Archive for April 2, 2013

Extended Temperature Range with LC Based Microdisplays

cookies and freezing

Extreme Car Temperatures

A reader, Doug Atkinson, asked a question about meeting extended temperature ranges with LC based microdisplays, particularly with respect to Kopin.    He asked the classic “car dash in the desert and the trunk in Alaska” question. I thought the answer would have broader interest so I decided to answer it it here.

Kopin wrote a good paper that is available on the subject in 2006 titled “A Normally Black, High Contrast, Wide Symmetrical Viewing Angle AMLCD for Military Head Mounted Displays (HMDs) and Other Viewer Applications”. This paper is the most detailed one readily available describing the how Kopin’s transmissive panels meet the military temperature and shock requirements.  It is not clear that Kopin uses this same technology for their consumer products as this paper is specifically addressing what Kopin did for military products.

With respect to LC microdisplays in general, it should realized that there is not a huge difference in the technical spec’s of the liquid crystals between the LC’s  most small panel microdisplays use and large flat panels in most cases. They often just use different “blends” of the very similar materials. There are some major LC differences including TN (twisted nematic), VAN (vertically aligned nematic), and others.   Field sequential color are biased to wanting faster switching “blends” of the LC.

In general, anywhere a large flat panel LC can go, a microdisplay LC can go. The issue is designing the seals and and other materials/structures to withstand the temperature cycling and mechanical shock which requires testing,  experimentation, and development.

The liquid crystals themselves generally will go through different phases from freezing (which is generally fatal) to heating up to the the “clearing point” where the display stops working (but is generally recoverable).  There is also a different spec for “storage temperature range” versus “operating temperature range.” Generally it is assumed the device only has to work in a temperature range in which a human could survive.

At low temperature the LC gets “sluggish” and does not operate well but this can be cured by various “heater mechanisms” including having heating mechanisms designed into the panel itself.  The liquid crystal blends are often designed/picked to work best at a higher temperature range because it is easier to heat than cool.

Field sequential color LCOS is more affected by temperature change because temperature affects not only the LC characteristics, but the switching speed. Once again, this can be dealt with by designing for the higher temperature range and then heating if necessary.

As far as Kopin’s “brightness” goes (another of Doug’s questions), a big factor is how powerful/bright the back light has to be. The Kopin panel blocks something like 98.5% of the light by their own spec’s. What you can get away with in a military headset is different than what you may accept in a consumer product in terms of size, weight, and power consumption. Brightness in daylight is a well known (inside the industry) issue for Kopin’s transmissive panels and one reason that near eye display makers have sought out LCOS.

[As an aside for completeness about FLC]  Displaytech which was sold the Micron and then sold to Citizen Finetech Miyota and the Kopin bought Forth Dimension Display (FDD) both use Ferro-electric LC (FLC / FLCOS) which does have a pretty dramatically different temperature profile that is very near “freezing” (going into a solid state) a little below 0C which would destroy the device. Displaytech claimed (I don’t know about FDD) that they had extended the low temperature range but I don’t know by how much. The point is that the temperature range of FLC is so different that meeting military spec’s is much more difficult.