International projectionist (Jan-Dec 1955)

of interchange of burner parts so that either 9-, 10-, or 11-mm regular, or 10-mm Hitex carbons can be burned effectively. Also, the improved 1614inch reflector lamps may be converted at any time to use the 18-inch reflector— an advantage not offered by any 16V2-inch reflector lamp manu factured up to the present time. Thus the highlights of the improved Strong "135" arclamp. A detailed description of the automatic positive carbon crater positioning system follows immediately, as a general response to numerous requests from projectionists for data thereon. Strong's 'Lightronic' Arc-Gap Control Here in word and illustration is a detailed description of a unit which contributes much toward the solution of a most important and long-standing projection problem. IT IS fitting that the same group of men who developed the automatic carbon-feed control motor for projection lamps should develop a system for automatically regulating the position of the positive carbon in relation to the lamphouse optical system. The automatic feed control at least partially freed the projectionist from being shackled to the arc in the days when operating was relatively simple as compared with today. Projectionists now must divide their attention not only to the burning arc but also to reel sequence, threading, sound, rewind, operation of the curtains by remote control, picture focus, and other details. The need for an automatic crater positioning system stemmed from the constantly-increased burning rate of the positive carbon as requirements for screen brightness have increased. Back in the days of low-intensity arcs, the positive carbon had a burning rate of, roughly, 4 inches an hour, and the angle of convergence, or optical speed, of the light path between lamphouse and aperture was low. Today, with carbon burning rates of 18 to 30 or more inches an hour, and efficient high-speed optical systems, a variation in the burning rate of as little as 2 to 3% can in 20 minutes change the position of the positive crater in relation to lamphouse optical system as much as ^4 inch and result in undesirable color patches on the screen. Crafer Positioning Tolerance Tests prove that the greatest amount of error that can be tolerated in the position of the positive crater in relation to the lamphouse optical system is under 1/32 inch. Exceeding this limit results in loss of illumination and change of color of the light on the screen to either blue or brown. It is impossible to maintain the carbon position within this close tolerance without automatic means, unless the projectionist devotes his entire time to supervising this essential detail during projection. Consequently, theatres using carbons with these higher burning rates encounter color change at the screen to a degree inverse to the amount of attention the projectionist possibly can give this detail. The Strong automatic crater positioning system controls the entire burning of the arc — both positive and negative carbons — even though the burning rate of the positive carbon at a given amperage may vary as much as plus or minus 8% of the normal value. Not only is a steady light of constant color-temperature maintained throughout the reel, but this automatic system insures that the quality of the light is exact for both machines at all times so that the audience will not be aware of changeovers. Any difference in light color-temperature is particularly noticeable when the incoming machine differs in a substantial respect from the outgoing machine in the quality of the light. Unvarying Color Temperature In the last few years projectionists and exhibitors have shown a great awareness of the importance of correct and constant color-temperature of the light on the screen. One of the tests for such constancy in illumination has been the new Cinerama projection technique. The light sources for the three projectors must be maintained absolutely constant in colortemperature and equally brilliant at all times during the 50-minute projection period so that the illusion of one continuous picture across the screen be preserved. To do this without an automatic crater positioning system would be utterly impossible. In the last several years Strongbuilt lamps of the rotating-positive carbon type have incorporated an automatic carbon-positioning system. This system had to be the essence of simplicity, both from the standpoint of minimum maintenance cost and of being consistent with the smallest number of operating parts that can be used, so that possibility of failure be reduced drastically, if not eliminated. All contact-making devices are of the highest quality, completely enclosed and hermetically sealed so that dust and oxidation cannot affect their operation. Since there is a particularly large amount of energy available in the carbon arc, it is not necessary to construct a super-sensitive detection device to pick up small amounts of energy and amplify them into useful quantities. Simple Control Required Since the source of energy being dissipated is measured in terms of kilowatts, it is only necessary to sample a small fraction of this energy to detect an error of position and directly utilize it to actuate the controls of the driving motors which feed the carbons. The control system further required that parts be easily interchangeable — particularly the electrical parts, such as contactors, relays, resistors, motors and optical elements. Simplicity of the currentadjusting control was also a requisite. It consists of a single control so that the projectionist will have the utmost degree of flexibility and ease in adjusting his lamp operation to meet any set of operating conditions. Furthermore, a good carbon-positioning system should not be sensitive to the diameter of a carbon, the brightness of the burning arc, nor to the current intensity being fed to the arc, but rather only by the position of the gas-ball in relation to the optical system. Because in a normal burning arc the position of the positive carbon crater in reference to the negative is determined by factors of voltage, current and arc-gap resistance, it will (Continued on page 26) INTERNATIONAL PROJECTIONIST • APRIL 1955 17