[Tom_Ryan]

I worked for Sony many years ago and knew several design engineers at the Trinitron display division. As I recall, Sony projection sets were manufactured in PA. They had several challenges but the three big ones were, first, achieving an acceptable field of view, the second was screen brightness, and the third, convergence.

Most people expect a bright screen image comparable to direct view CRTs. Achieving this in a rear projection set is a real engineering challenge. Driving a bright CRT is part of the solution; the real engineering marvel was the development of the high efficiency screen. It's basically the same problem encountered with a film projector screens - brightness fields are generally confined to narrow viewing angles. Solving this problem was essential to creating a screen with uniform high brightness over a wide viewing angle. Projection sets today use a specially constructed bipanel. It's a combination of a large fresnel lens matted to a lenticular light diffusing panel. The combination is designed to create a wide horizontal angle of view within a limited vertical aperture. This helps to concentrate the light to the most desirable viewing angles. This is a diffusion screen because light diffuses through it. Recall, that some of the earliest projection sets used reflected screens. The CRT assembly mechanically sat in front of the TV set and projected light onto a screen which then reflected light back to the viewer. Reflected screens do take up a lot of room. The use of diffusion screens made it possible for engineers to make projection TV sets more compact.

The second achievement was engineering a liquid cooled high intensity display tube that would resist phosphor burn. Operating a tube at high intensity does risk burning tube phosphor if sweep circuits suddenly stop working. Many early projection sets were prone to accidental phosphor burn with this type of circuit failure. Today's sets are better at sensing and preventing these problems - although they still happen. However, even with an efficient screen, to compete with direct view CRTs, projection set require high intensity projection tubes - things get expensive because 3 are required to create a colored image. I can recall once looking down into the CRT's of a projection chassis operating on a design test bench. The CRTs were so bright they can blind you! You are basically accelerating electrons onto a very small area with typically more 30KV accerating voltage. I saw a demonstration of a tube operated with liquid cooling removed and boy did it heat up quick. The faceplate got so hot that the phosphor was totally cooked. Projection tubes do tend to push CRT brightness technology to the limit.

The third problem was convergence. Early projection sets, without all the fancy computer alignment stuff found in todays sets, typically made the task of convergence overwhelming for most technicians - especially corner convergence problems.

Of course, there is trend to make modern projection sets light weight and portable - somewhat hard to do with traditional projection systems; however, DLP technology really changes all that now. Digital light projector use optical semiconductors instead of CRTs. This is a totally different technology that's revolutionized home theater and business applications.

Tom