d line has a wavelength of 587.56 nm; it gives a refractive index of nd = 1.523 for crown glass, and nd = 1.498 for CR-39.

"CR-39 does not have two different refractive indices, but the wavelength used to measure the refractive index is different."

The subscript ‘d’ tells us that the d line was used when determining the value. Thus, nd = 1.523 tells us that the speed of light with the wavelength 587.56 nm in a vacuum is 1.523 times faster than its speed in this particular glass material.

The other reference wavelength, preferred in Europe and Japan, is the mercury e line. This line has a shorter wavelength, 546.07 nm, so refractive indices quoted using the e line are slightly higher. CR-39, for example, has a refractive index of ne = 1.502.

CR-39 does not have two different refractive indices, but the wavelength used to measure the refractive index is different. The figures depend on whether the lens manufacturer uses the helium d line or the mercury e line as their reference wavelength.

Competing standards

The Europeans, in supporting a move to the e line as the international reference wavelength, argue 546.07 nm is closer to the peak sensitivity of the eye, and thus the logical choice as a reference wavelength for ophthalmic optics. The colour is chartreuse; the same as the bright yellow-green used in highlighter pens and ‘high-vis’ vests, jackets and other safety clothing.

large leaderboard
advertisement

In reply, the Americans argued that there is evidence to show that peak sensitivity is in the yellow band, closer to the d line’s 587.56 nm. They also point out that refraction is carried out using incandescent light, which is more in the yellow band of the spectrum. Some stock lens packets show both the d and e line values for the refractive index, while some only show one.

On an overcast day, which school road sign is easier to see, the one in yellow or the one in chartreuse?
On an overcast day, which school road sign is easier to see, the one in yellow or the one in chartreuse?

The International Organization for Standardization recently released new standards that make both reference wavelengths acceptable, while also suggesting that this may be reduced to one in time.

A betting man would wager on the e line winning this battle due to the eye’s peak sensitivity being the more logical argument. New refracting equipment has moved away from the older incandescent lighting, with modern systems utilising separate hi-resolution digital screens for projecting eye charts.

You may have also noticed the replacement of most of the older yellow signage around pedestrian road crossings with newer high-vis chartreuse signs. The new signs stand out in all weather conditions without the need for additional illumination.

So, what does this mean to us? It means that while both reference systems are in place, we need to be aware that common lens materials from different manufacturers may have a slightly different refractive index values shown on their packaging and information. For example, it is incorrect to assume that a CR-39 lens made by a European or Japanese manufacturer has a slightly higher index (ne = 1.502) than one made by an American manufacturer (nd = 1.498).

CR-39 has only one formula, and therefore only has one refractive index. Most people in our field agree to simplify this to CR-39 as having an index of approximately n = 1.5.

In answer to the title question: CR-39 has only one refractive index, it just depends what reference wavelength was used to calculate it. Despite what you may have heard, there is no such thing as a mid or high index CR-39 lens material.

There are two accepted wavelengths used to represent white light.
There are two accepted wavelengths used to represent white light.

Face A Face
advertisement





GLAUKOS
advertisement
Editor's Suggestion
Hot Stories

rectangle
advertisement


OR
 

Subscribe for Insight in your Inbox

Get Insight with the latest in industry news, trends, new products, services and equipment!