A. METALLIC FLUORIDE|
3A1. Purpose. Standards of quality and performance have been set up for evaporated metallic fluoride or other similar films which are
applied to optical elements to improve the
quality of optical images by reducing reflection
from, and increasing transmission through them.
3A2. Type of film. The reflection-reducing film
discussed is the metallic fluoride film produced by
evaporation in high vacuum. Other films include
those produced by chemical bleaching, acid etching, or by any means other than evaporation.
3A3. General requirements. a. Film thickness. The optical thickness of the film on each
treated surface should be 1/4 wavelength of green
light (5461 Angstroms). This thickness may be
considered as attained when a ray of light striking the treated surface at an angle of incidence
between 0 degrees and 30 degrees appears purple in the reflected ray.
b. Color variations over a treated surface.
Uniform graduations in the purple color-range
from purple-red purple to purple-blue purple over
the treated surface of an individual optical element are permissible.
c. Appearance. The film must not show evidence of crazing, spalling, or cloudiness which
might result in the scattering of light. Blotches,
spots, streaks, or other defects of dirty glass surfaces must not be present.
d. Durability. The film on each surface of an
individual optical element must be sufficiently
hard to withstand, without chemical or mechanical deterioration, the handling and cleaning required during the assembly or subsequent
overhaul of an instrument. This requirement may
be considered as fulfilled if the films remain unchanged after cleaning by washing in soap solutions and organic solvents (acetone, grain,
alcohol, and so forth), followed by drying with
lens tissue or a soft cloth.
e. Cemented surfaces. A glass surface which
is to be cemented to another glass surface is not
f. Samples for approval. Before making deliveries under any contract, sample coatings on
flat glass squares or disks 2 inches on the side or
diameter, and not less than 1/8 inch thick are
furnished to the Bureau of Ordnance for approval
of quality and workmanship. The samples submitted are representative of the types of films for
the range of indices of refraction of the glasses to
be used in the contract.
3A4. Detail requirements. Films must be smooth
and free from visible minute particles of solid
When the green light of wavelength 5461 Angstroms is incident upon a treated surface at an
angle of incidence between 0 degrees and 30 degrees, the percentage of reflected light should not exceed the
values shown in the following table:
|Index of Refraction (nd)|
of Material to Be Coated
Pure magnesium fluoride is used for treating
surfaces of elements which in an instrument are
the exterior surfaces of the optical system. For
treating elements which comprise the interior optical surfaces, other fluorides, mixtures of fluorides, or other materials may be used at the
discretion of the manufacturer. All films are
hardened by baking or other equivalent processes.
3A5. Inspections. To achieve the best results
possible from the treatment of optical surfaces
to reduce reflections, each optical element so
treated must be inspected.
A lamp and shield are used as a light source
in determining film thickness by noting the color
of the light reflected from the treated surface.
Small apertures (slits, crosses, or circles) cut in
the light shield provide an easy means of separating the images reflected from the two surfaces of
an optical element.
An optical element which, before treatment,
meets the requirements of all other applicable
specifications as to freedom from scratches or
other imperfections, is not rejected after treatment if fine hairlines and minute specks become
visible as a result of the coating process. However,
scratches or other film defects which under
conditions of normal use, are visible in the field
of view of the assembled instrument, are sufficient cause for rejection of the element or elements involved.
B. HANDLING TREATED OPTICS|
3B1. Detecting treated optics. The nonreflecting
properties of treated optics are derived from a
thin film of metallic fluoride deposited on the
glass as a surface layer. This film is not so hard
as the underlying glass, and since it is only four-millionths of an inch thick, it does not withstand
Before an optical instrument is disassembled,
it is examined to determine whether any elements in the system have been treated with nonreflecting films. This is done by observing the
elements of either the entrance or exit pupils of
the instrument and noting whether the light reflected is a distinct blue or deep purple color.
Such colors indicate that some of the elements,
but not necessarily all, have been treated, and
therefore require more care in handling.
3B2. Preventing damage. To prevent damage to
these films, avoid handling or rubbing as far as
possible. As the instrument is disassembled,
place the elements on clean paper or cloth, not
on dirty tables or benches which may be covered
with minute, gritty, abrasive particles. As each
element is removed from the instrument, examine
it to determine whether it has been coated by
noting the color of the light reflected from the
surface. Ordinarily, there is a thin uncoated rim
about 1 mm wide around the periphery of
the lens, from which the reflected light is white
Treated optics must be handled carefully if
the nonreflecting properties are to be retained.
Never use a rouge pad for cleaning because it
completely removes the film; use only a soft,
clean cloth wet with a solvent, or lens paper.
Paper or cloth used previously and left on a table
or bench must never be used, as grease and grit
which have been picked up leave oil streaks across
the lens and scratch the film.
Do not attempt to remove scratches. Scratches
in the film mean that the film has been removed,
thereby exposing the underlying glass. No
amount of rubbing or polishing can restore the
film. In fact, efforts to remove these scratches
only result in more scratches or even in complete
removal of the film. Therefore, avoid rubbing
and polishing coated lenses as much as possible.
Scratches do not impair the usefulness of the
instrument in service, as they are not noticeable
to the observer.
3B3. Cleaning treated optics. Organic solvents
such as alcohol, benzene, ether, and acetone do
not injure the film, and can be used in cleaning if
they are pure and do not leave a residue of gum
or grease on evaporation. Cold water and cold
dilute acids or salt solutions do not injure the
film if the film is not soaked in such solutions for
long periods. Do not boil the lenses in water or
dilute acids since this treatment not only softens
and sometimes removes the film, but may cause
an uneven etching of the glass.
Surfaces which are to be cemented are never
coated; only the outside air-to-glass surfaces are
treated. Heat does not harm the film, and lenses
therefore can be heated on hot plates for uncementing and again for recementing. Excess
balsam from the cementing operation is removed
with a clean cloth wet with a solvent. It is neither
necessary nor desirable to scrub the lens vigorously to remove the balsam. If sufficient solvent
is used, a few gentle sweeps across the surface
cleans the lens.
Lenses are now appearing in service which have
been cemented with a new type of cement that
is thermosetting rather than thermoplastic. The
procedure for uncementing these lenses is different from that for Canada balsam.
Equipment needed for uncementing these special lenses is a hot plate capable of attaining temperatures of 500 degrees F, special glass cloth, and a
pair of chenille or heat-resistant gloves (asbestos
glow are unwieldy and are not recommended).
The glass cloth as supplied is covered with invisible oil film. It is to be heated on the hot plate at
about 500 degrees F until all the oil has been evaporated.
The removal of oil is evidenced by the change
in appearance of the cloth upon heating. A change
from white to a scorched tan and thence to a
greyish white is noticeable during the heating.
The glass cloth and gloves are supplied to optical
repair activities by the Naval Gun Factory.
Lenses to be uncemented are placed on the
glass cloth on the hot plate after the latter has
reached a temperature around 500 degrees F. After
about 5 minutes, the cemented elements separate.
The separation may be evidenced by the sound
of the splitting or by reflection of light from the
new interface. If the elements have not completely separated, a slight tap on a wooden surface completes the separation. Generally, the
remaining cement may be stripped by hand from
the lens element. However, if this is not possible,
the remaining cement may be removed by playing a stream of hot distilled water on the cement
from a wash bottle. The uncemented lenses are
recemented by standard procedures with Canada
SPECIAL PRECAUTIONS. If the hot elements are picked up with cold gloves, cracking
may result. Therefore, the finger tips of the
gloves should be laid on the surface of the hot
plate for a few seconds before picking up the hot
lenses. Also, air drafts are known to crack lenses,
so all operations should be performed in relatively quiet air.
Stains from finger marks and Prussian blue do
not injure the film and can be removed with
solvent. It is best to remove such stains while
they are fresh, since they are more difficult to
eradicate without injury to the film when they
have thoroughly dried.
In summary, the best way to preserve the
nonreflecting properties of the film is to avoid handling and rubbing the elements. If rubbing is necessary, rub gently with little pressure. Use a clean
cloth or lens paper, and rub no longer than necessary to clean the element.
3B4. Determining thickness of films. Samples of
magnesium fluoride reflection-reducing films
tested by the Naval Gun Factory for compliance
with Ordnance Specification 1357 have invariably
been too thick. Similar samples submitted by
several Navy yards also were too thick. In terms
of color, the films are too blue when viewed in
reflected light. The tendency to make blue films
is greatest when coating glasses which have low
indices of refraction, such as common crown or
plate glass, and borosilicate crown glass. Such
low-index glasses are used as telescope windows,
sealing plates, clear rayfilters, prisms, and the
positive elements of doublets. Even highly
skilled filming operators may overcoat low-index
glasses. The error arises from the fact that the
colors produced are not distinct and cannot be
matched with any colors which appear in the
Munsell Color Charts.
However, the colors produced on high-index
glasses (commonly called flint glasses) are
quite distinct and it is easy for a filming operator
to follow the color changes as the film is being
made. The negative elements of most doublets
are usually made from high-index glasses.
It is recommended that the following instructions be followed by naval activities engaged in
the filming of optics to produce films which provide the greatest benefit:
1. When in doubt as to the proper color of
film, always make the film too thin rather
than too thick. In terms of color, this means
that the film should be too red rather than too
blue. This is in accordance with paragraph C-1
of Ordnance Specification 1357 which states that
the most satisfactory films will fall in the
color range (Munsell System) from purple
to purple-red purple.
2. Since the colors are most pronounced on
high-index glass, a rule should be made that at
least one piece of high-index glass should
always be placed under the bell jar for coating simultaneously with a group of low-index glass. The high-index sample is to be used
as the test piece for judging the correct color.
This can be done most conveniently by organizing the procedure so that a negative element of
one of the lenses of the instrument itself is used
as the test piece since they are invariably made of
3. Errors sometimes result in a lens being
given a double coating. The color of the reflected light is then green. The reflection from
such a surface is almost as much as from an
uncoated surface and the lens should not
be used in this condition.
This condition may be remedied by coating
the element a third time until the color returns
to purple-red purple. Such a triply coated lens is
less efficient in reducing reflection than one
which has been coated properly. The procedure
described is recommended for use by those
activities which do not possess facilities for
Do not attempt to remove a film by hand
polishing with a rouge pad under any
4. Reports indicate that the outside surfaces
of many coated binoculars have been badly
scratched by the indiscriminate use of gritty
cloth in cleaning. Do not recoat such binoculars unless all of the damaged film can be
removed. Some benefit is obtained as long as any
film remains. A second coating over the damaged
film does not improve the appearance and the
over-all reflection may actually be increased.
3B5. Removing damaged films of treated optics.
No satisfactory field method of removing
damaged films was known previous to the suggestion of the following method by the Navy
Yard at Puget Sound, Washington.
The materials required are: 1) a hot plate, 2)
any dish which is not attacked by hot concentrated sulfuric acid, such as a Pyrex glass beaker,
porcelain casserole, or nickel-chrome alloy dish,
3) sulfuric acid, concentrated, specific gravity
approximately 1.84, and 4) boric acid. All of
these materials should be available at repair
A solution of 2 ounces of boric acid per liter
of concentrated sulfuric acid is prepared. The
optics from which the film is to be removed are
immersed in the cold solution to prevent breakage. The solution is then heated to approximately
100 degrees to 110 degrees C and maintained at that temperature for 30 to 45 minutes, which is sufficient
to remove the film completely.
Higher temperatures are not dangerous, but
the proper temperature may be determined by
observing white fumes which appear at approximately 100 degrees C. The fumes are slightly acidic but
not sufficiently corrosive to require a fume hood
for safety. At the end of the acid treatment,
the lenses should be drained, cooled, and rinsed
in methyl alcohol or denatured alcohol. Acetone,
benzene, carbon tetrachloride, or other low
boiling point alcohols, may be used in the absence
of methyl or denatured alcohol. A thorough
washing with water and subsequent drying
should follow the alcohol rinse. If no organic
liquids are available for the first rinse, the lenses
are preferably left in the acid to cool. Each lens
is then removed individually and rinsed quickly
and thoroughly in a copious stream of cold
water after which it is immediately dried. Care
should be exercised in handling the highly corrosive solution to avoid burns resulting from
spilling or breaking of containers.
The optics should be cooled gradually to prevent breakage. Water should never be allowed
to come in contact with the concentrated sulfuric
C. VACUUM COATING APPARATUS, MARK IV, TYPE II|
3C1. Operation of vacuum coating apparatus,
Mark IV, Type II. These instructions indicate the
procedure used in the coating of optical elements
in their correct order and are to be followed in
the operation of the vacuum coating apparatus,
Mark IV, Type II. The machine should be
charged with clean optics and the bell jar
secured in advance of the following operations:
1. Close the vacuum valve located between
the diffusion pump and the mechanical pump.
2. Close the air intake valve.
3. Turn on the thermocouple gage switch
and adjust the input meter (tower meter) to the
value indicated for the instrument.
4. Note the reading of the output meter
(upper 0-200 microammeter) while the system
is at atmospheric pressure.
5. Start the mechanical pump.
6. Open the vacuum valve between the
diffusion pump and the mechanical pump.
7. Observe the reading of the thermocouple
output meter and turn on the diffusion pump
when this reading reaches plus 100 microamperes. (Diffusion pump switch: upper position
is high heat; lower position is low heat.) If high
heat is used, switch to low heat immediately upon
obtaining a reading above 100 microamperes.
8. The heater switch is ON in the lower position and the temperature is controlled by the
variac marked Heater Control. This operation
should coincide with the starting of the diffusion
pump. The heat control should vary directly
with the length of time required to obtain a
satisfactory vacuum. With average units requiring 1 hour to produce a vacuum, start the
heater control at 20 and increase 20 units at
intervals of 10 minutes until a reading of 80 is
reached. The ultimate reading and rate of increase of settings is inversely proportional to the
length of time required to reach a satisfactory
9. Satisfactory vacuum is reached when the
thermocouple output meter reaches its final
value (usually 190 to 200 microamperes), and
remains at that reading for 15 to 20 minutes.
Then turn on the ion gage switch and carefully
turn up the small variac control, watching the
upper microammeter. The upper microammeter
(plate) should not go off the scale and the lower
milliammeter (grid) should not read beyond 5
milliamperes. When a reading on the microammeter of 3 1/2 or less is obtained with a reading
of 5 on the milliammeter, the vacuum is satisfactory for coating.
10. Start the coating by turning the Heater
Control variac to zero and the heater switch
to the upper position. Watch the filament
through the side port while the heat is gradually
increased until a dull red color is obtained.
Allow the filament to burn at this temperature
for several minutes, unless it sputters, in which
case decrease the heat input with the variac until
the sputtering ceases. Then increase the heat
until a light red color is obtained. Observe the
optics by reflected light through the upper port
and continue coating until the proper purple
color is reached. Then turn the heat control to
the OFF position.
11. Turn the filament and diffusion pump
switch to the OFF position.
12. Cool the diffusion pump, preferably with
water, until the bottom of the pump is barely
warm upon being touched.
13. Close the valve between the diffusion
pump and the mechanical pump.
14. Turn off the mechanical pump.
15. Open the air inlet valve and admit air to
the system gradually.
16. Precautions to be observed. a) Always
heat and cool optics gradually. Sudden changes
in temperature may cause breakage. Large
optics should be heated and cooled more slowly
than small optics.
b) Do not use the ion gage until the vacuum
is sufficiently high. During normal operation,
the ion gage is used as a checking gage and is
turned on only while the reading is being taken.
It is not designed for continuous operation and
one or two checks per run should be sufficient.
c) Do not turn on the diffusion pump before a
satisfactory high vacuum is reached. The pump
oil (octoil) cracks if heated in too low a vacuum.
It is then necessary to replace the oil, an expensive, time-consuming operation.
d) If the filament is heated too rapidly, hot
particles of magnesium fluoride spatter from the
filament and imbed themselves in the optics
e) Keep the entire inner surface of the coating
chamber free of grease and dirt. Both the rate of
obtaining a satisfactory vacuum and the hardness of the coating are adversely affected by the
presence of dirt in this chamber.
f) Do not use castor oil on the bell jar gasket.
It is not necessary and may contaminate the
g) The base plate and the rubber gasket on the
bell jar should be absolutely free of small particles of foreign matter in order to assure a perfect seal when the bell jar is seated.
17. These instructions are for systems in
which a vacuum-type valve is used. If a vacuum type valve is no used, the air inlet valve should
be opened before turning off the mechanical
3C2. Cleaning optics for coating when afloat.
When afloat optics may be cleaned for coating
in the following manner:
1. Wash the optics in aerosol solution. A very
low concentration of aerosol is effective.
2. Test the dried optics to determine whether
water wets the entire surface.
3. If grease is difficult to remove, rub the surface of the optics with calcium carbonate and
rewash in aerosol solution.
4. After the entire surface is wet with water,
spray the surface with hot distilled water until
the optics become hot.
5. Use a small hand-aspiration bulb to blow
the remaining water from the surface.
6. If no water marks remain on the dry surface, the optics are ready for coating. Optics
should be recleaned if water marks remain on the
7. Precautions to be observed. a) This
method is more difficult than methods which can
be used ashore, but with practice the operators
b) Never touch the surface with anything
after it has been cleaned for coating.
c) Do not breathe on a surface after cleaning.
d) Water marks or grease spots lower the
quality of the coating; consequently, it is advisable to reclean the optics if there is any doubt
of their being absolutely clean.
Copyright © 2013, Maritime Park Association
All Rights Reserved
Version 1.10, 22 Oct 04