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The Nano Crystal Coat

Ghost and flare: Phenomena that occur due to reflection inside a lens barrel or camera. In the past it had been thought that it was impossible to eliminate the type of ghosts and flare that are caused by strong light entering the lens diagonally—even using various types of lens coating. Here we mark the arrival of the Nano Crystal Coat (as used on the AF-S NIKKOR 14–24mm f/2.8G ED, the first ever ultra wideangle lens)—a truly revolutionary technology that has greatly alleviated these problems—and focus on the men who developed it.

Issei Tanaka
Expert Staff, Design 2, 2nd Designing Department, Development Headquarters, Imaging Company
PROFILE:
Since joining the company in 1981, Issei Tanaka has been responsible for the optical design of steppers. He moved to the Imaging Company in 2003. Having proposed the innovative application to camera lenses of the nano particle coating used in steppers, he is a major figure in the birth of the Nano Crystal Coat. He is currently engaged in the development of future technology in the area of optical design of interchangeable lenses. In his childhood he was an astronomy buff who loved Nikon equipment.
Tsuyoshi Murata
Junior Executive Staff, Research, 2nd Development Section, Lens Engineering Development Department, Production Technology Headquarters, Core Technology Center
PROFILE:
Tsuyoshi Murata joined Nikon in 1991. Since 1996, he has devoted himself to the development of optical thin film. He has made Nikon coating technology renowned worldwide through the development of nano particle coating and its application to camera lenses and projection lenses for steppers. On the personal side, he is an outdoor type who loves to head for the mountains or the sea.

Nikon’s original photolithography equipment (steppers) technology for semiconductors has revolutionized the camera.

As the digital camera becomes the norm, there are probably many photographers who are beset with the problems of ghost and flare.

What is it that makes the Nano Crystal Coat so effective in successfully curbing reflection so thoroughly?

Coating samples (consist of 14
sheets of glass)
Left: Nano Crystal Coat has been applied
to 26 surfaces out of 28.
Right: No coating applied to any of the 28
surfaces.

Tanaka:

Ghosts are a lens reflection phenomenon conspicuous in dark areas that are diagonally opposite a strong light. Although coatings were applied to curb reflection, ghost and flare were difficult problems that existing technology had been unable to eliminate completely. The surfaces of standard coated lenses appear green or blue. This is due to the wavelength characteristics of the coating, which reflects various green and blue light wavelengths—or in other words, retains the wavelengths for those colors. The most difficult proposition is to prevent the reflection of light that enters the lens diagonally. Although existing coatings have been effective to a certain extent with light that comes in perpendicular to the lens, they have typically been very poor at coping with light with a large angle of incidence (that is, their angular characteristics were unsatisfactory). If you incline the lens as far as you can and look at it diagonally, you can see that it still reflects light like an ordinary sheet of glass. By comparing the samples here, you will probably be able to see clearly that the one with the Nano Crystal Coat applied to it does not look as if it consists of 14 sheets of glass. Although 14 sheets of glass amount to a total of 28 reflective surfaces, the reflection hardly changes at all—even when the lens is inclined.

It's plain to see, isn't it? With its transparent feel, the one with the Nano Crystal Coat is completely different.

Murata:

People often say “Did you forget to put a lens inside?” There are three main properties required of a non reflective film. The first is to reduce the reflection within the lens of light with a perpendicular incidence; the second is the ability to curb reflection over a wide range of wavelengths; and the third (which is regarded as the hardest) is to effectively prevent reflection, even when the light has a large angle of incidence. The Nano Crystal Coat is a revolutionary coating that can offer improvement in all three areas-the perpendicular incidence characteristic, the wavelength characteristic, and the angular characteristic.

I'm interested in the moment at which this remarkable technology was conceived. What was the original impetus for its development?

Tanaka:

A stepper consists of photolithographic equipment for reducing an original master circuit design diagram using lenses, and transcribing the design onto silicon wafer. This ultra high precision machinery has been dubbed “the most precise equipment ever,” and since developing the first domestically manufactured stepper in 1980, Nikon has been ahead of other companies in building original technology, of which the Nano Crystal Coat is one example. At the turn of the millennium, as semiconductors became ultra miniaturized and ultra detailed, the projection lens of the stepper was also required to be of extremely high resolution. As the number of lenses and the aperture were increased in order to improve the resolution, the barrel housing the lens grew to approximately 1 ton in weight and as much as 1 meter in height. With such large lenses, the angular incidence of light in the periphery of the lenses was extremely large, making it difficult to prevent reflection. Large hemispherical lenses resembling a fortune teller's crystal ball were also used; however, it was particularly difficult to form the coating, which was intended to increase the transmittance onto such rounded lenses, making it impossible to manufacture a coat with the desired capabilities using conventional methods; thus, success was virtually impossible.

Murata, holding a lens with Nano
Crystal Coat, expresses his delight at
the success in commercializing the
technology.

Murata:

Since developing optical thin film is my department, we naturally focused on this problem, as we knew, based on calculations alone, that existing coatings were insufficiently effective. Thus, in order to resolve this problem, I proposed that we manufacture an ultra low refraction film using a completely new process, and we began development. This is how the nano particle coating, on which the Nano Crystal Coat is based, was conceived.

Achieving the appearance of refraction with a coarse structure like a highly permeable pavement

Nowadays we often hear talk of nano technology; however, it is difficult to envisage it as part of everyday life.

Exactly what kind of structure does nano particle coating possess?

Murata:

The word “nano” denotes one billionth. Although the term evokes an image of hardness and precision, the reality is completely the opposite-the actual grains that comprise the material are extremely small nano particles which are arranged in a spongy construction, rather than being tightly arrayed. This coarse structure-which is in complete contrast to the densely packed composition of existing coatings-is a major strong point of the material.

Tanaka:

My impression, the first time I saw this, was of a high permeability paved road. It was reasonably strong and would not rut easily with the passage of cars-despite its coarse structure.

I see. So why does roughening the structure enable the material to curb reflection?

Murata:

The key point is the refractive index of light when it is reflected. Light that has been traveling in a dead straight line bends when it hits something with different properties. The angle by which the light is bent is determined by the difference in refractive indices. The more the refractive indices differ, the greater the angle through which the light is bent. Conversely, if there is only a slight difference in the refractive indices, the angle is small. The nano particle coating (which is also comprised of the air between the particles) exhibits the intermediary properties of the substance that comprises a film. The blending of the substance with air enables the apparent refractive index to be reduced so that it is not the same as the actual material of which the film is comprised.

Tanaka:

It is probably easy to understand this through the example of a contact lens. A contact lens in air is shiny and easily visible. This is because when the light that has traveled through the air strikes the contact lens, it is refracted and reflected. This is due to the fact that the refractive indices of the air and the contact lens are different. As soon as the lens is dropped in water, however, it becomes almost impossible to see. In other words, since the refractive indices of the water and the contact lens are almost the same, the lens blends into the water and becomes invisible.

So the reason why we can see a transparent object, then, is that light is refracted and reflected.
Is coarsening the structure so that it includes air enough to lower the refractive index?

“Soon after having developed lens
coats for steppers, I was transferred to
the design division for interchangeable
lenses, and this provided me with the
impetus to utilize this technology for
cameras,” says Tanaka.

Murata:

No, it isn't. Simply coarsening the particle structure is insufficient. Light's wavelength is one of its wave properties, and this is expressed in units of length. If a structure is manufactured where the unit size is greater than the wavelength of the light, the phenomenon of light dispersion will occur, and the light will end up pure white-exactly like frosted glass. This would be useless for a lens. As a “nano” technology, the greatest challenge with the nano particle coating was to manufacture a structure that was sufficiently coarse to prevent light dispersion from occurring. The way in which this type of structure has been created is as much the key to this technology as the kind of substance used.