FORUM

Demystifying Multicoatings

by

Rodger W. Gordon



Demystifying Multicoatings
by Rodger Gordon

Originally appeared in TPO Volume 8, Issue 4. ©1997 Typographica Publishing.
May not be reproduced for commercial purposes.

In the early part of this century, astronomers began noticing something curious about their refractors. They were transmitting more light after a 20 to 30 year period than when they were brand new! It was soon discovered that the exposed surfaces of the objectives had reacted with the air to form a natural coating on the glass, and this coating allowed more light to be transmitted. Experiments were soon made to artificially age glass. While such experiments were generally successful, the method was soon replaced by artificially applied coatings. Both Bausch and Lomb in the U.S. and Carl Zeiss Jena in Germany developed this process almost simultaneously in 1935. By 1939, Zeiss was successful in multi-coating their optics with a double layer and by 1942 were using a triple layer coating. After World War II, however, single layer coatings were used primarily and it was not until the early to mid 1960s when multi-layer coatings began to be used more commonly.

The advantage of applying coatings to optical surfaces are obvious. Not only do the optics transmit more light, they increase contrast and reduce or eliminate internal reflections ("ghost" images). Before coating, a typical binocular might lose 45 to 50% of the light entering it. The use of coatings on all the air-glass surfaces, however, might increase transmission from 65 to 80%! This was a noticeable gain and had both commercial and military applications.

Before coatings, binocular manufacturers like Ross of London and Zeiss sometimes designed the rear eyepiece element in the binocular in such a fashion that it could be cemented directly to the binocularís prism face, thereby increasing light transmission about 8%.

The most common substance used in ordinary single layer coatings is Magnesium Fluoride (MGF2) which has a refractive index of 1.38 and can be used on a variety of crown and flint type glasses. Many substances can be used, however, like Lithium Fluoride. Magnesium Fluoride, however, is suitable for internal or external surfaces and is reasonably hard enough to withstand ordinary, gentle, cleaning.

Multi-layer coatings can have as many as 60 to 70 layers, though the usual number is two or three, seven at most. Complex optical systems, like those found in certain camera lenses or medical equipment lenses, often require milticoatings to achieve as much light transmission as possible. There are, however, a few areas where single layer coatings are better than multi-layers. We will touch on this later.

Truth in Advertising

The use of coatings and multicoatings by various manufacturers has often resulted in a great deal of confusion among the buying public - and this has not been helped by the equally confusing advertising claims, many of which can be misleading if not downright fraudulent.

In the early 1950s, when inexpensive Japanese binoculars began to be imported in the U.S. in large quantities, their optical systems were advertised as "coated". However, this often meant that only the outer surfaces of the eyepieces and objectives were coated, while the interior surfaces had no coatings at all.

When some of the better brands started coating all the external and internal surfaces, they were advertised as "fully coated". Other manufacturers soon began doing the same and by the 1970s almost all but the very cheapest binoculars were using fully coated optics.

Around the mid-1970s, multicoatings came into wide use, both in binoculars and in telescope eyepieces. But often these optics, though advertised as "multicoated" frequently had only one or two air-glass surfaces with multicoatings and the rest with ordinary coating or perhaps none at all. On many eyepieces, the multicoating was usually applied only to the outer surface of the eye lens! Today, most of the more expensive eyepieces are fully multicoated, though cheaper ones may still have only the outer surfaces of the lenses with multicoatings.

Multicoatings

In general, a multicoated optical system (if it is fully multicoated) will have more light transmission than an eyepiece with a standard coating - but not always! Much will depend on the number of optical elements in the respective eyepiece and what types of glass are used. A three or four element (lens) eyepiece with standard coatings may transmit as much light as a seven or eight element eyepiece with multicoatings.

Multicoatings can vary in their own transmission characteristics. Some double layer coatings, for example, may lose only one half of one percent in the 500.0u to 700.0u part of the spectrum, but from 400.0u to 500.0u may reflect as much as 6 to 6.5% off each surface! When one considers that the average loss at an uncoated glass surface is around 49%, itís obvious that some multicoating can work better in some parts of the visual spectrum than other parts. A really good multicoating will have a pretty even range of transmission throughout the entire visual range.

In general, a three to four element eyepiece of high quality will have a 90 to 92% transmission with a standard single layer coating and this rises to 95 to 97% with a good multicoating. However, a higher light transmission does not necessarily mean the multicoated eyepiece is automatically of better quality. The surface finish of the eyepiece is an all important factor since a lot of cosmetic defects (bubbles, scratches, sleeks, etc.) may scatter a lot of light in spite the coating.

One problem with multicoating is a phenomenon known as narrow angle light scatter. This can sometimes be seen as a slightly greenish or purplish "glow" surrounding a bright object (moon, Venus, Mars, Jupiter). Narrow angle light scatter can greatly decrease contrast. Some years ago, I was present in the office of a major telescope manufacturer. We were discussing the advantage and disadvantages of coatings and multicoatings when the then-President of the firm told me that they had to replace a specialized multicoated industrial telescope of theirs used by a customer in very low level contrast work with another unit with regular standard single layer coatings. The narrow angle scatter from the multicoating was interfering with faint tonal contrast.

Several years ago, the noted deep sky observer Ronald Morales compared six or seven eyepieces of 12 to 12.5mm focal length. A multicoated eyepiece had better light transmission for detecting faint galaxies but a standard coated eyepiece gave the best contrast levels.

Durability

Iíve owned both multicoated and standard coated binoculars and eyepieces and have found that some perform better with multicoating and some better with single layer coatings. Multicoatings are often quite fragile and may scratch easier than standard coatings. Some military spec multicoatings are very hard, but commercial optics almost never have military spec coatings. Some optical repair firms wonít handle multicoated optics. Frequently, the multicoatings on the internal surfaces are "cold" coatings - that is, applied at lower temperatures than "hot" coatings. These can be easily damaged by ordinary cleaning. Some lens cleaners are bad news for a "soft" cold internal multicoating. "Hard" coatings applied at higher temperatures generally withstand cleaning better than soft coatings. Before cleaning any optic with a lens cleaning solution, be sure it states it is suitable for all coated or multicoated surfaces. Iíd advise against using lens cleaning solutions that are used primarily for eyeglasses.

I once owned a 1950s era Brandon eyepiece where a previous clean-crazy owner had put numerous scratches on the outer surface of the eyelens coating which scattered a great amount of light. Fortunately, these scratches were not on the glass itself. I decided to polish off the impaired coating and a solution of 100% lemon juice and Q-tip easily removed it. Iíve used lemon juice on other scratched coatings with varying success in removal.

The late, legendary Horace Dall recommended not coating the outer surface of the eyepieceís eyelens. This is a recommendation that I whole heartedly endorse! This surface is the one that receives the most wear. It can dew up; tears from the observerís eye may fall on it on cold nights (tears contain salt); eyelash dirt and grease may get on the lens; women who wear a lot of eye make-up can easily get this material on an eyelens, etc. Acid dew, often found in industrial areas, if allowed to remain, can easily ruin a coating or multicoating or perhaps even the glass itself!

Chasing Away Ghosts

Without coatings or multicoatings, modern eyepieces (six to eight or more elements) might not exist. Some designs are "ghost prone" and while many modern optical designers rely on good coating to subdue ghosts, older optical designers had to "design them out" and that wasnít always possible given other constraints.

I have an 8x40 uncoated Zeiss Delactis binocular from the 1930s that has far fewer ghosts and internal reflections than a multicoated Celestron Nova 7x50 (10 degree field of view) from the 1980s. As far as I can tell, some of the internal surfaces of the 7x50 may not have multicoatings. But while its a great daytime binocular, using it on a bright moon results in numerous internal reflections and ghosts. A bad design as far as these annoyances are concerned! Coatings cannot save a ghost-prone design. I should point out here that it isnít Celestronís fault directly as much as the Japanese manufacturer they imported them from. This binocular is no longer available and the current top of the line Celestron binoculars do not have this problem.

Unecessary Confusion

Manufacturers seldom provide detailed information on their multicoatings. This is unfortunate, although a few have provided light transmission curves for their eyepieces and some binoculars. Most also donít tell how many layers are used in their multicoatings.

You can usually tell if there is a multicoating present, however, by the greenish or purplish tint to the lens itself, but there are variations. Some multicoatings are now ruby or red colored - quite noticeable on some currently manufactured binocular lenses.

Haze Cutting

Multicoatings (or standard coatings) can be applied is such a way that they transmit certain parts of the spectrum while rejecting the rest. Often, these coatings will reject blue light and such optics are often advertised as "haze penetrating" or "haze cutting" optics. This is particularly useful in long distance viewing or on dull, cloudy days.

At one time, many binoculars were advertised with "amber coatings". These optics have a yellowish tinge and, in my experience, are not as effective in long distance viewing than those with a very heavy coating of Magnesium Fluoride, which has a very dark blue color. Some eyepieces back in the 70s were amber coated, but this has mostly fallen into disuse.

Buying Coated Optics

I own or have owned uncoated, standard coated and multicoated optics and, in my experience, the optical quality of the equipment is more important than what kind of coating it may or may not have. Multicoatings are something like the scoop of ice cream on a piece of pie - not necessary, but adds the "taste".

When purchasing any astronomical optics, one should be far more concerned with the optical quality of the manufacturer rather than the type of coatings used.

If all else is equal, choose the "fully coated" over "coated" and "fully multicoated" over "multicoated". Donít be afraid to ask questions, but donít be surprised if the sales clerk or the voice at the other end of the phone knows less about coatings and multicoatings than you do!

Without a spectrophotometer, you canít determine the exact transmission of a particular optic. However, you can get a general idea from the following rules:

  • Each uncoated air/glass surface loses about 1.5% at each air/glass surface.

  • Each multicoated air/glass surface loses from 0.25% to 0.5% though some as little as 0.1%.

As an example of how these general rules might be applied, letís consider an uncoated Abbe orthoscopic eyepiece with four elements and 4 air/glass surfaces. Each surface loses about 4%, thus we get (assuming 100% as a reference standard) .96 x .96 x .96 x .96 = 85%. Add another 1 to 2% for light loss through the glass and we get between 83 and 84% transmission.

Now letís look at the same eyepiece but with a standard single layer Magnesium Fluoride coating: We now have .985 x .985 x .985 x .985 = 94%. Again, take away 1 to 2% for light loss through the glass and we get 92 to 93%.

Next, letís look at the same eyepiece but with a multicoating: .995 x .995 x .995 x .995 = 98%. Again, minus 1 to 2% for light loss through the glass and we have 96 to 97% transmission.

Note the greater gap between the uncoated and coated version than between the coated and multicoated. With eyepieces having seven or eight elements, the same rules apply and transmission losses will be greater.

Given this, it is easy to see why before coatings that monocentric, Tolles and Hastings Triplet type eyepieces with only two air/glass surfaces were so desirable! Not only did they have higher light transmission than other uncoated eyepieces, their chances for ghost formation and internal reflections was far less.

Applying the same rules to the solid-type ocular as we did above, a Monocentric or Hastings with three elements and two air/glass surfaces yields: .96 x .96 = 92%; 90 to 91% with correction for light loss through the glass. A modern coated Hastings Triplet gives us: .985 x .985 = 98%; 95 to 96% corrected. This is more than a coated Abbe Orthoscopic (or Plossl) and compares very favorably to a multicoated four element, four air/glass surface ocular. One of the authorís favorite eyepieces is a 12.5mm Hastings Triplet. Another is a 16mm Steinheil Triplet.

Using this information, you can get a fair idea of the light transmission properties of any eyepiece if you know how many elements and air/glass surfaces it has and if it is uncoated, coated or multicoated.

In general, if you are looking for a high contrast image, the fewer the number of elements and air/glass surfaces the better (assuming, of course, the glass used is high quality and cosmetic defects are minimal). This is true regardless of the coatings.

Hopefully, this article will have shed some light on what can be a rather confusing subject!



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