The Other part of SpectraVue

As I mentioned in the original blog posting, there were actually two parts to the SpectraVue invention, The viewing film and the channel waveguide. Although the viewing film died a quick death after AlliedSignal folded the venture, the industry was mightily impressed with the channel waveguide and the design was widely adopted both in backlights for LCD as well as general lighting. The channel waveguide is something of the reverse of the viewing film, using a cons structure to columnate light rather than disperse it. The channel waveguide was rather problematic at the time as it did such a good job columnating light that you could see each individual channel element shining right through the display. The solution that was most obvious, giving the beams from each element some distance to integrate ran contrary to the trend of thinner displays. There was no appreciable LCD monitor business at the time; everything was notebooks. Finer structures and applications that could tolerate some thickness lead to a revival of the technique and AlliedSignal, after buying then changing its name to Honeywell, had a good time suing those that had adopted the idea.

Assuming the channel waveguide structure could be mad fine enough, there is an application for it on the front of some displays as well as in the backlight. If the optical pathway is run in reverse, you have a very efficient structure for capturing all off axis light and dumping it into the wavegude at sub TIR angles. This would give a display with extremely narrow viewing angle, virtually head on only, but the captured ambient light could be redirected to the LCDs own backlight or otherwise used to power the display.

A 12K Display

In other locations, I discuss some benefits of leaving out the color filter from LCD designs for signage: lower thermal radiation absorption, no cf fading, a brighter image. There is, of course a 4th benefit to removing the color filter in that you automatically get 3X more addressable pixels. That could give you a 3K display if applied to what would otherwise be an HDTV resolution screen or a 12K display if applied to the new 4K format.

Corning used to supply the glass for a 19” 12K monochrome CRT in the 1980’s. The customer used Corning’s standard 19” CRT glass to build the display although only about ½ of the glass supplied actually was within their spec. The 12K benchmark was important to the customer as it was a replacement for medical X-ray film. Because of liability concerns, the electronic replacement had to have at least the resolution of what it was replacing. I am unsure what X-ray film resolution is, but my understanding is that standard portrait film is the equivalent of 8K. With the transition to flat panel technology and the requirement of multi- billion dollar fabs, the opportunity to build such displays went away, replaced in part by the ability to pinch and zoom.

With the widespread availability of imagers well in excess of 12K, the market for 12K devices might extend well beyond the medical device market. Again, “digital signage “ is a possibility but there are a variety of other workstations where people look at images where 12K could be useful. … certainly defense as well. I would expect that whether they be field sequential color or monochrome, the product orders for 12K displays will start showing up.

Unbundling the Optical Stack for Better Environmental Performance

LCDs used outdoors as digital signage must be protected from the environment. Typically, they will have a cover glass to protect them from being poked and prodded. They may also have air-conditioning to protect the LCD from reaching its thermal clearing temperature or otherwise suffering thermal damage. Objects in direct sun can reach temperatures as much as 185⁰ F, and more than 70⁰ F above the ambient air temperature. The cover glass may be plate glass or chemically strengthened glass. Chemically strengthened glass has the advantage of being thinner and lighter. Plate glass has an advantage in that if it does fracture, it typically cracks rather than shattering, a feature known as "frangibility". This is especially true if a laminated solution is used. For public spaces, especially confined spaces such as in transportation applications, management of the broken glass hazard may be critical.

However, the cover glass may also be part of the thermal solution. The polarizers of an LCD, by definition, absorb about 50% of the light that hits them. They are agnostic as to which direction the light comes from absorbing 50% of inbound sunlight as well. Removing the outer polarizer from the LCD and laminating it between the layers of the cover glass puts a few millimeters of glass and an air gap between the absorbed thermal energy and the liquid crystal. This would cut the solar radiation load on the LCD by 50% and also provide the opportunity for passive, chimney, cooling rather than a powered air conditioner. Here, I discus removing the color filter as well. In addition to the brightness benefit, removing the color filter would have some solar radiation benefits as well. Certainly it will obviate any issues with solarization or fading of the color filter.

SpectraVue for Outdoor Signage

When LCDs had terrible viewing angle, many solutions were being developed to fix the issue prescriptively. Of course, the best solution is most always fixing the issue rather than applying a bandaid which is why most of these external fixes were dropped. One of these external fixes was a product called SpectraVue. It consisted of two components, and channel waveguide and a viewing film. The viewing film was an array of cones embedded in a black matrix. Light from the LCD entered the array columnated, bounced off the side of the cones a few times and left the viewing film very dispersed. The film produced an LCD appearance that was virtually the same as a CRT. It was described by Toshiba Labs as "Virtually Perfect viewing angle". Although I have seen several startups and other corporate projects where the intended function of the device depended on building a light funnel, basic geometry shows that funnel structures have exactly the opposite effect on bosons as they do on physical matter. Spectravue takes advantage of this effect to essentially provide something of a photon diode.

Although it was a great solution to the LCD viewing angle problem, it had two problems of its own. The first is that the maker, AlliedSignal, had problems actually manufacturing the product. The second, and more compelling issue was that it was a cost adder. It was being developed at the same time as IPS technology which fixed the problem in the LCD with no additional layers or costs.

Examining the diagram, there is another advantage that SpectraVue had, though it did not count for much at the time. The surface is virtually all blackfill. It adds inherently better sunlight viewability withoug significant loss of display aperture. As IPS development was the reconsideration of an old solution, as I note in my previous post, reconsidering the LCD optical stack might be in order for the digital signage market.