Glass for geeks: An in-depth tour of Nikon’s Hikari Glass factory

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posted Tuesday, April 17, 2018 at 2:30 PM EDT

 
 

I've been on a lot of factory tours with various camera and lens manufacturers before, but had never had a chance to see how the optical glass was made that goes into the lenses we use every day. So I was really happy to receive an invite from Nikon to tour their Hikari Glass factory in Akita Japan, following the annual CP+ trade show in Yokohama this year.

This was a pretty special tour, as we got to see the whole process, from start to finish, hosted by three of Hikari's top executives. Our hosts were Mr. Tatsuo Ishitoya, President-Director, Mr. Akio Arai, Corporate Vice President and Production General Manager, and Mr. Toshihiko Futami, Director and Management General Manager. Mr. Masaru Kobayashi, Assistant Manager of the Administration Section also accompanied us and contributed to the information we received. Arai-san is the person directly responsible for plant operations, and it was him who personally guided us on our extensive tour. All three executives briefed us before and after the tour itself.

 
Here are three of our hosts, Arai-san on the left, Futami-san in the middle, and Masaru Kobayashi, Assistant Manager of the Administration Section, on the right. It was pretty amazing, to have such high-level executives take us on a tour. I was in engineering-geek heaven; our guides were entirely up to fielding even the most technical questions I asked them. There were of course a lot of things that were too proprietary to share, but the knowledge (not to mention the degree of patience) they brought to the table was truly exceptional.

As I said, I've been on a lot of factory tours with various manufacturers, but this qualifies as one of the most interesting ever. I'd previously had only a vague idea of how glass was made; it turns out to be a lot more involved (and more fascinating) than I'd imagined. Here's the story of our tour…

 
This the road between the Hikari factory and Motoyu Ryokan, the amount of snow apparently pretty typical for early March. The snow was even deeper, closer to the ryokan.

The Hikari factory is in northern Japan, in Akita Prefecture, just a little south and east of the capital city of Akita itself. Akita is on the western coast of Japan, so the prevailing winds blow for hundreds of miles across the Sea of Japan before hitting the coast, picking up a load of moisture along the way. These winds drop a lot of snow even in Akita proper, but when they hit the mountains, they really cut loose. There was a LOT of snow around the factory, and even more as we wound our way further up into the mountains, to spend the night at the very traditional Motoyu Ryokan, built over a free-flowing natural hot spring.

 
There were actually a couple of entrances to the factory complex; this one was convenient as we ended our tour. The plant covers quite a large area, with multiple buildings.)
 
Even though I grew up with serious winters in New England, the amount of snow in Akita was truly impressive. Our host Arai-san told us that this is actually a public road in the summertime. In the winter, the town has to pick their shots in their battle against snow, so this road is left unplowed. That's a Japanese stop sign in the shot on the left, while the one on the right gives you some idea of how tall the snow piles were, compared to our guides. The two Nikon staff shown in this picture were pretty compact people, but the piles of snow towered over even my head, at 6' 2
 
This was one of our first views of the plant, as we started our tour. It wasn't terribly cold on the day that we were there, but it's obvious it stays below freezing a lot of the time. The Hikari factory seemed so much more ...

 

The basic recipe: Initial mixing and blending

Optical glass is a complex blend of ingredients, but some representative ingredients are quartz or silicon dioxide (SiO2). If you live near a beach, chances are a lot of the sand is quartz. All of Hikari's glass begins life as a blend of several basic ingredients, the main one being quartz, although Arai-san politely declined to list what they were. (He also asked that we not take photos of the area where the sacks of ingredients were piled up.)

(Deep geekery: We don't know the main components of Nikon's optical glass, but glass generally consists of SiO2, some sort of an alkali flux to lower the melting point, and stabilizers to make it insoluble in water and increase corrosion resistance. Modern glass frequently uses sodium oxide, typically added as sodium carbonate (Na2CO3) and a tiny amount of potash (K2O), added as potassium carbonate (K2CO3) for the flux. Finally, Lime (CaO) and Magnesia (MgO) are added as stabilizers, to increase corrosion resistance.)

It's important that the ingredients are blended thoroughly, which is the job of a pair of giant mixers like the one shown in the video below.

Ingredients enter the mixer from hoppers on the floor above, via the pipe you can see sticking down from the ceiling. The mixer handles batches of about 500kg at a time, or about 1,100 pounds. Once the ingredients have been delivered to the mixer, the operator sets it rotating for a fixed amount of time. The powdery mix is discharged from the bottom of the V-shaped mixing barrel into plastic bins, to be transported to the melting furnaces, in a nearby part of the facility.

 
Here's what the mixed raw-ingredient powder looks like, while waiting to be melted. It's pretty plain-looking, with a texture somewhere between table sugar and flour.

 

First Melt

We couldn't show the furnaces used to melt the mixed powers, as some parts of them were proprietary. (It's too bad, they were pretty dramatic structures!) The furnaces were quite tall, with steps providing access to a platform for servicing the dosing mechanism.

This was interesting: I'd expected that the melting process would just consist of dumping the mixed power into a crucible of some sort, then shoving the whole thing into a furnace. It turns out that this wouldn't work very well, as the unmelted powder doesn't conduct heat very well. So a huge crucible full of it would take a long time to fully melt, working from the outside in.

Instead, they start with the crucible empty, and a mechanism drops small amounts of powder into a metal box on the end of a long mechanical arm. A hatch opens in the side of the furnace, the arm extends and the box flips upside down, to dump a small amount of powder into the hot crucible. This small amount of powder melts relatively quickly, after which the next allotment can be dropped on top of it.

In this way, the crucible gradually fills with molten glass, in a process that Arai-san said can take several hours or so. 

 

The photo on the right has nothing to do with the furnaces at Hikari Glass, but it's at least some sort of a batch charger, albeit a part of a very high-volume commercial glass production facility (the kind that makes glass for windows, auto windshields, etc). The general idea is the same; a hopper above feeds the glass mix down to the charger, where a bucket slides in and out of the furnace periodically, to deliver doses of the mix into the furnace interior.

The primary melting furnaces were fascinating contraptions, but unfortunately, we weren't allowed to take photos of them. I found out why this was probably the case, when I went looking for an illustration image to use to break up the text here: It seems to be an unusual arrangement, or at least everyone else who uses it considers their solution proprietary was well: I couldn't find a photo of a similar dosing or "charging" system anywhere, despite a lot of Google-searching. The image at right was the closest I could come.

The crucibles used for this melting process is made of fused silica, or … quartz! But wait a minute, didn't we just learn that glass is made from quartz? What keeps the quartz crucible from melting as well?

 
We weren't allowed to photograph the crucibles, but they were massive, a good couple of feet in diameter by perhaps three feet long, and with walls more than an inch thick. They looked like they'd be very expensive consumable items! The photo above is from a web page by a maker of high-end kilns for glass and ceramics hobbyists. It gives you the general idea of what the crucibles look like; just imagine something that looked like the above, but was 2-3 feet tall. (Image from Paragon Kilns)

It turns out the quartz crucible does melt with each firing, but only a little bit, and the Hikari Glass engineers take this into account in their formulas for the powdery ingredient mix. They basically assume that they'll end up with a bit more quartz in their final glass than was present in the mixed powder.


 
 

Once the batch of glass has fully melted, a worker melts a hole in the bottom of the crucible, letting the molten glass (~1200C) pour into a large water tank. (~6 x 6 x 4 feet?). Note that once he's got the glass flowing, he turns on a water jet that sprays across the stream of glass, just as it hits the water surface in the tank. This fractures the glass into tiny, snowflake-like shards, called "frit". Having the glass in the form of frit helps the next step, of homogenizing the glass that's been produced.

 
Here's a shot of the frit, scooped up in a bucket by the worker running the operation, for us to look at. You can see how it's in the form of many fine, fractal-looking shards.

I always assumed that optical glass was made by just mixing together the various component, melting it, and pouring it out. It turns out though, that the composition of the glass can vary, depending on where it was in the crucible during melting. Parts that were up against the quartz crucible walls will have more SiO2 in them, and parts near the surface will have less of some more volatile components.

Something else I never knew about glass-making: Some of the compounds used have a higher vapor pressure than others at the melting temperature, so they actually evaporate away during the process. (Hence the need for the exhaust-gas scrubbing equipment shown at the beginning of this article.) So depending on the temperature cycle, you'll end up with less of some components than you initially mixed in, in parts of the melt that were near the surface.

 
If these look like cement mixers, it's because that's what they are! They're used to mix batches of frit, to make sure each batch is completely homogenous.

Between the crucible melting slightly each time and the evaporation of more volatile elements near the surface, there can be quite a bit of variation in frit coming from different parts of the melted glass. Consequently, after each melt is completed, the frit is tumbled for a while in a converted cement mixer, to homogenize the mix. The shot above shows two of the three huge mixers we saw in the room. (I'd estimate that the barrels were about 2 meters/6 feet in diameter.)

 
The problem with a stock cement mixer is the steel from the barrel would contaminate the frit, changing the glass' properties. Thick rubber liners prevent this from happening.

The problem with an off-the-shelf cement mixer is it has a steel barrel, and steel would contaminate the glass and change its optical properties. To avoid this, Hikari Glass fits them with thick natural-rubber liners, as shown above. Any tiny bits of rubber that abrade off into the frit mix end up burning off in the final melt so they have no effect on the glass itself.

I've shown the frit-mixing process as the next step, immediately following the melt and frit-production stage, but there's actually a step in between, where they melt a sample of the frit into a block of solid glass and measure its optical properties. Depending on where the refractive index of each batch ends up, they'll combine the output of different melts, to be able to hit the target refractive exactly on the money. (Although it occurs to me that there might be two mixing stages, one to make sure the frit from a given melt is homogenized, then a sample of it is melted and tested, after which frit from different batches is mixed together before the final melt.

 
This shows a generic three-zone glass melting furnace, similar in general concept to the proprietary and highly specialized ones used by Hikari Glass in their final melting process. This isn't what a furnace at Hikari Glass looks like, but it gives the general idea of a furnace with three temperature zones in it. (Image from British Glass & not from Hikari Glass)

Once the frit has been made and blended, it's time for the final or fine melt. The details of this are very proprietary, as it's the key to obtaining uniform, defect-free optical glass. Arai-san explained that every company makes its own final melting furnaces, and the details of them are very proprietary.

Unlike the initial melt, the final melt is done in platinum(!) crucibles. These must be extremely expensive, although the batch sizes for final melts are usually somewhat smaller than the initial melt. Still, a platinum crucible able to hold a couple hundred kilograms of molten glass must be a pretty pricey item! (In practice, I think they're platinum-lined rather than solid platinum. Still, they must be very costly!)

The reason they use platinum crucibles for the final melt is because the platinum won't dissolve into the molten glass and change its characteristics, the way the fused-silica crucibles do that are used for the initial melting.

Despite the use of platinum-lined crucibles, though, the composition of the glass will still change slightly due to the evaporation of some of its components, especially in the central, higher-temperature part of the furnace (see below). So this has to be taken into account, and the mixture adjusted to get the right final result.

 
Arai-san explains the thermal cycle in their final-melting furnace. The actual temperatures are proprietary and different than those shown, but the general idea is that the glass passes through three temperature zones, an initial melting, a higher-temperature zone, and then into a cool one before final casting.

 

Bubbles are trouble

One trick in final melting is making sure there are no bubbles in the glass as it's cast into its final form. A bubble in the middle of a lens element would obviously be a problem, so great care is taken to eliminate them.

I was curious how they did this. I thought they might perhaps use a vacuum furnace, so any bubbles would expand and come to the surface. The actual solution is a bit more clever than that, taking advantage of the natural properties of hot glass.

It turns out that air and other gases dissolve in hot glass, in much the same way that air dissolves in water (which is why fish can breathe underwater; they rely on the dissolved oxygen). As with water, cooler molten glass can hold more dissolved gas than hotter glass can. Hikari Glass takes advantage of this fact to eliminate dissolved gas, with a three-zone temperature profile in their final melting furnace.

 
(Note: The temperatures shown here are just for the sake of discussion; the actual temperatures are different and proprietary.)

The setup is shown above in the rough diagram Arai-san has drawn on the whiteboard. The temperatures shown aren't the ones Hikari Glass actually uses, but they serve to illustrate the concepts involved. On the left, glass is initially melted in an input chamber to a temperature of about 1,200C, a similar temperature to that in the first melting crucible we talked about earlier. From there, the glass flows to a second chamber, held at ~1,400C. Because it is so much hotter, dissolved gas is driven out of the glass, into the surrounding atmosphere. Passing out of the high-temperature chamber, the glass flows into a final crucible that's held at ~1,100C. At this cooler temperature, any bubbles left in the melt from the higher-temperature chamber are dissolved back into the glass, leaving behind perfectly clear, bubble-free glass that's drained from the bottom of the crucible onto the continuous casting conveyor.

As we'll see, the process isn't 100% perfect, because some bubbles and other defects still make it through, and are caught by a subsequent visual inspection.

 

Casting

The final casting process is pretty amazing; the glass flows very slowly from the bottom of the final melting crucible onto a conveyor belt in a long, long oven, where it's gradually cooled. The casting process is continuous, lasting until the batch of glass in the final melting furnace is exhausted. Arai-san was deliberately vague about specific details of the final casting process, as it is heavily proprietary.

 
Here's a long ribbon of glass, exiting the casting oven. The final melting furnaces are behind us in this shot, up on a second-floor mezzanine level, above the casting ovens. The details of those furnaces are so proprietary that we weren't allowed within 50 feet or more of them, and couldn't take any photos facing in that direction. It was kind of amazing to see finished glass creep out of the oven like this, a process that continues 24/7 until the entire batch of glass has been cast.

The glass is cast into ribbons of different widths and thicknesses, depending on the size of the lenses it will eventually be made into. We saw samples from ribbons that ranged from perhaps 125-150mm across and 15mm thick, down to maybe a 50mm across and 6-8mm thick.

At the end of the cooling tunnel, the glass ribbons very slowly inch along, propelled by an open-grid metal conveyor belt. When I asked how long it takes to complete the casting for one batch of glass, I was amazed to hear that it can take anywhere from a couple of days to a full month(!)

 
At the output end of the final casting line, a worker is waiting to label the glass and break it into 30cm-long chunks. He uses a small hammer and chisel to break off the pieces. (We were a little surprised that something as crude as a hammer and chisel would produce such clean breaks, without danger of cracking the slab into shards.)
 
Sometimes a slab of glass doesn't fracture all the way through from the chisel strike, so the worker uses a padded post to complete the break.

 

Visual inspection for defects

After the strips of glass come off the casting line, they're inspected visually for defects. This step involves checks for two different types of defect; bubbles and inhomogeneities. 

 
Bubbles are spotted by shining a strong light through the glass, peering through it at a dark background. Even tiny bubbles show up as bright specks within the glass ingot. (If you look very closely at the image above, you can see a few bright points of light within the glass that are the bubbles.) Bubbles are apparently a fairly rare occurrence, thanks to the special design of the final melting furnaces; the sample in the image above was one that Hikari Glass staff selected so we could see the defects clearly.
 
Once a bubble is identified by looking through the glass lengthwise, the worker turns the ingot 90 degrees and finds and marks each bubble's x/y position with red marker. This way, the defect-free parts of the ingot can be used, and the parts containing defects discarded.(This is the same sample a shown above, specially selected because it was easy to see the defects in it.)
 
Inhomogeneities in the glass are more subtle and a bit harder to detect. Once again, a bright light and human eyeballs do the trick.

The other thing to watch out for in optical glass is inhomogenieties caused by changes in the refractive index, resulting from the evaporation of component substances during the high-temperature portion of the three-step thermal processing used to eliminate gas bubbles. (Actually, evaporation occurs in all three thermal stages, but it is obviously most severe at the highest temperature. As mentioned earlier, parts of the melt close to the surface can become depleted of the more volatile components, and if that sort of glass makes its way to the final casting, its refractive index will be different than the rest. (Arai-san didn't give any but the most basic details of the final melting process, for obvious reasons of proprietary information, but I assume there must be some sort of mixing taking place within the three crucibles involved in the final-melt furnace. If the glass wasn't mixed, I would think there'd be a lot of homogeneity problems, or they'd have to waste a good portion of each melt, to avoid parts that had lost too much of their volatile components to evaporation.)

 
With a little Photoshop work, you can see what the inspection worker was looking for in the previous image. Note the curving line across the width of the slab here. That's an example of "striae", caused by variations in refractive index.

In the visual inspection, inhomogeneities are found by projecting light through the glass ingot onto a screen, and observing the light/dark patterns as the sample is rotated slightly about its long axis. The telltale optical artifacts are pretty subtle, so we cropped the image and radically adjusted the tone curve to highlight them. You can see the "striae" that the technician is looking for in the image above, as light/dark horizontal lines.

I asked whether glass ingots containing defects could be recycled by re-melting them, and was told that it depends on the type of glass involved. Some can be recycled, but my impression was that most could not. (I wonder if Hikari Glass could earn some additional revenue by selling rejected slabs of glass? I'd certainly pay a fair amount to have one as a keepsake/conversation-starter on my desk!)

 

Refractive index and light-transmission measurement

It's probably become clear from the preceding that refractive index is a key parameter that's controlled very precisely. It's no surprise then that it would be measured at various points throughout the production process. There's a separate room with precision optical instruments in it that measure both refractive index and optical transmission (how transparent the glass is).

 
This is one of the refractive-measuring machines; glass samples are loaded inside, via door on the right side (you can see the grey handle sticking up). This machine measures refractive indices at several different wavelengths, so they can tell the dispersion of the glass as well as its overall refractive index. As mentioned earlier, dispersion is a measure of how much the refractive index changes as a function of light wavelength/color. (If you're wondering about the wonkiness in the computer screen, it's because I blurred it in Photoshop, to avoid revealing any proprietary info.

There were two different machines used for measuring refractive index, one somewhat more sophisticated than the other. Both bounce light through a square block of glass, and read-out the refractive index, but one of them measures refractive index at a single wavelength, while the other measures refractive index at several different wavelengths, to also measure dispersion. These refractive-index measurements are performed on small test blocks melted directly from the raw frit we saw earlier, as well as on blocks cut from the continuously-cast glass slabs. The final check on refractive index is performed after the annealing step (see below), to make sure it precisely matches specifications.

 
This is the business end of the other refraction-measuring instrument, with a block of glass mounted in it. Both of us unfortunately missed getting a more interesting shot with the light shining through the sample :-/ As described throughout this article, glass samples like this may be collected at several points in the production process.

 

Light transmission measurement

In the same room with the two refractive index measurement instruments was another one that measured light transmission. I'm not sure what would cause glass to pass more or less light, but it's obviously an important parameter. The transmission-measuring instrument was just a large grey metal box, but there were three sets of glass samples sitting on top of it for us to see.

The problem with measuring light transmission is that light reflects off the surfaces of the glass you're trying to evaluate. And it's not just the front surface, where the light first strikes the glass; it'll reflect internally from the back surface, some of the internally reflected light will then bounce back off the front surface, etc, etc. When you're looking for very small differences in light transmission, any reflection will disturb the measurement.

 
Here Arai-san holds a couple of samples prepared for light-transmission measurement. By testing two different thicknesses of the same glass simultaneously, they can cancel-out the effects of reflection, and measure just the light lost to absorption within the glass itself.

The solution turns out to be pretty simple: Just prepare two identical pieces of glass, differing only in their thickness, and measure them both. The surface reflections will be the same between the two samples, so any differences in transmission will be due to the difference in thickness between them. 

 
Three sets of glass samples ready for transmission measurement. The thinner ones are 2mm thick, while the thicker ones are 10mm. The colored marks on them are just for keeping track which batch of glass they're from.

 

Cutting into "dice"

After the continuously-cast slabs of glass come off the line and are quality-checked, they're cut into chunks before being pre-formed into lens shapes. This was another surprise for me, in that the glass is fractured, rather than being cut.

 
Slabs of glass supported above strip-heaters, waiting to be fractured in two lengthwise. We were surprised by how perfectly clean a cut could be obtained so quickly and easily with this method.

The first step is to split the glass ingots in two lengthwise. (At last I assume they're always split just into two halves, as that's what we saw being done. I guess it's possible larger slabs might be split into thirds, but it looked like the slabs were always sized laterally to be twice the width needed for the final preforms.)

 
Depending on the thickness of the glass, it takes a little while for each piece to heat up to the point that it's ready to be fractured. There were two workers doing this task, each running about six stations simultaneously. They were constantly in motion, turning out a pair of glass strips every minute or so. (I'm sure they were also working very hard, with the big boss Arai-san looking on! :-)

The lengthwise splitting was done using thermal shock. The slabs of glass were laid on some sort of temperature-tolerant substrate, with a coil of nichrome resistance wire running along a slot in the middle. The slabs of glass rested over this coil for a matter of a couple of minutes, until the heat from it had had time to work through the thickness of the glass immediately above it. The worker would then just touch one end of the piece of glass with what looked like a pointed wooden stick that had been dipped in water. The sudden shock from contacting the tip of the cool stick would make the glass crack at that end, the crack instantly propagating down the length of the slab. It happened in the blink of an eye, producing smooth, straight edges every time.

Once the glass ingots had been split in two, each half was chopped up into little chunks or "dice", each approximately the right size to create a lens preform.

 
 
 
Amazingly, the lengths of glass were chopped into smaller pieces using a completely smooth steel "blade", with no abrasive on it, let alone teeth of any kind. As the workers held the pieces of glass against the spinning blade, friction would heat up the point of contact, producing a clean fracture in just seconds.

Here again, the chopping process was incredibly efficient, and relied on thermal shock to do the work. What looked like saw blades that the workers pressed the strips of glass against were actually smooth steel disks, with no teeth or abrasive on them at all. (Arai-san demonstrated how harmless they were by holding his hand against the edge of a "blade" while it was spinning.)


The video clip above shows a worker chopping the longer strips of glass into dice. It was interesting to watch, you could tell the moment that the fracture first formed, as a little line would suddenly become visible inside the block of glass, and the sound of the blade against the glass would change slightly. Very shortly after, the glass would split cleanly into two pieces.

Rather than using abrasive to cut the glass, pressure against the spinning steel disk produced heat from friction, concentrated at the point of contact. Thermal expansion of the glass in that immediate area resulted in a crack that then propagated almost instantly through the thickness of the block. As you can see in the video above, the process was pretty quick, with no kerf loss, powdered glass or expensive diamond blades required.

 
The little dice of optical glass were very pretty, glistening and sparkling in the light. Smaller scrap pieces might make nice earrings for female photography geeks ;-)

The little glass "dice" sparkled in their trays, thanks to their high refractive index. (The lead glass or "crystal" used in fine-dining glasses and chandeliers sparkles as it does because its high index of refraction bends light more, resulting in more internal reflections. The very high refractive index of cubic zirconia is why that gem sparkles so intensely as well. In fact, the too-high "fire" of CZ relative to diamonds is one give-away that this popular diamond substitute isn't the real thing.)

 

Weight-adjusting and rounding

While thermal and friction-cutting are very efficient, there's some variation, due both to the manual procedures involved, and slight variations in the width and thickness of the original glass ingots.

 
A row of large vibratory tumblers, used for grinding the glass dice down to final size. You can see a row of large rotary tumblers in the back, and there were a number of much smaller rotary-drum tumblers out of the shot to the right. There was a lot of tumbler capacity in this room; you're seeing only about a third of it here.

The final lens preforms have to be held to a fairly close weight tolerance, though, so there's a weight-adjusting step between cutting and pre-forming. This is done by grinding the glass dice in large tumblers, filled with smooth rocks, abrasive and a little water. The video above shows a large vibratory tumbler at work, performing this operation. As its name suggests, a vibratory tumbler uses vibration to grind its contents against each other, with the abrasive grit gradually abrading away the work pieces. These were pretty big units, with barrels that I'd estimate to be 2-3 feet (~70-100cm) across, and perhaps a foot (~30cm) deep. The smooth rocks that seemingly fill the barrel are just there to carry the grit and rub up against the optical glass dice that are being ground down.

 
Here's a shot showing a close-up of a tumbler barrel, letting you see a few of the squarish-looking dice that are being ground down, mixed in among the rocks.

Most of the tumblers being used were the vibratory types shown above. There were a number of smaller ones that we don't have pictures of, that were the more conventional rolling-drum type familiar to rockhounds, often used by hobbyists to smooth and polish colored glass, agates and semi-precious gemstones. (I had a couple of smaller versions of these as a boy, and have a tin of polished amethyst, quartz, jade and tigereye somewhere in the piles of detritus stashed in my basement. Vibratory tumblers were available even then, and were way faster than the drum-type ones that I had, but were priced way beyond my budget.)

 
Arai-san explains how the raw glass dice are sorted into four weight categories, which are then ground progressively, to bring them all within the necessary weight tolerance for preforming.

Arai-san told us that this tumbling process was used to adjust the weight of the dice prior to preforming, but I didn't see how just tumbling a bunch of glass dice together would work to homogenize their weights. As I suspected, it turned out that the dice are pre-sorted into weight groups. The heaviest group is loaded into the tumbler first, and once the average weight has reached that of the next-lighter group, that group is added. This process continues until all four weight groups have been added, and the lot of them reduced in weight to bring them within the final size tolerance.

 
Here's what the glass dice look like after they've been tumble-ground. Note the rounded edges and soft matte-finish.

 

Post-tumbling QC and repair

The smoothed dice are visually inspected after tumbling, to check for defects. It seems that common faults are chips, where a small fracture along an edge resulted in a chunk flaking out of the die. Provided the defects aren't too large, some of these chipped dice can be recovered by grinding-out the edges of the chip, as shown in the shot below. (I'd think that this would result in the die involved ending up under-weight, but perhaps there's enough slack in the tolerance that this sort of post-facto repair can still be applied.)

 
I was surprised to learn that chipped dice could be repaired to some extent, by manually grinding away the sharp edges of the chip, to prevent them from cracking during the preforming process. You can see the chip on the top edge of the die here, circled in red marker, and with its sharp edges ground into a smoother curve. Apparently, there's enough room in the weight spec to permit this sort of minor adjustment.

 

Preforming

Optical glass is delivered to lens companies as "preforms", chunks of glass having the general shape of the lenses they're to become. Without thinking much about it, I'd always just assumed that these preforms were made from rectangular chunks of glass by rough-grinding them.

Of course this is a glass factory, though, whose stock in trade is molten glass. So the preforms are of course made by pressing heated, softened glass into rough molds. (I mean duh, right?)


To keep the optical glass from sticking to the preforming molds, it's first coated with fine boron nitride powder. As you'd expect, this is a dusty operation, as the BN powder seemed to be about the consistency of coarse flour. The floor all around this area was a little slippery, thanks to a thin film of the powder that was ground into the cement. (They obviously kept it well-swept, but the powder settled into the fine pores of the concrete itself, making for a slick surface.

Soft glass can be kind of sticky, though, easily attaching itself to molds or tools. To prevent this from happening, the smoothed dice are covered with boron nitride powder, which acts as a mold release agent. Since preforming is carried out at a relatively low temperature (the glass is only soft, not molten), the BN doesn't contaminate the glass, and the outer layer containing it is ground away in the first stage of lens grinding.

 
Preform-pressing looked like something out of the Industrial Revolution, with huge, glowing ovens, open gas flames heating the preform press and molds, and buff, muscled workers laboring stripped to the waist. (Well, actually not the latter, but it certainly wouldn't have seemed out of place. While some areas of the factory were open to the outside air and quite cold, none of us felt a need for our heavy, Nikon-issued jackets in this section!)

Preform-pressing is done either automatically, by machines, or manually. Since the volume of finished preforms is made in Akita itself is lower than in the sister factory in China, most of the forming done there is manual. (Operational details of the one automated pressing machine we saw working while we were there apparently involved proprietary elements, so we weren't allowed to photograph the machine in operation.)


The preform pressing process was a closely-orchestrated dance between teams of either two or three workers. There's apparently some skill involved on the part of the press operator, who needs to judge how long to press each blank, depending on visual cues and monitoring the press operation itself.

As seen in the video above, the manual-pressing operation evoked images of early-industrial metal foundries or the like: Dull-red chunks of glass were flopped into a mold/carrier, and pressed by a pneumatic ram, which was surrounded by gas flames to keep it hot.

 
Here, Arai-san is showing the start of the preforming process. The tan blocks are ceramic holders that carry the boron-nitride-coated glass dice through the long oven you see the exit end of in the background. Pacing for the whole process is governed by the worker doing the pressing. After each die exits the mouth of the furnace and is handed off to the press operator, a worker puts a new die into the carrier and adds it to the head of the line, shown here. The dice travel in their carrier blocks up a conveyor, get pre-warmed along the way, then turn a corner at the far end of the furnace and proceed back down to the mouth.

Glass dice are placed in ceramic carriers, that cycle up and through a long heat-treating furnace. They exit glowing a dull red and are visibly soft and pliable as they're dumped into the bottom half of the mold. One worker pulls each softened die from the furnace and dumps it into a mold held by a second worker, who then places it beneath the heated ram, hits a treadle switch to trigger the ram, and then waits a little while, the duration determined by visual cues he's learned to judge, based on years of experience.

 
One of the preform-pressing stations was making two smaller preforms at a time. It may not have been clear in the movie or other shots, but lower halves of the molds that were attached to the handles had a gas tube running to them, and a ring of burners around each mold half, to keep it at the necessary temperature at all times.

I asked Arai-san how the press-operator knew how long to apply the forming pressure, and he replied that there were three factors: 1) the softness of the glass, based on its appearance, 2) how the glass feels while pressing it, and 3) how the glass feels when it drops from the mold. I can imagine that it takes a lot of experience, to be able to take all these cues into account, to produce perfectly-pressed preforms!

 
Here, a worker inspects just-pressed preforms after they've exited the press mold.

Once the appropriate amount of time has passed, the press-operator triggers the ram's retraction, removes the now-preformed lens element and transfers it to a third operator. From there, it seems it briefly goes into an intermediate-temperature furnace, to relieve the worst of its internal stresses, then is transferred to another, longer-cycle oven, where it's gradually cooled to room temperature.

Annealing

In metallurgy, "annealing" generally refers to a thermal process that reverses the effects of hardening. Annealing can also mean a process that relieves internal stresses caused by too-rapid cooling.

 
This is the row of huge annealing ovens at Hikari Akita. They're stacked two-high; the tops of the bottom level are perhaps 6 feet (2 meters) tall. Notes on the front of each furnace tell: 1) What the type of glass is that's being annealed, 2) What its glass-transition temperature is, 3) What the cooling rate is, and 4/5) two other things I forget :-0

In optical glass manufacturing, though, annealing has a much different purpose, namely adjusting the refractive index. (Stress-relief is important as well, but that would occur with much shorter cooling cycles. The most important function of annealing is to change the refractive index.)

This was the first time that I'd heard that refractive index could be adjusted by thermal processing, so I asked Arai-san how it works. He deferred the question until we could be back in the conference room, with a whiteboard available for him to diagram the process for me.

 
The shot above shows Arai-san and the diagram he drew for us to explain annealing. The critical temperature involved is Tg, the "glass transition temperature". In simple terms, this is the point at which glass goes from being a hard, brittle substance to one that's pliable and can flow. (The full definition of Tg is beyond both the scope of this article and my own understanding, but the preceding is close enough for this discussion.)

In an annealing cycle, the preforms are heated to Tg, held at that temperature for some period of time, and then slowly cooled to some temperature beyond which no further changes in optical characteristics would occur. During this process, the refractive index will change, depending on how quickly or slowly the cooling occurs. The density of the glass is the ultimate controlling factor, and different cooling cycles affect the refractive index because of the influence they exert on density. Slower/longer cooling cycles result in more dense, higher refractive-index glass, while faster/shorter ones produce less-dense, lower refractive-index glass. The annealing cycle needed for each batch of glass is determined by the refractive-index measurements made after the melting process.

 
The annealing ovens were pretty big, towering over our small group. (I think two or three of us could have comfortably sat inside one, without feeling too claustrophobic. The green panel on the front is a chalkboard, where operators would make notes about annealing temperature, cooling schedule, etc, so anyone could tell at a glance what was going on. This was an empty furnace, so had no notes inscribed on it, and hence was one of the only two we were allowed to take detailed photos of.

It's not clear to me just why slower cooling cycles result in greater density; the details of that were beyond the scope of questions I was able to ask during the tour.

Take it as given, though, that slower cooling = higher refractive index, and that annealing gives Hikari Glass very fine-grained control over refractive index.

OK, so much about refractive index, but what about dispersion?

I was struck by how much emphasis was placed on fine-tuning refractive index, and how little discussion we had about dispersion. When I asked later, it turned out that this was because dispersion is a much more complicated topic, and a full discussion of it wouldn't remotely have fit in the time we had. (Especially given how many questions I ask ;-)

Dispersion refers to how much the refractive index varies based on color/wavelength. Dispersion is why a prism projects a rainbow from white light; all else being equal, high dispersion means you'll get a very wide rainbow, low dispersion means you'll see a much narrower rainbow. So-called "ED" glass is characterized by low dispersion.

 
This is the standard "Abbe Diagram", showing dispersion vs refractive index. It looks a bit like a map of the Japanese islands, so optical engineers will often talk about "Hokkaido", "Tokyo" or "Nagasaki" glass. The different regions labeled on the graph give some idea of how different ingredients affect the glass' properties, with barium and lanthanum appearing in the names of some glass types.

The image above shows the standard graph of Abbe number (a measure of dispersion, the thing that makes ED glass "ED") vs. refractive index that will be immediately familiar to any optical engineer. As you can see, there are a lot of different glass types, and this only shows the major categories! As an interesting side note, the general shape of this diagram calls to mind the shape of the Japanese archipelago, so Japanese optical engineers will often refer to a type of glass as a region of Japan. For instance, if an engineer is looking for a glass with low Abbe number but high refractive index (the extreme upper right of the diagram), they'd say they're looking for a "Hokkaido" glass. (Hokkaido is Japan's northernmost island.) On the other hand, a glass from the lower left-hand side of the diagram would be referred to as a "Nagasaki" glass. (Nagasaki is the capital of Kyushu province, and located at the far southwestern tip of Japan.) 

When I asked about dispersion, it sounded like it's a fairly basic quality, affected only slightly by process variations. Apparently, dispersion is somehow set by the overall mix of components in the glass recipe, while the overall refractive index is subject to fine-tuning, by mixing different batches of frit, or (seemingly more routinely) by adjusting the annealing cycle. As noted above though, even a basic understanding of how dispersion is controlled would have required far more discussion than we had time for.

(Dispersion really does seem like a very deep subject, I wasn't able to find much in Google searches beyond dozens upon dozens of pages with the same basic description of what it is, vs how different glass ingredients affect it. I'd really like to learn about it and write up an article on it at some point; maybe I can convince a glass engineer to teach me about it, on another visit to Japan someday ;-)

Visual preform QC inspection

After pressing, the lens preforms go through a visual inspection. Using a bright light in fairly dark surroundings, the workers look for any chips, cracks or other flaws. The shot below shows a preform for a binocular prism that has a crack on one side of it. (Hikari makes optical glass for all Nikon products. Camera lenses are a big part of that, but they also make glass for everything from huge semiconductor stepper lenses to microscope lenses to prisms for binoculars.)

 
After the preforms are pressed, they all go through a visual inspection stage, with a very bright light shone through them against a black background. This will highlight any cracks or imperfections. We have some shots of flawed lens blanks as well, but this photo of a binocular roof prism blank with a crack in it was the best example of how defects might appear.

Packing and shipping

At the end of the whole process, the finished preforms are packed for shipment to Nikon lens factories. It's a long, complex and fascinating process (at least if you're techno-geeks like us), and very impressive that Nikon maintains this entire operation, just to satisfy their internal needs for optical glass. (Or at least 90% to satisfy their internal needs; as mentioned earlier, 10% of Hikari's output is sold on to other manufacturers.)

 
The finished preforms are packed in vinyl trays, with a number of preforms in each carrier - at least for smaller lenses like these. I imagine that the huge front elements for the like of a 600mm f/4 might call for more robust packaging. Then there's glass for the enormous lenses inside Nikon's semiconductor "stepper" exposure systems. Those are so large they must have to be crated individually!

 

Epilogue: Motoyu Ryokan

It was a really long day by the time we were done; we'd departed from Tokyo's Haneda Airport fairly early in the morning (at least for a jet-lagged night owl like me), and it was a pretty intense day of asking questions and absorbing information. So we were pretty happy to roll into our overnight digs that evening, especially since it was a pretty long bus ride to get there from the Hikari factory.

Japan is on the Pacific "Rim of Fire", and many of its mountains are of relatively recent volcanic origin. (Recent in geologic terms, at least.) So there's a lot of magma close to the surface in many places, and hence a lot of natural hot springs as well. So Onsen (hot spring spas) are a significant cultural thing there, and a lot of ryokan (traditional Japanese inns) are built around them. This was probably the fourth or fifth time I've stayed in a traditional ryokan (yes, I'm truly fortunate, and realize it :-), but this was a particularly nice one. I don't know its history, but it's apparently a pretty well-known one, and as always was a great experience.

 
 

Although I don't have many photos to show from the dinner that evening, it was an unusually lavish affair, with more than a dozen little courses/small plates, all delivered more or less simultaneously to the table. We also had some of the best sake that I've ever tasted. The best was a "raw" sake, meaning it still had live yeast in it. It was one of the tastiest liquids I've ever put in my mouth, and I so wanted to bring a couple of bottles home with me. Unfortunately, though, the live yeast meant that it had to be drunk within a week or so of its manufacture, so it would have been past its prime before I even left Japan. (And I had a series of meetings scheduled for a full week following our visit to Akita, so even with my prodigious sake capacity, I wouldn't have been able to do justice to it from my hotel room in Shibuya :-/ )

 
Our not-so-humble abode for the evening, Motoyu ryokan.
 
 

All in all, our tour of Nikon's Hikari glass factory was an extraordinary experience. As I said at the beginning, it was easily one of the most interesting factory tours I've been on, and that covers a lot of ground. It was certainly a tour in which I learned an incredible amount, about things that I had no knowledge of previously.

Nikon obviously wanted us to draw from the experience the impression that they have a unique ability among camera manufacturers, in that they exercise an unparalleled amount of control over the quality of the most basic material that goes into their lenses, namely the glass itself. Even allowing for the obvious PR intent for the trip, though, we came away very impressed with just that: Nikon really does have a unique ability to control their own destiny and optical designs, all the way from the raw materials to their finished lenses.

Entirely apart from the intended PR message, this was a remarkably informative tour, that left us knowing far more than we did before it began. Many thanks to Nikon Tokyo and Hikari Glass for their hospitality and patience, in answering all our many (many!) questions!

 
A finished lens blank from the Hikari factory, ready to be polished and built into an amazing Nikkor lens. (We know it's destined for an amazing lens, because of how big this blank is; this is probably the front element for a long, large-aperture tele :-)

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