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REFLECTION-REDUCING FILMS
 
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 treated.

  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 coating material.

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
Percent
Reflection
1.521.5
1.571.2
1.601.0
1.650.8
1.700.6

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.

 
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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 rough treatment.

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 and brilliant.

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).

 
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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 balsam.

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 high-index glass.

3. Errors sometimes result in a lens being given a double coating. The color of the reflected light is then green. The reflection from

 
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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 repolishing.

Do not attempt to remove a film by hand polishing with a rouge pad under any conditions.

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 activities.

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 acid solution.

 
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

 
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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 vacuum.

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 being coated.

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 system.

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 pump.

 
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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 surfaces.

7. Precautions to be observed. a) This method is more difficult than methods which can be used ashore, but with practice the operators become proficient.

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.

 
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