Necessity

Cathode ray tube technology was very limited in the early days of television. It relied on conventional glass blowing methods largely unchanged in centuries. Since the tube had to contain a very high vacuum, the glass was under considerable stress, together with the low deflection angle of CRTs of the era, the practical size of CRTs without increasing their depth was limited.^[2] The largest practical tube that could be made that was capable of being mounted horizontally in a television cabinet of acceptable depth was around nine inches. Twelve inch tubes could be manufactured, but these were so long that they had to be mounted vertically and viewed via an angled mirror in the top of the cabinet. In 1936, the British government persuaded the British Broadcasting Corporation to launch a public high definition (for the era^{[lower-alpha 1]}) television broadcasting service.^{[lower-alpha 2]} The principal driver for the British government's move was to establish cathode ray tube production facilities which it believed would be vital if the anticipated World War 2 was to materialise.

The ability to correct the deflection signals for aberrations in tube geometry had not yet been developed, and it was necessary to make tubes that were relatively long compared with their screen size to minimise distortion. However, because the tube face had to be convex to provide resistance to air pressure, this mitigated the problem but only if the apparent deflection centre was more or less at the centre of curvature of the screen. This necessitated a tube that was relatively long for its screen size. The accelerating voltage used for these tubes was very low by later standards and even a twelve inch tube only ran from a 5000 volt supply. The early white phosphors were not as efficient as later offerings and these early televisions had to be watched in subdued lighting.

Solution

In 1937, both Philips and HMV put on display at the Radiolympia show in London, television sets that had a screen size of 25 inches based on the same MS11^{[lower-alpha 3]} Philips/Mullard^{[lower-alpha 4]} tube.^[3] These had been the subject of an advertising campaign prior to the show which generated much interest. The television back projected the image from a 4+1⁄2 inch tube onto a 25 inch etched celluloid screen sandwiched between two sheets of glass for protection. The tube size was dictated by the fact that it was the largest tube that could be made with a flat screen. It had not been appreciated at this time that a curved screen was optically better if the centre of curvature of the screen was in roughly the same place as the centre of curvature of the mirror. The tube was mounted vertically in the bottom of the cabinet with the screen pointing downward towards a concave mirror which reflected the image upward toward an angled mirror at the top of the cabinet onto the 25 inch screen, The top of the tube mirror box had a Schmidt lens^{[lower-alpha 5]} to correct aberrations. Because the picture had to be magnified to illuminate a screen that had about 100 times the area of the picture on the tube face, the image on the tube had to be very bright indeed. To achieve the required brightness, the tube was driven from a 25,000 volt accelerating supply.^{[lower-alpha 6]} As betrayed by the tube type number, the phosphor was green which was brighter for a given beam current that the contemporary white phosphors.

Unfortunately, both Philips and HMV had to withdraw their sets from exhibition by the afternoon of the first day as the cathode ray tubes had failed in both cases. Customers who had purchased these sets were disappointed to discover that their tubes rarely lasted longer than a few weeks (bearing in mind that there was only one hour of television broadcasting each day). By November 1937, Philips decided that it was more economic to buy the sets back rather than keep replacing tubes under warranty, which were becoming harder to source as the demand outstripped supply.^{[4][lower-alpha 7]} No information is available as to how HMV handled the problem.

By 1938, Philips had substantially overcome the shortcomings of the previous cathode ray tube to produce the Philips/Mullard MS11/1^{[lower-alpha 8]} projection tube.^[5] This new tube was basically similar but had a larger cathode that required more heater power which was able to support a higher beam current.^{[lower-alpha 9]} This new tube retained the green phosphor screen of the earlier tube. The television set also had a smaller 21 inch screen which was roughly three quarters of the area of the previous year's model which meant that the tube did not have to be driven so hard. Purchasers of this later model only got to use it for a year or less as television broadcasting was suspended in 1939 for the duration of the Second World War. Both models of the television had a problem in that the high accelerating voltage on the tube meant that it produced substantial X-radiation. This was not widely looked at as a concern in the 1930s.^{[lower-alpha 10]} Fortunately most of this radiation passed through the bottom of the set from the downward pointing tube.

In the United States of America television broadcasting became more widespread at the end of the Second World War.^[6][7] Although cathode ray tube technology had improved during the war such that tubes became shorter for their size, as it was now possible to correct distortions, twelve inches was still the practical limit on size. However, it was now possible to mount a twelve inch tube horizontally in an acceptable cabinet size. As a result of these size limitations, rear projection systems became popular^[8][9] as a way of producing television sets with a screen size larger than 12 inches.^[10] Using a 3 or 4 inch monochrome CRT driven at a very high accelerating voltage for the size (usually 25,000 volts^[11] though RCA did produce a larger five inch tube that required 27,000 volts.^[10]), the tube produced the extremely bright picture which was projected via a Schmidt lens and mirror assembly onto a semi translucent screen of typically 22.5 to 30 inches diagonal in size using an optical system practically identical to the earlier Philips system described above. The only change was that RCA used the optically superior convex screen on the tube having figured out that the Schmidt lens did not have to correct for the curvature of the tube face but only the spherical aberration of the mirror. The resultant picture was darker than with a direct view CRT and had to be watched in very subdued lighting. The degree to which the tube was driven meant that the tube had a relatively short life.

When British television broadcasting resumed in June 1946, television production was slow to resume mainly due to shortages of materials following the war. As already noted, twelve inches was still the practical upper limit for direct view cathode ray tubes. In response, in 1950, the Philips company via its Mullard subsidiary introduced a new projection tube, the MW6/2.^{[lower-alpha 11][12]} Although the basic idea of the tube had not changed, it was smaller at just 2+1⁄2 inches and now featured a convex screen face, taking advantage of the intervening American developments. It was also around four inches shorter and now featured a more efficient white phosphor developed during the war. This tube allowed for a more compact rear projection system. The tube was mounted horizontally and directed toward a concave mirror as before, but this time the reflected image was turned through ninety degrees by a plane mirror with a central hole for the tube. It was then reflected upward through a Schmidt correcting lens before being reflected through a further ninety degrees to strike the screen.^{[lower-alpha 12]}

This new tube and optical system offered several advantages over the previous system. The set cabinet was able to be smaller. Previously the screen was on top of a substantial piece of furniture but this new system allowed the screen to occupy a similar position as a direct view television's screen in a regular console sized cabinet. The Schmidt was still required to correct the image for spherical aberration from the mirror.^{[lower-alpha 13]} The use of an additional plane mirror allowed the deflection coils and the focussing magnets to be positioned behind this mirror out of the light path. Previously they had partially obstructed the image from the concave mirror being somewhat larger than the tube's screen. The optical box that housed the tube was also designed to shield the X-radiation produced by the tube. The optical boxes were produced in three versions for 15+1⁄2, 17+3⁄4 and 19+7⁄8 inch [diagonal] screens. Two further sizes were available for front projection onto 44 or 52 inch screens.^[13] The difference was solely the position of the tube's screen relative to the concave mirror and the optical characteristic of the Schmidt lens. This new system provided acceptable pictures that were bright enough when viewed in subdued lighting. However, the bright image in the tube's screen along with it still being driven hard meant that the tube's life was still substantially shorter than contemporary direct view tubes. A rear projection set would require at least one or two replacement tubes during its lifetime. This inconvenience was somewhat offset by the tube's relatively low price compared with the larger direct view versions, partly due to the quantities in which they had to be produced, plus the fact that they were fairly easy to replace.

As the 1950s unfolded, there were several major advances in cathode ray tube technology. Pre stressing the bulb of the tube with steel bands around the outside of the screen for implosion protection allowed larger tube diameters to be produced. Improvements in correcting for deflection aberrations on those screens allowed larger deflection angles and consequently shorter tubes for a given screen size. Further: much simpler deflection systems had been developed that could generate the large currents required without consuming the power of earlier circuits. By 1956, the ability to produce near rectangular faced tubes was developed. This was facilitated by the pre stressing, but still required the walls to have a convex shape to withstand the atmospheric pressure.^[14] Although 17 inches in size was the largest size at this time, it was large enough to render rear projection technology obsolete for the immediate future. Using the superior white phosphor of the post war period and higher accelerating voltages,^{[lower-alpha 14]} televisions were larger and brighter.

As television technology developed and picture quality improved, limitations in cathode ray tube sizes became an issue once again. Even though larger screen sizes with short tube lengths were available, there was a revival of interest in rear projection systems to achieve picture sizes that were beyond the capabilities of direct view cathode ray tubes of the time. Modern color rear-projection television had become commercially available in the 1970s,^[15][16][17] but at that time could not match the image sharpness of a direct-view CRT.

Given their already large dimensions, projection TVs sometimes included larger speakers and more powerful built-in audio vs direct view CRTs and especially depth-limited flat panels, as well as basic surround sound processing or emulators such as Sound Retrieval System (SRS) by SRS Labs, similar to a sound bar.