Kopin Displays and Near Eye (Followup to Seeking Alpha Article)

Kopin Pixel compared to LCOS

Kopin’s smallest transmissive color filter pixel is bigger than nine of the smallest field sequential color LCOS pixels

After posting my discovery of a Himax LCOS panel on a Google Glass prototype, I received a number of inquiries about Kopin including a request from Mark Gomes of SeekingAlpha the give my thoughts about Kopin which were published in “Will Kopin Benefit From the Glass Wars?”  In this post I am adding morel information to supplement what I wrote for the  Seeking Alpha article.

First, a little background on their “CyberDisplay® technology would be helpful.   Back in the 1990’s Kopin developed a unique “lift-off” process to transfer transistor and other circuitry from a semiconductor I.C. onto a glass plate to make a transmissive panel which they call the CyberDisplay®.  Kopin’s “lift-off” technology was amazing for that era. This technology allowed Kopin to apply very (for its day) small transistors on glass to enable small transmissive devices that were used predominantly in video and still camera viewfinders. The transmissive panel has 3 color dots (red, green, blue) that produce a single color pixel similar to a large LCD screen only much smaller. In the late 1990’s Kopin could offer a simple optical design with the transmissive color panel that was smaller than existing black and white displays using small CRTs.  This product was very successful for them, but it has become a commoditized (cheap) device these many years later.

CyberDisplay pixel is large and blocks 98.5% of the light

While the CyberDisplay let Kopin address the market for what are now considered low resolution displays cost effectively, the Achilles’ heel to the technology is that it does not scale well to higher resolution because the pixels are so large relative to other microdisplay technologies.  For example Kopin’s typical transmissive panel is15 by 15 microns and is made up of three 5 by 15 color “dots” (as Kopin calls them).    But what makes matters worse; even these very large pixel devices have an extremely poor light throughput of 1.5% (blocks 98.5% of the light) and scaling the pixel down will block even more light!

While not listed on the website (but included in a news release), Kopin has an 8.7 x 8.7 micron color filter pixel (that I suspect is used in their Golden-i head mount display) but it blocks even more light than the 15×15 pixel as the pixel gets smaller.    Also to be fair, there are CyberDisplay pixels that block “only” 93.5% of the light but they give up contrast and color purity in exchange for light throughput which is not usually desirable.

There are many reasons why the transmissive color filter LCOS light throughput is so poor.  To begin with, the color filters themselves which are going to block more than 2/3rds of the light (blocking the other 3 primary colors plus other losses).    Because it is transmissive, the circuitry and the transistor to control each pixel block the light which becomes significant as the pixel becomes small.

But perhaps the biggest factor (but most complex to understand, I will only touch on it here) is that the electric field for controlling the liquid crystal for a given color dot extent into the neighboring color dots thus causing the colors to bleed together and loose all color saturation/control.  To reduce this problem they can use less light throughput efficient liquid crystal materials that are less susceptible to the neighboring electric fields and use black masks (which block light)  surrounding the each color dot to hide the area where the colors bleed together.

Field Sequential Color – Small Pixels and 80+% light throughput

With reflective LCOS, all the wires and circuitry are hidden behind the pixel mirror so that non of the transistors and other circuitry block the light.  Furthermore the liquid crystal layer is usually less than half as thick which limits the electric field spreading and allows pixels to be closer together without significantly affecting each other.  And of course there are no color filters which waste more than 2/3rds of the light.    The down side to field sequential color is the color field breakup where when the display move quickly relative to the eye, the colors may not line up for a split second.   The color breakup effects can be reduce by going to higher field sequential rates.

Kopin’s pixesl are huge when compared to those of field sequential LCOS devices (from companies such as Himax, Syndiant, Compound Photonics, and Citizen Finetech Miyota) that today can easily have pixels 5 by 5 microns and with some that are smaller than 3 by 3 microns.   Therefore FSC LCOS can have about 9 times the pixel resolution for roughly the same size device!  And the light throughput of the LCOS devices is typically more than 80% which becomes particularly important for outdoor use.

So while a low resolution Kopin CyberDisplay might be able to produce a low resolution image in a headset as small as Google Glass, they would have to limit the device in the future to a low resolution device – – – not a good long-term plan.  I’m guessing that the ability to scale to higher resolutions was at least one reason why Google went with a field sequential color device rather than starting with a transmissive panel that would have at least initially been easier to design with.  Another important factor weight in advantage of LCOS over a transmissive panel is the light throughput so that the display is bright enough for outdoor use.

I don’t want to be accused of ignoring Kopin’s 2011 acquisition of Forth Dimension Displays (FDD) which makes a form of LCOS.  This is clearly a move by Kopin move into reflective FSC LCOS.   It so happens back in 1998 and 1999 I did some cooperative work with CRL Opto (that later became FDD) and they even used I design I worked on for their silicon backplane in their first product.  The FSC LCOS that FDD makes is considerably different in both the design of the device and the manufacturing process required for a high volume product.

Through FDDs many years of history (and several name changes) FDD has drifted to a high end specialized display technology with a large 8+ micron pixels.   For a low volume niche applications FDD is servicing, there was no need to develop more advance silicon to support a very small device and drive electronics.  Other companies aiming more at consumer products (such as Syndiant where I was CTO) have put years of efforts into building “smarter” silicon that enabled minimizing the not only the size of the display;  reducing the number of connection wires going between the display and the controller; and reduced the controller to one small ASIC.

Manufacturing Challenge for Kopin

To cost effectively assemble small pixel LCOS devices requires manufacturing equipment and methods that are almost totally different from what Kopin does with their CyberDisplay or FDD with their large pixel LCOS.   Almost every step in the process is done with an eye to high volume manufacturing cost.   And it is not like a they can just buy the equipment and be up and running, it usually takes over a year to get the yields up to an acceptable level from the time the equipment is installed.  Companies such as Himax have reportedly spent around $300M in developing their LCOS devices and I know of multiple other companies having spend over $100M and many years of effort in the past.

Conclusion

For at least the reasons given above, I don’t see Kopin as currently positioned well to build a competitive high volume head mounted displays that are to meet the future needs of the market as I think all roads lead to higher resolution, yet small devices.  It would seem to me that they would need a lot time, effort, and money to field a long-term competitive product.

6 comments

  1. magskyic says:

    I agree with your thinking about kopin. But what still bothers me is the rainbow effect from a Field Sequential Color based projector. I just wonder which limit the size of the color filter design, the liquid crystal? can they be smaller? or the color filter, just because it is actually designed for cameras and can only pass 80% by one way, if it is smaller, than will cause problem?

    • admin says:

      Definitely there are issues with both color filter and field sequential.

      The biggest issue with color filter display is that liquid crystal is the electric fields from one color dot crosstalking/bleeding into the other color dot. Transmissive is worse in this respect than reflective because the liquid crystal is nominally twice as thick (because with reflective, the light passes through the liquid crystal twice). Typical LCOS devices have a LC thickness of 1 micron to 1.5 microns thick (depends on the LC and mode etc.). Transmissive devices are as I said, about twice as thick that or 2 to 3 microns thick (big flat panel displays are 4 to 8 microns). If you consider that the thin side of the smaller Kopin pixel is about 3 micron across (give or take a near 1 to 1 ratio of width to height), the electric field extends well into the neighboring cell. Reflective LCOS has the a big advantage in this regard in being thinner and thus less spreading of the electric field, but still there is a lot of color crosstalk even in LCOS color filer devices.

      By putting a black mask over the area of the worst electric field overlap, the color saturation can be improves at the expense of light throughput. Of course no matter what you do, you have to have at least 3 primary colors per pixel; well, not really as you can “cheat” and get away with having green in full resolution and red and blue at half resolution (as Samsung did with their S3 display) and it will work almost as well due to the way the eye sees (green has most of the resolution).

      There is no easy answer as the rainbow effect is a definite drawback with FSC. I do find it interesting that Google Glass went with FSC as it is not what I would have expected.

      Karl

  2. bill says:

    hi Karl, thanks for your articles and thoughts. what about OLED microdisplays? I’d love to hear your thoughts. thanks in advance

    • Karl says:

      Since I first started working on LCOS back in the late 1990’s, I have been hearing about OLED and OLED microdisplays. I have not followed it closely but OLED has had two major problems, cost and stability/lifetime. Kopin claims they have 98% of the military market with OLED (I assume) having the other 2% (so they maybe have 2% of a low volume market).

      There are problems of getting a perfect seal as the least bit of oxygen will cause rapid degradation. One of the toughest problems is that the various colors of OLED age at different rates and thus you get color shifting with age (this is what killed Sony’s OLED televisions about 5 years ago). Even the giant companies have had trouble keeping the colors stable. There are a new set of challenges when you are making very small devices.

      The only volume I have seen in OLEDs microdisplays have been in applications where they are not on constantly (thus increasing their lifetime through non-use). For example a camera viewfinder is only on while you are looking at the camera. The viewfinder is typically a total of a few hours a year at most which is very different from a HMD that could be on thousands of hours a year.

      The smallest OLED pixel I have heard of is my MicroOLED and it has about a 7 micron (three color) pixel pitch. This is still about 5 times the area of the smallest FSC LCOS pixels. This at least for now makes OLED panels about 2 times larger (in two dimensions) than LCOS for the same resolution. I should point out that being too small can have its drawback in near eye as smaller at some point drive up the cost of optics.

      OLED advantages are well known in terms of efficiency, contrast and a much simpler optical path (than reflective technologies of LCOS and DLP) and it would not have the “rainbow effect”. The drawbacks right now clearly outweigh the advantages. The going theory for the last 15 years has been that once they solve all the problems with OLED it will dominate the near eye space. As I point out in a recent post on LCOS and OLED the one long term advantage LCOS has is that it can use laser illumination for very high depth of focus (something that long time HMD expert Steve Mann things is very important).

  3. skyboy10000 says:

    hi Karl, thanks for your introduction. I found 5 by 5 micro pixel is based on 3.3V device. Do you know what kind of device (2.5V or 1.8v) is used for 3 by 3 micro pixel? Will this effect the display quality since the voltage cross LC is lower? Thanks.

    • admin says:

      There are a number of factors that will affect the display quality including voltage include the cell gap (gap between the glass and the metal mirror on the silicon), the liquid crystal “blend” used, and compensating films used to improve the contrast. The compensating films are used to give a good black level but they can do so at the expense of light throughput. All things being equal at lower voltages, there is a tendency to give up contrast and/or light throughput.

      Karl