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- Take Care With 'Exact Equivalent' Glasses
Take Care With 'Exact Equivalent' Glasses
- By Eddie Judd
- Published 13 February 2007
- User Articles , Glass and Refractive Index
-
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Introduction
This article is also available in Japanese.
A few years ago the manufacturers of optical glasses started the process of reformulating their glasses to remove arsenic and lead. The driving forces for this programme were environmental and the health issues at various manufacturing stages. To most end users there are few differences between the old and new glasses [and I’m sure no photographer ever suffered from lens poisoning!]
At first glance, most of these glasses appear to be the exact equivalents of the original glasses – for example Schott N-F2 seems to be the same as F2 – after all, they do have the same catalogue glass number - 620364 – which suggests that the refractive indices at the F, d and C wavelengths are the same. Indeed they are; to about 1 or 2 in the 5th decimal place. And that is where ‘exact equivalence’ ends! However, examining the indices in the near UV and the NIR shows that the indices can differ in the 3rd decimal place.
Furthermore, many other properties of the glasses differ significantly: the new formula glasses absorb earlier at each end of the spectrum, and there are massive differences in the thermal properties between the old and new formulations: for example, the dn/dT for Schott SF14 and N-SF14, at the e-wavelength for 20 to 40°C, are 9.4 and 0.4 respectively!
I am sure there are many other properties that will be found to differ between the old and the new formula glasses.
The ZEMAX manual does carry a warning under the heading Obsolete Catalog Data that states: "Because some new glasses may have the same name as old glasses, although the exact composition may have changed, optical engineers need to be especially vigilant about checking the index data predicted by the software against the melt sheets of the glass which will actually be used." How true!
The Schott web site does make a statement along the lines that ‘ if only arsenic has been removed then the new glasses can be considered exact equivalents’. Considering that the majority of glasses have also had lead removed, in my opinion a more appropriate statement would have been ‘the majority of new glasses cannot be considered as exact equivalents of the glasses they replace’.
You will notice that ZEMAX refers to 'obsolete' glasses in the glass catalogs. However, a publication by Schott shows that several of the glasses will not necessarily become obsolete and will be melted in both old and lead-free forms. And, just to ‘help’ matters they are introducing HT (High Transmission) variants in some cases, so, for example, there will be SF6, SF6HT, N-SF6 and N-SF6HT.
Schott have produced a document entitled “Positive List of Optical Glasses – Update December 2006” This is attached to the end of this article.
So, let’s look at some consequences of replacing the old glasses with their lead-free equivalents!
A few years ago the manufacturers of optical glasses started the process of reformulating their glasses to remove arsenic and lead. The driving forces for this programme were environmental and the health issues at various manufacturing stages. To most end users there are few differences between the old and new glasses [and I’m sure no photographer ever suffered from lens poisoning!]
At first glance, most of these glasses appear to be the exact equivalents of the original glasses – for example Schott N-F2 seems to be the same as F2 – after all, they do have the same catalogue glass number - 620364 – which suggests that the refractive indices at the F, d and C wavelengths are the same. Indeed they are; to about 1 or 2 in the 5th decimal place. And that is where ‘exact equivalence’ ends! However, examining the indices in the near UV and the NIR shows that the indices can differ in the 3rd decimal place.
Furthermore, many other properties of the glasses differ significantly: the new formula glasses absorb earlier at each end of the spectrum, and there are massive differences in the thermal properties between the old and new formulations: for example, the dn/dT for Schott SF14 and N-SF14, at the e-wavelength for 20 to 40°C, are 9.4 and 0.4 respectively!
I am sure there are many other properties that will be found to differ between the old and the new formula glasses.
The ZEMAX manual does carry a warning under the heading Obsolete Catalog Data that states: "Because some new glasses may have the same name as old glasses, although the exact composition may have changed, optical engineers need to be especially vigilant about checking the index data predicted by the software against the melt sheets of the glass which will actually be used." How true!
The Schott web site does make a statement along the lines that ‘ if only arsenic has been removed then the new glasses can be considered exact equivalents’. Considering that the majority of glasses have also had lead removed, in my opinion a more appropriate statement would have been ‘the majority of new glasses cannot be considered as exact equivalents of the glasses they replace’.
You will notice that ZEMAX refers to 'obsolete' glasses in the glass catalogs. However, a publication by Schott shows that several of the glasses will not necessarily become obsolete and will be melted in both old and lead-free forms. And, just to ‘help’ matters they are introducing HT (High Transmission) variants in some cases, so, for example, there will be SF6, SF6HT, N-SF6 and N-SF6HT.
Schott have produced a document entitled “Positive List of Optical Glasses – Update December 2006” This is attached to the end of this article.
So, let’s look at some consequences of replacing the old glasses with their lead-free equivalents!