A remarkable contribution of the United States to searchlight science was the production of a satisfactory metal mirror for projecting the beam. The metal mirror not only weighed a little less than the glass mirror, but it cost only one-third as much as the glass one, could be produced in one-fifth the time, was much less fragile, and extended the possibility of manufacture to a wide number of industries. The metal mirror possessed 97 per cent of the reflectivity of the glass mirror. This slight dullness is inappreciable in searchlight work and more than compensated for by the other qualities of the metal reflector. This type of mirror, however, had not yet been put in production when the war ended.
Our inventors during the 19 months of hostilities succeeded in reducing the size of carbons used in 200-ampere lamps from 2 inches in diameter to 1⅛ inches. This cut the cost of carbons in two, but the improvements tripled the amount of light developed.
In November, 1918, we were working with assurance of success to develop a simple system whereby field searchlights could be pointed and controlled from a distance. Such controls had been used in experimental work prior to 1917, but the mechanisms were complicated and not suitable for field service.
The searchlight section of the Corps of Engineers also developed optical finding devices, which doubled the range of all searchlights without requiring any modification of the lights themselves. Neither the ordinary telescope nor night glass is suitable for target finding by searchlight. The result of our investigation was the development of a combined observer's chair, eye protector, and searchlight target finder, the new equipment adding only 10 per cent to the cost of the searchlight unit.
The range of our modern high-power searchlight, whose target is a ship at sea, is about 15,000 yards; the range of this searchlight when its target is an airplane is about 15,000 feet.