refractive index

[achilles]

:hammer: Ahoi,

Can some tell me the difference between refractive index d line en refrective index e line.

Thanks for the help.


Achilles

[Darryl Meister]

Hi Achilles,

The refractive index of a given material is the ratio of the speed of light in air (or a vacuum) to the speed of light in the material. Different colors (or wavelengths) of light actually travel at different speeds through transparent media. Blue light, for instance, travels slower than red light. Consequently, the refractive index of a material actually varies for each color of light. This is also why prisms and lenses disperse white light into its component colors; since each color has a different refractive index, they are all refracted (bent) differently.

When manufacturers state the "refractive index" of a lens material, they have to pick one particular color (wavelength) to measure. Ideally, they want to select a standard color that is close to the center of the spectrum, relatively easy to isolate and measure, and near the peak sensitivity of the eye. One common way to do this is to "excite" a gas, such as helium or mercury, that produces some discrete wavelengths of light close to the center of the visible spectrum, which can then be measured. When helium is excited, it emits several colors, including a yellowish color at 587.6 nm. This is referred to as the "d line." When mercury is excited, it emits several colors including, a greenish color at 546.1 nm. Some countries, such as the United States, use the helium d line to measure the refractive index of lens materials. This is our reference wavelength. Other countries, such as Japan, use the mercury e line. (Years ago, a sodium line was used.)

A given lens material will have a slightly higher refractive index when measured with the mercury e line than when measured with the helium d line. Consequently, it is important to know which reference wavelength was used when the refractive index is stated for a material. For lenses with lower Abbe values, which disperse light more, the differences between the two refractive indices are more significant. There was a move towards a single reference wavelength for all countries a few years ago. However, it is rather expensive to convert the entire industry over to a different wavelength, so most countries could not agree on a single one. The effort was later dropped indefinitely.

This does have some practical implication for dispensers and laboratories, though. For instance, some automatic lensmeters need to be set to the correct wavelength to provide accurate results. Also, products that have a stated refractive index based upon the mercury e line need to be relabelled or recomputed.

Best regards,
Darryl

[achilles]

Thanks for the reply

Indeed the e line is often the index for lenses made "in" japan

So some companies are selling, in Belgium , lenses with the index 1.60 , but this is than a index 1.58 in the d line ?? Or not what we refere as an higher index lens

Once again thnks for your help.

Kind regards

Achilles :cheers:

[David Wilson]

Achilles,
Darryl has given another great explanation. As an aside to what Darryl has said, the Europeans also prefer the mercury e line and you will note refractive indices from European countries with the subscript e after the n. There is (and has been for some time) a debate within the International Organization for Standardization (ISO) between the followers of the e line and the followers of the d line (sounds like a cult war). The ISO wants a single reference wavelength but has been unable to reach any agreement. The current ISO standards say that both are acceptable. This is also true in the ISO standards for focimeters, which, as Darryl points out, must be calibrated to a particular reference wavelength, as do the test lenses used to calibrate all focimeters.
The 'lines', incidentally, refer to the telltale black lines first found in a close examination of the solar spectrum by Fraunhofer. As a result they are referred to as Fraunhofer lines and it was he who named them after letters of the alphabet (both lower and upper case).
Regards
David Wilson

[Darryl Meister]

Hi David,

Good stuff, as always. We should probably clarify that these elements don't actually produce "lines" of color. The line is actually the image of a slit that the light passes through before it is dispersed or diffracted by the prism or grating being used to observe the spectrum.

Also, when an element absorbs a certain color it creates a dark line, and when it emits a certain color it creates a bright line. The color (or wavelength) of these lines is related to the energy levels of electrons orbiting the nucleus within the atom of the element. When an atom absorbs energy (a photon), electrons are "bumped" up to higher energy levels (or orbitals). When they drop back down to the lower energy levels, they can release the same amount of energy in the form of a photon. The atoms of each element have a particular configuration of energy levels and electrons, which gives the elements their characteristic discontinous spectrum of colors when excited.

Best regards,
Darryl

[David Wilson]

Hi Darryl,
Thanks for making clear my rather brief and potentially misleading comment. The whole reference wavelength thing sounds a bit esoteric to many, but manufacturers are well aware of the consequences for the eventual 'loser' in the ISO debate. You referred to some of the consequences above and I have heard Daniel Torgersen speak of this issue in the past.

It is interesting to follow the arguments used by both sides. The Europeans claim that the mercury e line, at 546.07nm, is closer to the eye's peak sensitivity for photopic vision of 555nm (our peak for scotopic vision is an even shorter 507nm). The Americans (and Aussies) would argue that the helium d line, at 587.56nm, more closely represents the lights used in optometry.

The current ISO standards permit both reference wavelengths, reflecting the current stalemate, but they do indicate a desire to eventually have only one.

Given the forces behind both sides, I can see this going on for some time. Mind you, the combined forces of ISO optical committee were unable to have dioptre (diopter) registered as an SI unit. The protectors of SI units arguing that it is merely the reciprocal metre and that the reciprocal metre, as an existing SI unit, is perfectly suitable.

Regards
David

[Rich R]

Wow, what an interesting subject, for all the years I've been in the optical field this is the first time I've heard this, makes me think there's always so much more about optics to learn about.
Rich R

[David Wilson]

Hi Rich,
Join the club. If you think you know it all, then you don't. Anyway, how boring would it be if there were nothing to learn. I'm sure that, desipte his obvious knowledge of the field, Darryl would agree.
Welcome to the 'oasis', Rich.
Regards
David

[Darryl Meister]

Originally posted by David Wilson
Hi Darryl,
Thanks for making clear my rather brief and potentially misleading comment.

Don't worry, I make the exact same assumptions when I talk about Fraunhofer lines and that kind of esoteric stuff (and did so in my earlier post). Sometimes we just take stuff like that for granted.

Countries may argue over the scientific merits of choosing one reference wavelength over the other, but it really comes down to economics. It just costs too much to switch.

Interesting to hear about the SI. Most of the SI's units are derived units that have been formed from the more fundamental base units (such as length, time, mass, charge, etcetera). However, I guess you could argue that the diopter really isn't a derived unit, since it is simply equal to a reciprocal meter. But, hey, they gave Hertz to the reciprocal second! I don't know that the term "diopter" is used much outside of ophthalmics though. Maybe they think we just made it up! ;)

Best regards,
Darryl

[Darryl Meister]

Originally posted by Rich R
Wow, what an interesting subject, for all the years I've been in the optical field this is the first time I've heard this, makes me think there's always so much more about optics to learn about.

Strangely enough, you probably hear more about Fraunhofer lines in chemistry and astronomy than in optics! ;)

David is exactly right, though... The more you learn about this stuff the more you realize that there is so much more to learn. But that's what keeps it interesting.

Anyway, I'm going to go finish watching Soylent Green for the night. A good film for anyone not aware of the dangers of our exponential population growth and limited natural resources.

Best regards,
Darryl "Soylent Green is People" Meister

[Jim G]

[QUOTE]Originally posted by Darryl Meister
[B]Hi Achilles,

This does have some practical implication for dispensers and laboratories, though. For instance, some automatic lensmeters need to be set to the correct wavelength to provide accurate results. Also, products that have a stated refractive index based upon the mercury e line need to be relabelled or recomputed.

I'd like to reinfore Darryl's quote (above) in his first posting. I believe that when ordering some automated lensmeters, the default is the mercury-d. When used in the U.S., all your reading would be off.

Caveat emptor.

[sandeepgoodbole]

Originally posted by Darryl Meister
Hi David,

Good stuff, as always. We should probably clarify that these elements don't actually produce "lines" of color. The line is actually the image of a slit that the light passes through before it is dispersed or diffracted by the prism or grating being used to observe the spectrum.

Also, when an element absorbs a certain color it creates a dark line, and when it emits a certain color it creates a bright line. The color (or wavelength) of these lines is related to the energy levels of electrons orbiting the nucleus within the atom of the element. When an atom absorbs energy (a photon), electrons are "bumped" up to higher energy levels (or orbitals). When they drop back down to the lower energy levels, they can release the same amount of energy in the form of a photon. The atoms of each element have a particular configuration of energy levels and electrons, which gives the elements their characteristic discontinous spectrum of colors when excited.

Best regards,
Darryl

In India, we use Pink coloured glass known as Sp2. Since last about 18 years I was wondering if that name has some thing to do with absorption of frequencies realting to Electron Orbit named as Sp2. I had stopped asking this question 17 years ago.