Dual-Layer Diffractive Lens - The Issue Was Whether It Could Be Made

--- But what exactly is the Multi-Layer Diffractive Optical Element? Does it mean an optical element with several diffractive layers? Nakai: By multi, we mean two layers attached together, rather than a single layer. An optical element refers to an individual lens. In other, friendlier words, the Multi-Layer Diffractive Optical Element can be called the Dual-Layer Diffractive Lens.--- That clears things up a little bit. Nakai: As you know, light is a phenomenon of wave motion. Diffraction can be used to superimpose or eliminate waves, precisely controlling the direction in which light travels.--- Yes, I think I understand. Nakai: Optical elements making use of this principle have been adopted in measuring instruments and the lens of optical disc pickup systems. But this is the world's first example of such optical elements being used in a camera lens.--- In my research for this interview, I noticed on the Web that you had received a major Japanese academic award for this lens, and that you were invited to present your achievement. I didn't understand a thing after reading the brief for your presentation, but I got the idea that this was a revolutionary discovery. You were also listed as Professor Nakai and from this point, is that how I should address you? (Laughs) Nakai: It was very embarrassing. (Laughs) My face turned purple under the spotlights, but most of the response was praise for the manufacturing. (Laughs) After all, this optical element was truly difficult to produce.
The diffractive lens

The diffractive lens has a wide diameter of 10 cm. In development, close attention was paid to visual appeal, such as the attractiveness of the striped pattern, in addition to optical performance.
--- This is the element itself? Nakai: Yes. At first glance, it looks like your average concave lens, but if you look closely, you should see a concentric pattern of stripes.--- I see it. Yes, I see it. They are engraved at rough intervals at the center, but the striped pattern becomes finer and almost nonexistent as you reach the outside perimeter... Nakai: There are about 150 stripes in all. Adjusting the width of the gratings converges light in the same way as lenses. The finer the width, the greater the level of light bending.--- You mentioned two layers attached together, so what I am seeing here is two layers times 150 stripes? Nakai: No. The stripes of the two layers match perfectly.--- A perfect match? But the two lenses are produced separately, and then attached, right? How perfect can the match be? Nakai: To the micron level. The difference can't be detected by the naked eye.--- Making two individual lenses with such a perfect fit is difficult enough in itself, but I imagine exactly matching them must be a superhuman effort. Nakai: The hard parts were figuring out which striped pattern would best produce the desired lens performance, and then actually producing the lenses in line with the design.--- I have to admit that this is quite an achievement. What was your objective at the design stage? Nakai: Objective? First I had to confirm whether the lenses could be produced at all.--- That would be the basic problem. (Laughs)