OPTICAL GLASS INDUSTRY—GLAZE 
glass surfaces disappear and the imper- 
fections in the interior become evi- 
dent under proper illumination. The 
blanks are also gaged for conformity 
with specifications, those undersize 
being marked for salvage into smaller 
blanks and those oversize for grinding 
tovesize pune |) rest! are Yready’ for. 
annealing. 
The annealing of optical glass is a 
critical process. Blanks of widely 
different weight require a different 
annealing schedule even though they 
are both made of the same glass. 
Also, each type of glass has its own 
annealing schedule except where two 
glasses are similar in composition. 
The blanks are loaded into an iron 
box on perforated iron trays with an 
air space between the layers. The use 
of the box and trays gives much better 
temperature distribution throughout 
the annealing furnace. The furnace 
is gradually raised to the annealing 
temperature and held at that tempera- 
ture until, from experience, it is known 
that the glass in the furnace is free of 
strain. Now it becomes necessary to 
cool the furnace at such a rate that no 
strain will be introduced because of 
temperature gradients within the glass. 
The cooling rate is very slow at first 
and is gradually increased as the 
temperature drops. If properly an- 
nealed, an optical element should 
show no distortion throughout its life. 
The final process before shipment or 
grinding and polishing is the inspection 
for strain. The blanks are again im- 
mersed in a liquid of index of refrac- 
tion similar to the glass and examined 
with polarized light. If the glass is 
perfectly annealed, the area of the 
glass appears the same as the adjoining 
background. Otherwise, an interfer- 
ence figure is obtained. This inter- 
ference figure is compared with that 
for standard strain samples to deter- 
mine if the annealing is satisfactory 
for optical purposes. If not, the blank 
must be reannealed. 
And so, after about 1 month, the 
batch that was put into the pot appears 
as a satisfactory finished blank, ready 
221 
for grinding and polishing into an 
optical element. In other words, 
there is present in a plant producing 
optical glass a month’s production of 
glass. Also, only about 15 to 18 per- 
cent of the glass melted is satisfactory 
for grinding and polishing; hence the 
high cost of good quality optical glass. 
At the National Bureau of Standards 
28 different types of optical glass have 
been produced with indices of refrac- 
tion ranging from 1.511 to 1.754 and 
with nu-values from 64.5 to 27.7 (see 
fig. 1). The nu-value is the reciprocal 
of the dispersive power of the glass. 
There are three wave lengths that are 
especially important in the measure- 
ment of refractive index, i. e., the 
sodium D line, 5893 angstroms, in the 
central region of the visible spectrum, 
the hydrogen C line (H-alpha), 6563 
angstroms, in the red, and the hydro- 
gen F line, 4862 angstroms, in the blue. 
When the mean refractive index is 
given, that for the sodium D line is 
meant. For each type of glass there is 
a definite relationship between the 
indices of refraction for the three 
spectral lines mentioned. The recip- 
rocal of the’ dispersive power instead 
of the dispersive power itself is used in 
order to avoid fractional values. 
A high nu-value represents a low- 
dispersive power and a low nu-value a 
high dispersive power. 
For glasses of index of refraction 
less than about 1.555, the allowable 
tolerance is 0.001 of index either way; 
for those above 1.555 it is 0.0015. 
The allowable tolerances for the 
nu-value vary somewhat. For nu- 
values above approximately 50.0, it 
is 0.4 to 0.5 either way, depending on 
the type of glass. Below 50.0 it is 0.3. 
The light absorption of all types of 
optical glass except dense and extra 
dense flints and dense barium crowns 
must not exceed 1 percent per centi- 
meter of light path measured in white 
light (generally it is only half of that 
value). For dense and extra dense 
flints and dense barium crowns, 2 
percent is allowed, although it prac- 
tically never exceeds 0.8 percent. 
