Optical Coatings

[Jim Fusco]

Hi Andrey,

There is often much talk about coatings. I know Astreya does their own coatings and others in house, so maybe you could clear some things up. What are the purpose of coatings? Reflection reduction and transmission of all light? I heard of hard coat and soft coat, is soft coat still used.
I have seen blue and red coatings and some that seem very dark. I was wondering if you can shed some light on this subject

Jim

[astreya]

Hi Jim,

When light passes through glass, it partly reflected from a surface of a glass and in partly absorbs by it. On each border air - glass is lost approximately 4 % of light. To reduce losses by reflection, apply coatings. The principle of action of these coatings is based on an interference of beams. At the certain thickness of layer, the beams reflected from external and internal surfaces of a thin film, develop in an antiphase and extinguish each other, therefore the factor of reflection becomes equal to zero. Speaking about monochromatic light, the factor of reflection can be lowered to 0,2 % and even lower.
Broadband coatings (for example, coatings for all visible spectrum) are not so effective, but all the same, provide reduction of reflection factor up to a level, approximately of 0,8 %. Coating always has colouring. Color of a coating can be miscellaneous. All depends on in what area of a spectrum the maximum of reflection lays. The ideal coating should have grey color. It means, that the factor of reflection is identical to all waves (colours). Really it not so, and any coating has a maximum of reflection in this or that area of a spectrum. So, if the maximum of reflection is in green area it will be green if in dark blue - dark blue.
Cheap coatings can have 2 big maxima of reflection in dark blue and in red area. In this case the coating will be purple. Therefore the main thing is not color of a covering, but the factor of reflection, which this covering can provide.

[Jim Fusco]

Thanks Andrey,
Another question, how strong is the bond of the coating to the glass. I often become concern with cleaning and worry of removing the coating during this process. Can there be a reaction between the cleaner and coating itself, maybe effecting the coating life.
Jim

[astreya]

Hi Jim,

For coatings usually use oxides of metals (Al2O3) or fluorides, for example MgF2. These substances have high hardness, therefore they not only decrease the reflective factor of a surface, but, in many cases also strengthen it.
Such solvents as spirit, acetone or gasoline do not represent harm for coatings, therefore rubbing coating just be sure that there is no abrasive particles on the surface.

[Jim Fusco]

So there are different types of coatings, and the type of coating that is selected is matched with the type of light that will be passing. Does the coating effect the refraction of the element and overall correction of the optical design?

Jim

[herbhighstone]

Hello to Jim and the folks --

My impression is that optical coatings are too thin to make any noticeable change in the actual optical performance of a lens. Something that is only a few waves of light thick won't affect the ray paths very much.

In the good old days before vacuum coating chambers were invented, optical anti-reflection coatings were applied chemically. This all began about 100 years ago when people noticed that refractor lenses that were stained by smog gases such as hydrogen sulfide had a lower loss of light. If you're interested in antique optics, you really need to look out for the chemical coatings because they're so soft that you can wipe them away with a soft rag. This has been an unpleasant surprise, for example, to collectors of the large Japanese "battleship binoculars" which are WW II surplus.

One disadvantage of the modern "hard" coatings is that you can't easily remove them if they're damaged. They actually have a chemical bond with the glass, and you'll need to repolish the glass surface and then recoat it in a vacuum chamber. This will probably cost more than buying new optics.

Some older optical glass will sag and change its shape in a modern, hot vaccum coating chamber. In one case, a lens element from a classic refractor sagged so badly that it wouldn't even fit inside its cell anymore. It's often best to leave classic lenses in their original condition.

Best wishes from Herbert Highstone

[Jim Fusco]

Hi Herbert,

That's interesting, I recall that in the navy, they apply the coating on board ship. I have seen good coatings and bad. The bad coatings were on eyepieces and small scopes that was made in china. The material would flake off and I could drag my nail across and the rest would flake off. This also happen to me when I sent a lens to a US manufacture for a cleaning and came back missing dime size coatings. They told me it happens and enjoy the view.
Anyway is coatings on a retractor apo lens is the same say as for eye glasses. Since the surface accuracy of a apo lens is much greater.

Jim

[astreya]

Hi Jim, Herbert and the folks,

as for chemical coating, reflective index of this coating is more than physical (vacuum) coating's one, so chemical coatings are used now only for cheap optics.

Optical coatings are very thin and won't affect the ray paths at all, but it makes noticeable change in the contrast of a lens' actual optical performance.