How the Generic Optical Glass Code Works
Like all products on the market, manufacturers attempt to differentiate their products by assigning each one with a registered trade name. The military on the other hand has tried not to designate the requisite material for a particular application by a trade name. It has attempted to establish a generic system in its procurement process.
To establish a standard for optical glass and to create a generic designation for optical material that the government might require for an optical system, the military has promulgated different guidelines over the years. Currently, the guideline that defines the grading of optical glass and the generic designation is Mil-Std. 174-B.
Mil-Std. 174-B establishes guidelines for designating optical glasses by their general chemical composition and by a generic numerical code. The following is excerpts from the military standard that is relevant to the generic designation:
3.3.2 Type name. Type name shall be specified by indicating the nonproprietary trade designation, or in its absence of such designation, the characteristic constituent of the glass.
3.3.3 Type number. Type number shall be specified by two sets of numbers separated by a dash. The first set shall consist of three numbers corresponding to the first three round digits after the decimal point of the refractive index of the glass for the sodium D line (589.3 nm). The second set after the dash shall consist of three numbers corresponding to the first three digits of the Abbe constant (see 3.3.5) (e.g., nd =1.511 and vd = 63.5 would be code as 511-635).
3.3.4 Index of refraction. Standard glass shall have an index of refraction correctly identified to the fourth decimal place, and shall conform to the nominal value specified in the contract within the following tolerances: below 1.600, + 0.0010; from 1.600 to 1.730, + 0.0015 and above 1.730, + 0.0020.
3.3.5 Abbe constant. Standard glass shall have the Abbe constant correctly identified to the first decimal place, and shall conform to the nominal value specified in the contract within the following tolerances: below 30, +0.2; from 30 to 45, + 0.3, between 45 and 55, +0.4; over 55, +0.5.
3.3.6 Partial dispersions: Standard glass shall conform to the partial dispersion values specified in the contract.
As a footnote, in the past optical glasses did not have a refractive index higher than 1.xxx. However, in recent years manufacturers have formulated glasses that have indices in the 2.xxx. The procedure cited above in paragraph 3.3.3 assumed that all glass would have a 1 before the decimal place for their refractive indices. To handle the development of refratvie indices starting with a 2 in front of the decimal point, the industry has adopted its own procedure of subtracting 1 from index of refraction. Therefore, if the glass had an index of 2.135 it would be coded as 1.135 (4 numbers) ahead of the vds numeral numbers under the military's numeric code. (for example, nd = 2.135 and the vd = 63.5 than the generic code would be 1.135-635)
In the compendium the - was removed between the nd and vd numericals.
Remember that even though two manufacturers may have glass with the same generic code, they may not be equivalent. The potential user or buyer should request the manufacturers detailed specification sheets on each glass to see if there is a difference between them. There is not warranty express or implied that glasses with the same generic code are equivalent or that they meet all of the requirements of mil-std 174B.
Also, remember that the military has established the minimum standards that optical glasses have to meet. Many manufacturers make products that exceed the minimum specifications established in Mil-Std 174-B. Therefore, one should request the detailed technical data sheets on glasses that are required.
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